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16MMC500 INDIVIDUAL PROJECT SPORTS TECHNOLOGY (BSc)

STUD CONFIGURATION DESIGN AND THEIR EFFECT ON TRACTION

FINAL REPORT

2016/17

MICHAEL MEIKLEHAM B321195

Supervisor: Steph Forrester 2nd Reader: Séan Mitchell

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ABSTRACT

Traction is a factor within sports equipment design that has important implications for

performance and injury risk. Different stud types have different traction properties

which allows the athlete to perform sporting movements to a higher level. Artificial

turf was introduced to imitate natural grass and FIFA instilled testing procedures to

ensure the playability of these surfaces from a performance and injury risk

perspective was similar to natural grass. This includes a rotational traction test to

quantify the rotational resistance of the surface to studded footwear.

The objectives of the study were:

To obtain a greater understanding of the traction that occurs at boot – surface level

on artificial surfaces by conducting FIFA testing on different stud types and analysing

the quantitative data that is outputted.

The second objective is to gain a further insight into the mechanical testing

procedure and how different variables affect peak torque and stiffness.

The final purpose of the study is to conclude on what stud type has the best traction

properties out of the studs selected.

Testing highlighted that the longer the stud length the higher the peak torque. There

was minimal difference between the material types metal and plastic. The triangular

stud types portrayed that orientation and shape is an important factor as these

variables had the smallest difference between early stiffness (between 0-5°) and

rotational stiffness found later on (between 25-40°).

There were many variables that were tested within this study however some were

non-conclusive and more need to be done in future proceedings to fully understand

the effect stud types have on traction at the boot surface level.

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ACKNOWLEDGEMENTS

I would like to take this opportunity to thank the Sports Technology Institute and the

Wolfson School of Mechanical Manufacturing Engineering at Loughborough

University, for providing the facilities and apparatus to make this project feasible. I

would also like to thank my mentor Dr Steph Forrester, who designed the project and

gave the chance to direct the research. Steph’s provision and guidance throughout

the duration of the project was invaluable and deeply appreciated. Finally, I would

also like to thank Sports Technology Technician Max Ferrand with assistance of the

initial equipment set up during the early phases of the testing period.

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ABBREVIATIONS

3G – Third Generation Artificial Turf

COT = Coefficient of Traction

FG = Firm Ground

SG = Soft Ground

HG = Hard Ground

AG = Artificial Ground

LPC = Long Plastic Conical

SPC = Short Plastic Conical

LMC = Long Metal Conical

SMC = Short Metal Conical

FST = Flat Side Triangular

PST = Pointed Side Triangular

CSV = Comma Separated Value

FIFA = Fédération Internationale de Football Association

Kg = Kilograms

s = seconds

SBR = Styrene Butadiene Rubber

Hz = Hertz

m = metre

GRF = Ground Reaction Force

PE = Polyethylene

PP = Polypropylene

2EW = Silica – based sand

FR = Force Reduction

VD = Vertical Deformation

v = Speed

r = radius

rev = Revolutions

min = Minute

μ= minimum coefficient of traction

N = Newtons

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CONTENTS

TITLE

STATEMENT OF ORGINALITY

ABSTRACT

ACKNOWLEDGEMENTS

CONTENTS

Chapter 1 – INTRODUCTION 1.1 Introduction

1.2 Flowchart

Chapter 2 – LITERATURE REVIEW 2.1 Artificial Surface Introduction 2.1.1 History 2.1.2 Construction 2.1.3 Fibres 2.1.4 Infill 2.1.5 Cost 2.1.6 Maintenance 2.1.7 Natural vs Artificial Surfaces 2.1.8 Testing 2.1.9 Surface Summary 2.2 Traction Introduction 2.2.1 Defining Terminology

2.2.2 Measuring Traction

2.2.3 The Player

2.2.4 FIFA Rotational Traction Device

2.2.5 Alternative Mechanical Testing Devices

2.2.6 Vertical Load

2.2.7 Constraints of Testing Device

2.2.8 Velocities and the Test Foot

2.2.9 Measurements

2.3 Football Boot History 2.3.1 Pre 19th Century

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2.3.2 1940’s / 50’s

2.3.3 1960’s / 70’s

2.3.4 1980’s

2.3.5 21st Century

2.3.6 Boot Summary

2.4 Stud Types 2.4.1 Hard Ground

2.4.2 Artificial Ground

2.4.3 Soft Ground

2.4.4 Firm Ground

2.4.5 Stud Design on Traction

2.4.6 Stud Length effect on Traction

2.5 Literature Summary

Chapter 3 – Equipment Methodology 3.1 Introduction

3.2 Rotational Traction Device

3.3 Studs

3.4 Artificial Surface Test Piece

3.5 Lab Testing

3.5.2 Test Procedure

3.6 Pilot Testing

3.7 Data Processing

Chapter 4 – Results 4.1 Pilot Results

4.2 Stud Type vs Peak Torque

4.3 Stud Type Vs Stiffness

4.4 Stud Length Vs Average Peak Torque

4.5 Angular Velocity Vs Peak Torque

Chapter 5 – Discussion

5.1 Introduction

5.2 FIFA Traction Device

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5.3 Velocity Testing

5.4 Studs

5.5 Confidence in Testing

5.6 MATLAB Script

Chapter 6 – Conclusion 6.1 Overview

6.2 Summary of Findings

6.3 Future Research

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Chapter 1

INTRODUCTION In terms of optimising an athlete’s performance it is vital to obtain maximum traction

at the boot-surface level. Certain governing bodies of sports, such as FIFA for

football, have introduced test standards in order to ascertain the level of quality that

the playing surface is at. This is exceedingly applicable as in recent years the

number of artificial pitches that have been installed around the world has increased

(Sport England).

Variables that affect traction are predominantly the boot, surface and the player.

Factors within these variables are outsole design, stud type, stud configuration,

surface type and surface infill. Many researchers have conducted experiments into

how each of these different factors affects traction, however trying to quantify the

interaction between the boot and the surface can be very complex. Throughout this

project each stud will be compared to another stud that has only one differing

variable. For example, long metal conical vs short metal conical (with length being

the differing factor), long plastic conical vs short plastic conical (with length being the

differing factor), flat edge triangular and pointed edge triangular (with orientation

being the differing factor). Then material comparisons can be made between the

conical studs that have similar dimensions. Academics and various sporting

governing bodies have designed and manufactured a range of mechanical test

devices with the intent of quantifying traction.

The primary aim of this study is to obtain a greater understanding of the traction that

occurs at boot – surface level on artificial surfaces by conducting FIFA testing on

different stud types and analyse the quantitative data that is outputted.

The second objective is to gain a further insight into the mechanical testing

procedure and how different stud variables affect peak torque and stiffness.

The final purpose of this study is to conclude on what stud type has the best traction

properties out of the studs selected. The outcomes have relevance for boot

manufacturers such as New Balance, to help inform their future designs.

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Football

Boot History

Traction

Chapter Two Literature Review

Artificial Surfaces

Stud Types

Chapter One Introduction

Chapter Four Results

Chapter Three Methodology

Test Procedure

Equipment

Data Processing

Chapter Five Discussion

Chapter Six Conclusion

Figure 1.1 - Flow diagram highlighting how the chapters interact throughout the report

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Chapter 2

LITERATURE REVIEW

2.1 Artificial Surface Introduction The most common type of artificial pitch in the United Kingdom is 2G (Sport England

2005). However, as technology and government funding increase, more 3G pitches

are being installed and forecasted to become the most popular.

Artificial surfaces provide a great alternative to grass pitches. Both pitch types are

subject to many variables such as their construction, how they deteriorate over time

and their exposure to weather. Artificial surfaces allow an increase in playing time

due to a reduction of maintenance costs. Large amounts of studies have been

conducted into what effects artificial turf has on boot – surface interactions. This

allows the author to draw stark comparisons between synthetic and natural grass

pitch types.

2.1.1 History The introduction of artificial turf was first brought in by the Americans during the

1960’s. The synthetic carpet was first used as a substitute for natural grass in

baseball stadiums such as the Astrodome in Houston, Texas and the Comiskey Park

in Chicago (Brady, 1972). Prior to this installation, the field conditions of the natural

grass were below satisfactory and inadequate for elite players, with sometimes the

dead grass being painted green. This lead to 1200 artificial pitches being installed in

the US during 2013 and even more around the world (Weeks, 2015). This message

transcended to the UK as numerous top flight football teams opted for an artificial

pitch, the first was at Loftus Road, the home of Queens Park Rangers in 1981 (BBC,

2014). Since then, most elite clubs have resorted back to the traditional option of

natural grass and in more recent times hybrid surfaces have been made use of

(Desso, 2007).

2.1.2 Construction In order for an artificial pitch to provide traction and stability various materials are

layered together to make the artificial playing surface (Villwock MR, 2008). As shown

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in Figure 2.1 the surface typically consists of a rubble foundation and drainage pipes,

all the way up to the shock pad and grass. Some of these constituents such as grass

length and amount of infill vary depending on what type of pitch it is (Alcantara E

Gamez, 2009). But on the whole the layers remain the same throughout the different

models.

Figure 2.1 - Cross-sectional view of the construction of an artificial surface

The foundations are designed to provide overall support and a level base for the

artificial turf to be laid upon. Usually, this is rubble and compact soil and drainage will

be integrated to take surplus rainwater away from the sub base layer and prevent

flooding (Prestige Civil Engineering, 2014). A 250-320mm layer of gravel and stone

(*Depths and dimensions of each layer vary depending on model and application

(Blakedown, 2014)) is laid on top of the foundations with a ‘geo-synthetic’ membrane

on top of that which assists drainage away from upper layers. The shock pad is

usually produced from a synthetic rubber or open cell foam, below this is a binding

layer of asphalt, typically 60-80mm thick which gives stability.

2.1.3 Fibres The fibres on a 3G pitch are commonly manufactured from polyethylene (PE) or

polypropylene (PP) which is coloured green to give the surface a grass like aesthetic

(Anderson, 2008). First and second generation pitches are made from nylon which

can be very abrasive on the skin. However, the advancements in material selection

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of PE and PP are much less abrasive than its former counterparts. These PE or PP

fibres can be monofilament or fibrillated during construction. Monofilament fibres are

textured and / or twisted and come in a variety of thicknesses designed to help the

individual fibres stay vertical. The fibrillated fibres are slit during their processing to

create a honeycomb effect that is claimed to reduce the movement of the infill and

therefore increase the performance of the surface (Alcantara E Gamez, 2009).

2.1.4 Infill In 3rd Generation surfaces there are typically two layers of infill used. There is a

stabilising infill which consists of silica-based sand (2EW), this is used to add weight

to the carpet and help keep the fibres upright (Severn KA, 2010). The carpet and the

shock pad are not bound together with any adhesive or alike so the carpet is reliant

on its weight in under to stay fixed to the shock pad. These granules of silica sand

typically range between 0.25 – 0.39 mm in size (Aggregate Industries, 2006). As

there is little space for the sand to fall into, these granules build up and become

incompressible and extremely difficult to penetrate, giving the stabilising sand infill a

very low density. This then has an effect on the outcomes of the testing procedures

such as an increase in force reduction and a decrease in vertical deformation

(Alcantara E Gamez, 2009). The stabilising infill has little effect on the traction

properties as stud penetration is unlikely to go as deep as the sand. Although, as

previously mentioned the weight of the sand helps keep fibres upright and this may

affect traction if it were to change.

The second type of infill used is performance infill which consists of broken down

recycled car tyres and made of styrene butadiene rubber (SBR). These granules

range in size but are typically 1 – 2 mm in diameter. Due to the SBR granules being

bigger than the silica sand and having a more irregular shape it creates air voids

which are spaces of air in between each granule. This allows greater stud

penetration as there is more room for the rubber to compress (Severn KA, 2010).

The performance infill is a huge variable in the outcomes of mechanical testing.

Severn et al (2010) found that when using the rotational traction device, the density

of the rubber granules can change under compression and compaction. Meaning

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dependant on the vertical load applied, the properties of the infill can change which

ultimately has an effect on traction.

2.1.5 Cost The typical area of an artificial facility is around 7000 m2 which can price anywhere

between £400,000 and £1.2million. For a senior football 3G surface on average the

installation costs are around £700,000. However, these costs include floodlights, car

parks, roads and paths.

The aforementioned costs were taken from a 2011 report from Sport England so it is

possible that the current costs of purchasing raw materials have increased since

then, making artificial surfaces more expensive.

2.1.6 Maintenance Many organisations install artificial pitches with the belief that the surface needs little

or no maintenance. Albeit they need less time and care than elite level grass pitches

but they still need a substantial amount of upkeep to prevent surface degradation in

the future. Alcantara et al (2009) found that a lack of maintenance lead to a

deterioration in surface properties such as vertical ball rebound height and ball roll

etc. Similarly, conveyed through the study of McLaren et al (2014) it was shown that

if the surface is maintained correctly the deterioration effects will be minimal and the

surface will meet the standard requirements. According to McLeod’s survey of recent

developments in 2008, the average annual maintenance spend on artificial turf is

£8000. The average weekly use is 44 hours and it costs on average £3.49 per hour

to maintain. Jan Kieft (2009) observed 50 artificial 3G pitches over a seven year

period and by the end of this timeframe only one of the pitches managed to meet the

FIFA Quality standards. Foreign debris such as leaves, twigs, mud and litter can

cause migration of performance infill and leave the rubber granules unevenly spread.

Mechanical wear like player loading or machinery loading can cause a decrease in

force reduction (FR) and vertical deformation (VD). As shown in Jan-Kieft’s study

both mechanical properties decreased; FR lowered by 10% and VD dropped by 5

mm. In addition, weathering is also a contributing factor as a high ambient

temperature can cause the colour of the carpet to fade which can be perceived as a

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decrease in value (Schoolenburg G.E, 1991). Surface moisture can cause a

decrease in traction as severe rain inhibits the fibres from remaining vertical which

leads to them bending and flattening meaning an increase in slippages (Gulminea,

J.V, 2003).

Leaving a synthetic facility without maintenance even in the first 12 months can

affect its longevity significantly. During its first year the constituents of the assembly

are still in the bedding process so maintenance is vital. A complete preservation

programme would include drag brushing the surface at least weekly. Drag brushing

(shown in figure ?) keeps the infill evenly distributed over the surface as during use

the rubber granules migrate from high use areas (goal mouths, centre circles, exits

and entrances) to areas of low use (pitch edges and corner flags). Grooming also

plays an important part in maintenance but is very similar to drag brushing as it

keeps the performance infill evenly distributed. Power sweeping removes finite

elements such as dust from the surface, these small contaminants cause the infill to

harden as the dust acts like a cement which in time will cause systematic failures

with drainage and performance.

2.1.7 Natural vs Artificial Surfaces The main comparison found in recent literature when evaluating the difference

between artificial turf and natural surfaces is how the player loads their foot. During

human testing, Ford KR (2006) found that there were considerably higher peak

pressures in the central and lateral part of the forefoot on artificial turf compared to

grass. The reasons for this remain unclear but it is obvious the players adopt a

different technique dependent on the surface and pitch conditions.

Drag Brushing Grooming Power Sweeping

Figure 2.2 - Visual representation of different maintenance methods

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A study by Torg JS et al (1996) found that an increase in air and turf temperature

(ranging between 52°F and 110°F) had an effect on shoe – surface traction for

artificial turf. This is mainly due to the styrene butadiene rubber expanding upon heat

exposure and reducing the air void in between granules. This makes it more difficult

for the studded boot to penetrate the performance infill due to an increase in density.

In contrast, in more recent literature it has been portrayed that there is no evidence

gathered regarding a relationship between the ambient air temperature and its effect

on traction on a natural playing surface. The only relationship between an increase in

ambient temperature and natural grass was an increase in injury frequency. A major

contributor to this relationship was ground hardness, which occurs in drier and

warmer conditions during summer. This has a great effect on contact sports such as

rugby when athletes have a large impact with the surface (Villwock, 2009).

Other readings using mechanical testing instruments have found substantial

increases in rotational traction on artificial surfaces compared to natural grass.

(Villwock, 2009). Currently, there isn’t a standardised performance test for both

natural and artificial turf making it difficult to compare directly.

2.1.8 Testing Current studies all use mechanical test devices to get quantitative data and analyse

shock absorption and the performance infill. This can represent consistent data but

isn’t truly representative as it may not accurately show the ways in which the surface

is loaded and interacted with by the player. Garcia et al (2001) conducted an

experiment which consisted of integrating sensors into individual studs to try and

quantify what happens below surface level but the outputted data from the testing

was inconclusive. This test shows that the sensors used on the FIFA rotational

traction device are sufficient enough and further sensors on the studs are not

necessary.

The FIFA rotational traction device entails six studs equidistant apart in a circular

shape with 46 Kg of weight being loaded vertically on top of the test foot. This

doesn’t truly represent player loading and traction as it is much different to that of a

human. More than 6 studs are used, a player’s boot isn’t circular it is in a

configuration

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2.1.9 Surface Summary It is evident that the surface and infill play a role in the amount of traction created

with the boot. Variables such as pile length, fibre type and infill can drastically

change the outputted results of the mechanical testing. Maintenance is vital in order

to obtain longevity and a reduction in surface degradation. The literature illustrates

that the maintenance procedures reduce compaction of the SBR granules and allow

greater penetration from the studs.

2.2 Traction Literature Traction is a crucial factor when designing footwear and also surfaces. Traction is a

variable that can have a huge impact on athletic performance. Football boot

manufacturers such as Nike, Adidas, Puma and New Balance ensure that their

soleplate designs incorporate the most efficient stud types in the necessary

configurations in order to optimise performance. Artificial surface designers and

manufacturers are also extremely attentive towards the traction characteristics

between boot and surface, as they must permit the athletes to perform without risk of

injury from the artificial turf.

2.2.1 Defining Terminology Throughout the chapter ‘traction’ is a word that will be used excessively. For

clarification of the term the definition is; “The action of drawing or pulling something

over a surface, especially a road or track.” (Oxford Dictionary, 2016).

However, traction can be broken down into different categories, as follows:

• Rotational Traction – The traction force is applied to generate rotational

motion.

• Linear Traction – The traction force is applied to generate linear movement.

• Static Traction – The traction force is not sufficient to generate movement.

• Dynamic Traction – The traction forces are sufficient to cause relative

movement of the boot over the surface.

• Available Traction – The potential traction that could be used during the

movement.

• Utilised Traction – How much traction is utilised during the movement.

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When discussing ‘boot-surface’ interaction it can be described as; “resistance to

relative motion between a shoe outsole and a sports surface that does not

necessarily obey classical laws of friction.” (Annual Book of ASTM Standards, 2011).

2.2.2 Measuring Traction Calculating the coefficient of traction is one of the most popular and important

methods when defining traction (Shorten M, 2003). The formula for this calculation is

outlined below:

COT = 𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹

It is simply the ratio of the tractive force (𝑥𝑥) against the normal force (𝑦𝑦). So if the

usable traction is defined as the available traction without slippage, when θ is at its

maximum then the usable traction =

COT x 𝐹𝐹𝑦𝑦

To increase the usable traction either the vertical force or coefficient of traction must

be enlarged. The angle of the athlete’s body is also a factor that can determine the

traction between the boot and surface as COT = tan θ. If an athlete can lean further

whilst maintaining feasible boot- surface traction then the acceleration when they

turn can subsequently increase (Luo G, 2011).

θ

Traction Force (Fx)

Normal Force (Fy)

Applied Force

Figure 2.3 - A New Balance athlete applying a force at angle θ from the vertical. (Figure Adapted from Shorten M (2003))

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Kugler and Janshen 2010 found that the capability to orientate the ground reaction

force towards the horizontal direction is also decisive for turning performance. When

running at a top speed (v) along a curve of radius (r), a minimum traction coefficient

(μ) is required in order to generate an adequate centripetal force in the horizontal

direction.

µ = 𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹

= 𝑣𝑣2

𝑟𝑟.𝑔𝑔

Where F c is the magnitude of the centripetal force; F n is the magnitude of the

normal force; and g is the magnitude of the gravitational acceleration.

Luo G (2011) named different traction coefficients MT 0.2, MT 0.5, MT 0.8 and MT

1.1. These different values were tested against the athlete’s body angle when

sprinting. It was found that an increase in the traction coefficient lead to a decrease

in body angle, which can be shown in the figure below.

MT 0.2 = 80.7° ± 13.6°

MT 0.5 = 72.0° ± 12.9°

MT 0.8 = 71.0° ± 13.6°

MT 1.1 = 71.0° ± 13.4°

During linear acceleration, as the mechanically available traction increased from MT

0.2 to MT 1.1 the athletes leaned more forward and oriented the ground reaction

force in a more anterior direction.

Figure 2.4 - Illustrating how much the athlete leans forward in an anterior direction depending on how much available traction there is. (Adapted from Luo G (2011))

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2.2.3 The Player By understanding the variables of the biomechanics and boot – surface interaction, it

will help to identify how aspects of the mechanical testing procedure can be

improved to further replicate human movement. Artificial turf properties can change

depending on the environment and climate, for example surface moisture content

and temperature. These variables then change the player’s technique in order to

obtain a suitable level of traction. When analysing player performance it is difficult to

measure contact time, impact velocity and foot rotation at the same time. These are

all factors that affect traction but are more easily obtainable through the FIFA

rotational traction device. This study is assessing the affect traction has on

performance as well as injury risk. It would be unethical to get players to perform

cutting and turning movements with the chance of injury occurring.

2.2.4 FIFA Rotational Traction Device The rotational resistance device is FIFA approved and used in industry to measure

traction; artificial surfaces must pass traction tests in order to achieve FIFA and IRB

quality standards. The apparatus comprises a test foot which consists of a circular

metal disc with dimensions of 150 ± 2 mm in diameter. There are six football studs

on the base of the foot equidistant apart and 46 ± 1 mm from the centre. Above the

test foot are three frictionless weights that have a combined mass of 46 ± 2 kg.

Surrounding these components is a tripod to give the entirety of the apparatus

stability when in use. A shaft with connected lifting handles is attached to the centre

of the studded disc to allow the test foot to be raised from the artificial surface. A

two-handled mechanical torque wrench is attached at the top of the shaft to record

peak torque; the scale goes from 0 to 80 Nm with a maximum of 2 Nm increments.

The plates and test foot are then raised by the lifting handles to a height of 60 ± 5

mm before being dropped onto the artificial test piece allowing the studs to penetrate

the surface. The user then applies a rotational torque of 12 rev / min (72° / sec) until

the test plate has rotated at least 45°. The result is displayed as the maximum torque

on the wrench.

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2.2.5 Alternative Mechanical Testing Devices As previously aforementioned, many researchers opt away from the FIFA testing

standards and decide to design and manufacture their own traction measuring

devices. Some of the apparatus outlined in Table 2.1 are automatically functioned,

which contrasts the device utilised by FIFA as it entails human involvement.

Figure 2.5 - Different components of the FIFA Rotational Traction Device

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Table 2.1 - A list of alternative mechanical testing devices from previous literature.

Author Description Measurement Vertical Load

Manual / Auto

Lab / Field

Livesay et al

(2006)

This apparatus makes use of a rotary potentiometer and a torque trust sensor which connects to a vertical shaft attached to a firm stiff structure as the forefoot part of the outsole is rotated. (Figure 2.6)

Rotational torque

34 kg Manual Both

Villwock et al

(2008)

Villwock’s testing involved a lower leg assembly rotated by dropping a suspended weight.(Figure 2.8)

Rotational torque

102 kg Manual Both

Severn et al (2011)

An outsole is attached to the device and pushed onto the surface with a high pressured pneumatic force.

Rotational torque

44 kg Auto Lab

Ballal et al (2014)

There is a synthetic foot attached at the end of the shaft. Which connects to a axial torsional hydraulic actuator. (Figure 2.7).

Rotational torque

102 kg Auto Lab

Kent et al (2012)

There is an inner support frame that allows the test foot to position itself horizontally and vertically.

Rotational torque

285 kg Auto Both

Below is visual representation of the test devices used in some previous literature:

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Figure 2.6- The Rotational Traction Device used in Livesay's study (2006)

Figure 2.7 - The Rotational Traction Device used in Ballal's study (2014)

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2.2.6 Vertical Load The vertical load that is applied onto the artificial surface is to simulate an athlete’s

body weight / loading force. The frictionless weights that are used on the rotational

traction device are to help the studs penetrate the surface and gain traction. As

shown in the above table, the testing apparatus can vary in loads. The reason

behind the researchers’ choices of what vertical load to use is so the trial can

emulate a specific person’s bodyweight. Although, Kuhlman (2010) found that during

an athlete’s performance on an artificial surface, the weight expressed on the

surface is more than double during running and cutting movements. Cawley (2003)

portrayed in his study that due to very little variation in the COT when the load

is >90kg, it is not necessary to have anything more than 1000N. Other researcher’s

also expressed this view, as it can be very difficult to load and unload the weights on

and off the apparatus. Another reason to avoid excessive weight loading is that when

testing on smaller samples of artificial turf, it can rotate the test piece with the studs

once they have penetrated the infill.

Figure 2.8 - The Rotational traction Device used in Villwock's study (2008)

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2.2.7 Constraints of Testing Devices One of the constraints for manual based testing devices is that errors can occur

during the rotation of the torque wrench due to human error. The velocity of rotation

should be 12 rev / min (72° / sec) as outlined in the FIFA testing requirements, this

can vary a lot depending on the user’s experience. Also, some of the weights that

have to be placed onto the test foot can be very heavy, the loading and unloading of

these can be time consuming and sometimes unachievable depending on the user’s

strength. The testing devise used by different researchers all have different

parameters and differing methods of data capture when measuring the traction of the

surface, irrespective of the mechanics behind the machine, the parameters must be

assessed to see how they affect the output.

2.2.8 Velocities and the Test Foot Many test devices utilised in the studies of the researchers allow the football boot or

soleplate to be attached to the rotational traction apparatus, which contrasts the

FIFA device as this is difficult to do. As the velocity stated by FIFA is 12 rev / min

(72° / sec, it is difficult to determine how representative this value is of a sporting

movement as no literature has commented on it to date. Villwock (2005) used 180° /

sec in a study which is more than double the FIFA requirements and the data

outputted still showed a high peak torque compared to devices using a lower

velocity. However this could be because an outsole was incorporated to the rotation

instead of just a test foot with studs screwed in. The traction rises considerably when

using a full outsole, but when the boot’s last is attached to the device it can provide

more weight and stiffness to the rotation which in turn can increase traction.

2.2.9 Measurements Peak torque (Nm), rotation (°), rotation at peak torque (°), velocity of rotation (° / s),

and stiffness (Nm / rad) are measurements that are taken during the testing

procedure. Figure shows a typical torque vs rotation plot, this is taken from Severn

(2010) and it illustrates different curve gradients throughout the course of rotation.

Between 0-5° the curve is very steep but as the rotation starts to increase in angular

velocity the gradient decreases. Once the rotational angle at peak torque is found (in

this case it is around 50°), the curve starts to decrease. Stiffness is calculated in Nm

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/ rad and is done in MATLAB. The first stiffness region is measured between 0-5°

which is the steepest part of the curve, then it is again calculated at around 20-45°

when the curve is less of a gradient. The difference in values outputted shows the

stiffness of the stud. Then throughout the testing, time (s) is recorded as well as the

speed of rotation of the torque wrench.

Figure 2.9 - A torque vs rotation plot taken from Severn’s 2010 study

The depth of the rubber infill varied between 18 and 32 mm depending on the area of

the test piece and if the SBR granules had been compacted or not. When fully raked

the rubber infill depth averaged at 24 mm.

2.3 Football Boot History Henry VIII was the first recorded individual in history to possess a pair of football

boots; they were specifically manufactured for the King as he ordered them from his

‘Great Royal Wardrobe’ in 1526 (BBC 2004, The Guardian 2004).

2.3.1 Pre 19th Century Throughout the 1800’s people wore work boots, which were rigid and dense and not

designed for running or kicking. The majority had steel toe caps, which were not

favourable to the player receiving the tackle.

Before 1891 it was forbidden to have projections from cleats, a revision closely

followed which admitted players the use of small studs or bars, which lead to the

birth of football boots (National Football Museum, 2015).

2.3.2 1940’s / 50’s Boots were made of leather and went right up to the wearer’s ankle. Even though

they were a vast improvement on work boots, they were still thick and heavy.

27

The period between the start of the 20th century and 1940 many countries were at

war so innovation was scarce. Brands like Valsport and Gola emerged onto the

manufacturing scene in the late 40’s and became very popular, but the traditional

silhouette remained the same.

Interchangeable screw-in studs made from plastic or rubber immerged onto the

market in the 1950’s with the help of sporting giant Adidas. They were engineered to

allow players to adapt to vast weather conditions and playing surfaces.

In South America and Southern Europe this was immensely popular as pitches were

less muddy and much more firm (Football Boots History, 2010).

2.3.3 1960’s / 70’s During the post-war period between 1945-1960 football boot design transformed

significantly. The ideology of creating boots that were solely for protection was

eradicated and a focus on a boot that would be light, agile and give the user an

enhanced sporting performance were now considered as a focal point.

In the 1970’s Hummel’s Marketing Director Brian Hewitt revolutionised football boots

by introducing a variety of colours. This was unheard of as only black leather boots

had been fabricated in the past. Competing brands then saw Hummel’s

advancements and this was considered as a catalyst towards innovation (Sporting

Memories, 2014)

2.3.4 1980’s A new concept was introduced into the footballing world during the 1980’s. Brands

started to pay elite players to wear their boots. This was a marketing scheme that

saw the public mimic their footballing idols buy purchasing whatever boots they wore.

At the end of the 1980’s ‘The World’s most famous boot’ was designed; The Adidas

Predator (Football Boots History, 2010).

2.3.5 21st Century Post 2010 the advancements in technology have been rapid and innovative. Nike

has brought out an anti-clog boot that forbids clumps of mud sticking to the sole plate.

28

Dr Jeremy Walker analysed the molecular structure of mud and looked at developing

a hydrophilic solution to prevent mud sticking to the sole plate (SportBible, 2016).

This is just one of many futuristic developments alongside knitted uppers from

Adidas and waterproof TPU’s from New Balance.

2.3.6 Boot Summary Football boot design and manufacture is forever changing due to new research and

development, as illustrated in the above figure. It has been proven the design of the

outsole / soleplate / studs can affect traction properties however the extent of its

affect is still determined and to some extent limited by the surface condition. Football

boots become more of an issue with traction at lower league levels, as teams play on

differing surface conditions week in week out, but can only afford one pair of boots.

2.4 Stud Types At an elite level athletes utilise different stud types to increase the traction between

the boot and the surface. Footwear is often changed based on pitch conditions and

weather types. Below is a list of the most common boot types used in football.

Figure 2.10 - A football boot timeline of how technology has advanced from 1930 to 2002

29

2.4.1 Hard Ground (HG) When the ground is firm, it is not necessary for long studs to penetrate the surface

for grip, this will hurt the user’s sole. Numerous small moulded studs usually

between 10 and 15 and between 8-14 mm in length (Clarke JD 2010) are

incorporated to the sole plate to allow distribution of pressure. These boots are also

used on 3G surfaces.

2.4.2 Artificial Ground (AG) These are very similar to hard ground boots, but often have minute rubber studs

instead of moulded plastic. They can contrast in size and shape, from different

diameter circles to hexagons and triangular shaped studs. Typically they are just

rubber projections from the sole plate and each boot has usually between 20-35

studs.

2.4.3 Soft Ground (SG) Soft ground configurations are only ever used on natural surfaces. These boots have

a minimal amount of studs usually between 6 and 8 and are usually long and metal,

with dimensions of 14-18mm in length. When the pitch is heavily watered or muddy,

long metal studs help infiltrate the ground and avert slippages (McGhie D, 2013).

2.4.4 Firm Ground (FG) Firm ground boots are now often a combination of studs and blades, predominately

moulded plastic. The studs are much shorter in contrast with the SG outsole, but a

bit longer than AG and HG boots. It is deemed as a hybrid for the majority of

surfaces as it can be played on all surface types (Arrons E, 2013).

2.4.5 Stud Design on Traction Stud design depends solely on the manufacturer and can vary massively depending

on what the manufacturer deems effective. As previously mentioned the Adidas

Predator boot changed the game in the late 20th century as they introduced the

bladed stud. Before this innovation the majority of boots were long metal conical

studs as they were renowned for their good traction properties. However, there are

some published studies that have shown blades improve traction performance, these

30

then have been disputed by McGhie (2013) and Bentley (2011) as they state that the

loading of the foot is unnatural and decreases traction, so the evidence provided is

not clear cut. However, there is a possibility that the general public still purchase

bladed boots as endorsed athletes and role models wear them which portrays the

blades in a positive light.

2.4.6 Stud Length Effect on Traction Stud penetration on the surface of the artificial turf is one of the major keys when

trying to obtain traction. Zanetti (2012) stated that traction is heavily based on the

length of stud and how much of the stud penetrates. They went on to identify that

short studs do not penetrate the surface enough, and long studs don’t penetrate the

surface fully. This can be a reason to why players either exploit artificial ground or

firm ground boot types when playing on synthetic surfaces.

If stud design is diverse it can help maximise stud penetration. For example conical

studs tend to penetrate the infill more than a stud with consistent width would. This is

because the smaller the tip width the easier the stud finds it to find space between

the SBR granules. As the stud is then pressed into the infill, the stud width widens

(typical conical shape) and the granules disperse because the tip has already

infiltrated the rubber. If the tip width is the same as the main body of the stud then it

can be difficult to penetrate through the granules.

If the stud has low traction then there is a chance of injury due to translational

slippages when pushing off from a turn. Howver if there is too much traction then the

boot will not be able to slide and allow the ankle to move in sequence with the rest of

the body, also resulting in an injury. Therefore, a balance between the two has to be

found.

Clarke and Carré (2010) found that sometimes when testing long studs on the

rotational traction device, the weighted plates are not enough to allow the studs to

penetrate the surface. Therefore, the test foot (soleplate) does not come in contact

with the surface, in turn, reducing friction. This conveys the importance of the outsole

and how traction isn’t entirely dependent on stud type.

31

2.5 Literature Summary It has been concluded that when designing a football boot or artificial surface, the

safety of the user must be a priority, therefore, numerous variables must be

considered with traction being one of the most important. Each sporting surface will

have to have different traction properties to tailor to the competitive nature of the

game, for example a low traction for skiing and high traction on formula one tracks. A

balance always needs to be found between reducing the risk of injury and optimising

performance. When measuring traction between two surfaces it is expressed as a

ratio between the vertical load and the horizontal resistance. This can sometimes be

difficult due to intricate interactions of the two surfaces. The vertical load is quite

simple and easy to quantify, which contrasts the complexity of the horizontal

resistance and many researchers have expressed traction as surface stiffness as

well as torque.

Different stud types and configurations were introduced by manufacturers to combat

traction issues presented by natural grass. Natural pitches can differ in surface

properties from high use areas (centre circle and goal mouth) to low use areas

(wings and corner flag areas), and differ even more depending on varying locations

and climates. The introduction of artificial surfaces meant that one boot type can be

worn on this surface in all conditions. This was favourable to grass root players and

the rest of the wider population who did not have a large disposable income to buy

multiple pairs of boots to suit each surface like the professionals do. Even though

artificial surfaces still need maintenance, they require a significantly less amount that

natural pitches do.

Artificial surface properties are tested at a high standard in order to maintain quality

and become FIFA approved. One of these tests is the rotational traction tests. This

test outputs values such as peak torque and velocity of rotation which can then be

compared against test standards. The problem with this test is that it lacks validity as

it is not truly representative of human movement.

The literature review provides an in-depth insight into different factors that can affect

traction. Focusing on aspects such as stud type, stud configuration, artificial surfaces,

football boot history, the physics of traction and measuring devices. The data

32

collected from testing in the Sports Technology Institute at Loughborough University

has used to determine which kind of stud type provides the user with the most

traction.

33

Chapter 3

EQUIPMENT METHODOLOGY

3.1 Introduction The test device that was utilised throughout this methodology was the recommended

instrumentation from FIFA. This was selected as it has guidelines and a quality

concept that allows you to compare your findings against what is deemed right from

a governing body. It has a very simple set up and a straight forward data capture.

Slight variations have been made to its original protocol, as sensors are used to

gather the data instead of readings from the torque wrench.

This chapter describes how the different pieces of instrumentation were used to

gather raw data and process it into meaningful information that was used to make

decisions on stud types and artificial surfaces. The key components to this process

were the rotational traction device, the rotational traction rig, LabView and MATLAB.

3.2 Rotational Traction Device As previously mentioned in the description of the rotational traction device (Chapter

2.2.3) the torque wrench usually displays the peak torque. However during testing

this was not the case as the wrench did not provide any information during the

rotation. Instead, three electronic sensors were used to capture the data. These

wires lead from the apparatus to the rotational rig and the sensor cables did not

obstruct the rotational movement or hinder any characteristics of the device. In figure

3.1 a linear displacement sensor is included in the setup to measure the penetration

of the studs during the rotation of the test foot. A strain gauge and a potentiometer

were clipped into the shaft (below the torque wrench and above the lifting handles) in

order to measure the rotation angle (°) and the torque (Nm). The data obtained by

these sensors fed into a rotational traction rig that then transferred it via USB cable

into the laptop. The data was then captured using software called LabVIEW. This

program allowed each sensor to be plotted on an individual graph against time.

Before the data was captured the sensitivity values of the sensors were calibrated as

these values were attained from a previously conducted experiment. The sampling

frequency was then set to 250 Hz and the time to 10 seconds. Each sample was

saved as a .CSV file which meant it could be further processed in different softwares.

34

Pilot testing was done before collecting the actual data. This allowed familiarisation

with the apparatus and ensured recording devices were working properly. One

element of the testing that had to be repeated several times during the pilot test was

the speed of rotation. As FIFA state in the quality concept guidelines velocity must

be 72 °/ second. Once the data collected from the pilot study matched similar results

found in the literature, e.g similar stiffness values and peak torques (Webb 2015,

Livesay 2006), testing proceeded. During the pilot test the sampling frequency was

changed from 300Hz to 250Hz as random errors started to appear in the data

capture. However, sampling time always remained at 10 seconds throughout.

Laptop for data capture

USB to transfer raw data to LabView software

Rotational potentiometer Strain gauge

torque wrench Linear displacement sensor

Figure 3.1 - The components used on the rotational traction device during data collection

35

3.3 Studs The studs that were used during testing are illustrated in the Figure 3.2 overleaf. The

idea behind the testing was to change just one variable in between each test, so it

was then easier to highlight and compare what effects traction the most. The first set

of tests that were conducted was done by using the long metal conical (LMC) and

short metal conical (SMC) stud types, which would compare the effect on length. The

dimensions of the LMC are 18mm in length, the base width is 16mm and the tip

width is 6mm. The dimensions of the SMC are 12mm in length, the base width is

16mm and the tip width is 6mm. These dimensions show that the only differing

variable was the length of stud.

Secondly, a comparison was drawn between the long plastic conical (LPC) and the

short plastic conical (SPC). The dimensions for the LPC are 16mm in length, the

base width is 16mm and the tip width is 9mm. The dimensions for the SPC are

12mm in length, the base width is 16mm and the tip width is 9mm. These

dimensions show that the only differing factor within this comparison is length.

Table 3.1 - Dimensions of the studs during traction testing

Stud Type Length (mm) Base Width (mm)

Tip Width (mm)

LMC 18 16 6

SMC 12 16 6

LPC 16 16 9

SPC 12 16 9

FST 9 16 6

PST 9 16 6

36

The third and final stud that was compared was the triangular stud. However, in this

instance, it was not compared against another stud; it was compared against its

orientation. A set of rotations occurred with the flat side of the triangular face (FST)

directing towards the circular motion of the test foot. Then the second set of rotations

was using the pointed side of the triangular stud (PST). The dimensions of this stud

are 9mm in length, the base width is 16mm and the tip width is 6mm. The orientation

of the triangular stud is shown in Figure 3.3 on the next page.

16mm

16mm

16mm

18m

m

6mm

16m

m

9mm

9mm

6mm

Figure 3.2 - Dimensions of the studs used during traction testing

37

As shown in the left image in Figure 3.3 the direction of rotation is clockwise and the

flat side of the triangle lines up in a path with stud in front of it. In contrast to the right

image, each pointed side of the triangle is lined up with the stud in front of it. The

intention is to see if there are any differences in peak torque due to orientation of the

stud.

3.4 Artificial Surface Test Piece The artificial test piece that is used for this study, was a sample taken from

Loughborough University’s Holywell Pitch. It was initially designed with the intention

of American Football to be played on it, hence the thickness of the shock pad and

turf height being high. The properties that are detailed in the below table convey very

good shock absorption characteristics for the rugby and American football players,

but also allow respectable ball roll for football making it a very good multi-purpose

facility.

Pointed Side Leading Flat Side Leading

Figure 3.3 - How the triangular stud was orientated on the test foot to monitor the difference in traction values

38

Table 3.2 – A specification of the artificial surface used during testing

Component Material Length / Depth / Thickness

Additional Information

Carpet Polyethylene 60 mm Monofilament

Shock pad SBR Rubber –

Recycled rubber,

originally from car

tyres.

25 mm Laid in situ bound

with polyurethane

binder

Performance Infill SBR Rubber

Granules

30 mm Approx. 15 kg / m2

Stabilising Infill 2EW Silica Sand 10 mm (individual

grain size ranges

from 0.25-0.71 mm)

Approx. 15 kg / m2

3.5 Lab Testing The artificial surface samples that were used in the Sports Technology Institute for

testing, were handmade and measured one metre squared in size. The sand infill

was ensured that it was 10mm in depth, as this was measured in weight first and

then consistently raked through to give an even depth throughout the sample. The

spreading of the rubber granules took the same approach as the sand infill, with it

being weighed and distributed but this time it was to a depth of 30mm. Quality

checks then ensued to ensure the infills were done correctly. This was achieved with

a FIFA approved depth gauge shown in Figure 3.8 .Compaction levels were also

measured to determine the consistency of the data captured. Nine equidistant points

across the surface were sampled.

39

Figure 3.4- The instrumentation used in order to measure the depth of the infill

When taking traction samples, it was made sure that the sample was taken at least

half a metre away from the previous sample, and 10cm away from the edge of the

carpet. Figure 3.5 shows that the testing area (white dots within the black box) is

approximately 0.8m2 in order to minimise any possible errors that may occur from the

edges. The white dots represent the points on the test piece that the studs will

penetrate; they are within a 0.6m2 and equally spaced from each other.

3.5.1 Compaction of Infill After each trial, the weight of the load from the traction device meant that the rubber

infill would become compact and not fully recover to its original state. This had to be

Figure 3.5 - Diagram of the sample used for testing and where the drop locations were

40

reconditioned after every nine drops by reconditioning it with a rake which made all

of the individual granules loose again, allowing the studs to penetrate deeper on the

next drop. If this servicing of the surface did not happen, then the peak torque would

be considerably lower, due to the studs following a pre-defined path. If the rotational

track is already there, then it means there is much less resistance when trying to

rotate the torque wrench.

3.5.2 Test Procedure Each trial started by ensuring that the correct studs were attached to the test foot.

The torque wrench was screwed in tight to the apparatus. The two sensors

(rotational potentiometer and strain gauge torque wrench) must be plugged into the

rotational traction rig to capture the data. The correct weight (46 ± 2 kg) was loaded

onto the machine then the initiation of the ten second time window would begin and

the mass would be dropped. Once the studs have penetrated the surface the user

will rotate the torque wrench at 72 °/ second and wait for the information to pass

through the rig via the sensors and onto the LabView software. The data was then

presented in the format of a graph, but also saved as a .csv for further processing

which is detailed in Chapter 3.3.

3.6 Pilot Testing The results from the first round of testing were deemed unusable as the majority of

trials conducted were taken from more or less the same point on the surface. Which

meant the peak torque would be much lower than it should be due to the studs

creating a pre-defined path for them to move in. Furthermore, the studs and mass of

the test foot meant that the SBR granules were compacted and the studs could not

penetrate properly.

As well as this testing error, the sensitivity the sensors were not calibrated to the

correct values, which meant the data captured was invalid.

During the second round of testing it was ensured the sensitivity values were

calibrated correctly. The hand rake (shown in Figure 3.9) was used in between each

trial to recondition the artificial surface and redistribute any SBR granules that had

migrated, making sure that they were not compacted to one spot.

41

Figure 3.6 - The instrumentation used to recondition the surface in between trials

Once these errors from the main testing had been eliminated in the second round

testing, the actual date collection process could begin. The pilot tests gave great

user experience and enabled insight into proceedings and how to alter and modify

the apparatus set up to obtain the results needed.

3.7 Data Processing As aforementioned in chapter 3.2 each trial was stored as a .CSV file, the data

consisted of torque, rotation and displacement which were all measured against time.

Plotting this data with the use of graphs in excel would be very time consuming so a

script was formatted in MATLAB for further processing. The script depended on the

user finding key benchmarks such as peak torque and also when the rotation angle

went over its range. MATLAB was utilised to ensure the processing of the data was

self-sufficient and autonomous to help safeguard the results from human error and

increase the reliability. Although this was difficult to do due to different quality and

inconsistency of data captures, averages were taken and any anomalies were ruled

out which helped the dependability of the findings. Once the data had been gathered

and configured calculations were then made to work out variables such as velocity of

the torque wrench and stiffness.

42

Once the script had been written (this can be found in the appendices), it was time

for it to be tested. This was done with data from the pilot study initially. The first user

input that is needed is to select which trial should be run first; this is shown below in

Figure 3.4.

Figure 3.7 - The first user input that the MATLAB script requires

Then once the script has been ran, a graph appears where the user must select

stiffness regions. The user must click on a point on the peak torque curve where the

rotation has been initiated and becomes evidently stiff. The below graph shows

which part of the graph the user would click on.

Figure 3.8 - Selecting the point on the graph at which peak torque starts being recorded

During this data capture the user must roughly select the start of movement and then

MATLAB will improve the accuracy of it automatically. This must be done manually

43

because even after filtering, the data still has some peaks and troughs from the

remaining noise and that would trigger MATLAB to select the wrong point along the

curve if it was done by a computer. This user input was relatively accurate, there was

some dependence on user experience but was very simple when having to select a

start point.

Once this point on the graph has been selected then the stiffness curve appears

(shown in Figure 3.6 below) which enabled the start and end positions to be

specified.

Figure 3.9 - Selecting the stiffness regions in MATLAB

The first stiffness point (‘Stiffness A’) is usually found between zero and five degrees

of rotation and can be identified by having the steepest gradient on the curve. The

second point of stiffness (‘Stiffness B’) occurs just before the gradient begins to

plateau at the peak torque. The Stiffness B is usually less of a gradient as this point

ensues between twenty and thirty five degrees of rotation and as the studs on the

test foot are in a circular position, each stud will have already created a pre-defined

path meaning the stud behind it following its motion will have less SBR granules to

penetrate.

44

Once these two regions have been selected, MATLAB processed the data and

outputted the given values into the MATLAB command window (shown below in

Figure 3.7).

This was repeated for every .csv file that was captured for every single stud type.

Each result (Peak Torque (Nm), Angle of Peak Torque (°), Angular Velocity (° / s),

Stiffness’ (Nm/ °)) was then saved into an excel file where means, standard

deviations and comparisons were made.

Figure 3.10 - MATLAB outputting the results for the selected trial into the command window for the user to log

45

Chapter 4

RESULTS

4.1 Pilot Results As aforementioned the results produced from the first round of testing were deemed

unusable due to incorrect sensitivity values of the sensors and repeated testing on

the same part of the test piece. However, the values outputted by the rotational

traction device were still saved as .csv files and ran through MATLAB just to make

sure the script was functioning. This also gave greater user experience and widened

the understanding of the process.

4.2 Stud Type Vs Peak Torque The first set of data that was processed was finding out what effect the stud type had

on peak torque. Through receiving the data on LabView it was evident that the studs

with more length seem to have a higher peak torque. The long metal conical stud

type was tested 10 times and had a mean peak torque of 42.1 ± 4.9 Nm. The main

observation that was intriguining was looking at the traction properties between

metal and plastic. Figure 4.1 illustrates how the peak torque varied between each

different stud type rotation.

05

1015202530354045

Long MetalConical

Short MetalConical

Long PlasticConical

ShortPlasticConical

Flat EdgeTriangular

PointedEdge

Triangular

Peak

torq

ue (N

m)

Stud Type

Stud Type Vs Peak torque

Figure 4.1 - Stud Type Vs Average Peak torque

46

4.3 Stud Type Vs Stiffness The initial rotational stiffness between 0-5° was always larger than the stiffness in the

20-35° regions. This was due to the circular path of studs dispersing the rubber infill

during the rotation making the movement easier the longer it occurs. The 2 different

orientations of the triangular stud had interesting stiffness values. The initial stiffness

of the Flat Edge Triangular had a mean of 2.1 ± 0.3 Nm / ° which contrasted the later

stiffness as that had a mean of 1.2 ± 0.3 Nm / °. Even though the Flat and Pointed

Edge Triangular Studs had the lowest stiffness values out of the rest, they had the

smallest difference between early stiffness and later stiffness. This potentially

suggests how the triangular shape maintains a greater amount of stiffness under

rotation than the traditional conical shape does.

4.4 Stud Length Vs Average Peak Torque Stud length has been the main variable that effects traction within this methodology

as shown by the below table. The x-axis of Figure 4.3 has been designed so that the

lengths of the different stud types descend in numerical order. This allows a more

visual representation of the negative linear trendline that illustrates the decline in avg.

0

0.5

1

1.5

2

2.5

3

3.5

LongMetal

Conical

ShortMetal

Conical

LongPlasticConical

ShortPlasticConical

Flat EdgeTriangular

pointedEdge

Triangular

Stiff

ness

(Nm

/ °)

Stud Type

Stud Type Vs Stiffness

Stiffness A

Stiffness B

Figure 4.2 - Stud Type Vs Stiffness

47

peak torque as stud length decreases. The 18mm length stud obtained an avg. peak

torque of 42.0 ± 4.8 Nm in comparison to the 12mm stud which was 29.7 ± 2.4 Nm.

The 12mm stud was selected for comparison as it had the same conical shape,

although it differed in material the length was still the main key factor.

4.5 Angular Velocity Vs Peak Torque A question that arose during the testing stage was; whether the manually controlled

angular velocity affected the peak torque? As shown in Figure 4.4 below, there is a

positive linear trend line for the angular velocity and a negative linear trend line for

the peak torque. When the torque wrench is rotated at a slower rate (58.6 °/ s on

average for the long metal conical stud) the peak torque is high (42.04 Nm for the

long metal conical stud). When the torque wrench is rotated at a faster rate (77.2 °/ s

on average for the pointed edged triangular stud) the peak torque is low (26.4 Nm for

the pointed edge triangular stud).

05

1015202530354045

18mm 16mm 12mm 12mm 9mm 9mm

Peak

torq

ue (N

m)

Stud Length in Descending Order

Stud Length Vs Average Peak Torque

Figure 4.3 - Stud Length Vs average Peak Torque

48

Figure 4.4 - Angular Velocity Vs Peak Torque

0102030405060708090

Long MetalConical

Short MetalConical

Long PlasticConical

Short PlasticConical

Flat EdgeTriangular

PointedEdge

TriangularStud Type

Angular Velocity Vs Peak Torque

Angular Velocity (°/s)

Peak Torque (Nm)

72 / sec – FIFA Target

49

Chapter 5

DISCUSSION

5.1 Introduction This chapter will discuss the changes found in the different stud types and what has

lead to the results found. Having previously discovered in Chapter 4 there were clear

differences in peak torque when the length of the stud was altered, the reasons

behind this is discussed.

The only variables that were changed throughout the testing procedure were the

stud types. The FIFA regulation studs that were provided with the rotational traction

machine were tested initially. Once the peak torque matched the values found in

other literature, the shape, size and material could then be changed to note the

differences. The artificial surface remained the same at all times, it was ensured that

the SBR granules were raked before each trial was conducted. The testing

apparatus also remained the same throughout the duration of the data capture.

Although the rotational traction device was kept the same, it was changed from the

original FIFA design having added rotational potentiometer and strain gauge sensors

which allowed the whole rotation to be measured. The data processing allowed the

rotational movement to be broken down into time frames (in degrees of 5) so the

stiffness could be analysed at certain regions. This also acted as a quality control

check as it conveyed velocities, meaning it was evident if the values were out of the

suggested guidelines.

5.2 FIFA Traction Device The main issue that was found with the rotational traction device was trying to make

it truly representative of a sporting movement from an athlete. As stated in the

literature review, the ground reaction force of football players when changing

direction can be up to twice their bodyweight. This is more than double what the

rotational device uses based on a 75 kg athlete. Although, if this weight of an

athlete’s exerted load was to be used it could not be done manually. It would not only

be extremely difficult to manoeuvre around the test piece, but also to rotate due to

the increase in torque (Livesay, 2006). With normal load Kuhlman (2010) found the

50

slope of the traction coefficient to change in steepness, which contrasts Livesay

(2006) which highlights how the peak torque and stiffness points have a linear

correlation with compressive load.

Another aspect of how the device does not represent human movement is the angle

of rotation. FIFA specify in their Quality Concept guidelines that the test foot should

be rotated at least 45°, but if a player’s foot was to rotate 45° in one position this

would result in injury. Perhaps if the peak torque was taken between zero and five

degrees it would be more sensible.

What has to be investigated further is that when a player is running, their foot is

further than 60mm from the floor. This is the height that the test foot is dropped from

onto the artificial surface. This would make the vertical velocity different as more

mass and momentum is moving with the player than there is from the frictionless

weights. Potentially, the player’s studs would penetrate the surface deeper due to a

greater load and a higher drop. If the stud penetrates deeper then more early

stiffness becomes available, leading to a greater peak torque. This is something that

could be taken into consideration when taken the investigation further.

The advantages from collecting data from the traction apparatus is that it allows the

user to collect kinetic and kinematic data which would not have been as accurate

when gathering it in its natural environment.

The limitations in the results come from using the rotational traction device and being

in a lab environment. The dropping of the load onto the artificial surface was done at

a slow controlled pace which is not realistic in comparison to an in-play game.

Using laboratory data allowed quantitative results which lead to in-depth analysis

and a greater understanding of what happens during stud rotation.

5.3 Velocity Testing Initially, the velocity of rotation was very difficult to maintain or get close to 72 °/s;

especially in the pilot testing as the device was wrongly being tested on the same

position of the test piece. This meant the first two times were quite stiff, then the third,

fourth and fifth were easy which meant rotational velocities would vary dramatically.

51

Once the positioning of the test foot had been amended and the hand rake was

utilised for reconditioning, the velocity of rotation was improved. When testing on the

6 different types of stud variable, the average of all of the velocity rotations came to

72.4 °/s, which is very close to FIFA’s test standard (FIFA).

There is a common theme in the angular velocity of the testing, which highlights that

initially on the LMC the rotational velocity average was low. Then when moving onto

the SMC it was high. As the testing went on through the different variables the

velocity average got closer to 72°/s. This showed that the user input adapted to the

variable conditions and stayed consistent with the rotation speed. To improve the

validity of the testing the LMC and SMC could be re-done at a better velocity to

closer match the Quality Pro Concept.

5.4 Studs The LMC and SMC had the biggest difference in averages of peak torque. This could

be due to a few reasons. The main one being that the difference in length between

the LMC and SMC is bigger than the rest of the studs, making the traction properties

of the LMC seem far superior to all the other studs. Within this comparison another

factor is that the average velocity of rotation for the LMC was much lower than the

SMC, meaning that the peak torque would be higher as there is less force helping

the studs rotate through the fibres and SBR granules.

The difference in length between the two plastic studs was only 4mm which showed

in the results as there was not much difference in peak torque values. Both short and

long plastic studs had an angular velocity average of approximately 75 °/s which is

very close to the FIFA guidelines of 72 °/s (FIFA) and shows consistency in testing

and of the user input.

The results from the flat side triangular stud against the results from the pointed side

triangular stud is difficult to compare. Due to the length of the stud only being 9mm, it

was hard to tell how much of the 9mm got through the fibres and penetrated the SBR

granules. With the triangular stud being so short in length, the peak torque average

was very low in both flat side (26.9 Nm) and pointed side (26.4 Nm). Although the flat

52

side was 0.5 Nm higher than the pointed side, this amount is still not enough to

determine if it has better traction. The same triangular stud shape needs to be tested

with greater length, so it has the potential to penetrate the rubber infill and allow for a

more realistic analysis.

5.5 Confidence in Testing The main variable within the testing procedure that causes doubts of confidence is

the velocity of rotation that the torque wrench is turned at. The user has no gauge or

speed tracker when turning the wrench, which makes it difficult and can only, be

improved with repetition and experience. The user cannot find out the velocity of

rotation until the post-processing of data when the potentiometer provides feedback.

Without the inclusion of this sensor the user would not be able to check the velocity.

Twomey (2014) developed a machine that performed the rotational movement of the

wrench autonomously at the FIFA suggested value of 72 °/s. This custom machine

showed more consistency than ten manual users. If Twomey’s device could be

incorporated into this testing procedure then there would be more confidence in

testing and reliability in results.

Using the hand rake in the second and final round of testing instilled a lot more

confidence in the procedure and outputted results. It was essential to try and limit the

confounding variables and this meant that the rubber infill compaction and direction

of fibre must be the same for each trial. So between each drop the artificial surface

was raked and the depth was checked with the digital gauge to make sure it

matched the guidelines. When continuously raking, sometimes the SBR granules

would migrate onto the lab floor and off the test piece. When this was noticeable they

would be returned back into the infill.

5.6 MATLAB Script The data was processed with the help of the MATLAB script which was efficient

when calculating the variables. May errors arose during the testing of data that was

found in the second round testing. Before the final script was confirmed many

changes had to be made to the code in order to eliminate errors and gain correct

values. Once the code was correct and outputting sensible values, all the data

collected was re-run through the script to ensure that all the .csv files were

53

processed the same way. When selecting stiffness regions on the curve, the same

operator chose the points of ‘Stiffness A’ and ‘Stiffness B’. Due to the variation of

users’ input and subjectivity of the task, the same operator processed the data to

maintain consistency.

5.7 Results During testing, the idea was to only change one variable per trial. So, between the

long metal conical and short metal conical studs, the variable was length. Between

the long plastic conical and short plastic conical stud the variable was also length.

Then between the flat edge triangular and short edge triangular the variable was

orientation. The outputted results from MATLAB showed that the longer the stud the

higher the average peak torque and early stiffness. There is some uncertainty

towards this hypothesis as the highest peak torques collected was perhaps due to a

lower angular velocity. As the angular velocity increased and got closer to the FIFA

recommended value, the average peak torque decreased, but this may have been

due to a decrease in stud length. In order to get further confirmation on whether

angular velocity has a large impact on average peak torque further testing would

have to be done with a more consistent speed of rotation. Perhaps incorporating

Twomey’s (2014) automatic device would increase the reliability of the angular

velocity.

The two studs that were identical in length and base width were the short metal

conical and the short plastic conical (only differing factor was the tip width). This

allowed comparisons to be made between the materials and not just the dimensions

as the angular velocity differed by 5 °/s and the peak torque differed by <1 Nm. This

meant the two stud types produced very similar results and could possibly be

identical when under the same angular velocity. This meant it was hard to draw a

conclusion between which material had better traction properties as both plastic and

metal were very closely matched. Further testing would have to be done with studs

of the same dimension and rotated at the same velocity for a clear conclusion.

The flat edge triangular and pointed edge triangular had the shortest difference

between ‘Stiffness A’ and ‘Stiffness B’ which meant that the orientation of the stud is

54

a factor that has an impact on traction. This comparison would have been better if

the length of each stud was greater than 9mm, which would have ensured full

penetration of the rubber infill and obtained higher peak torques. However, this is

something that can be taken into consideration future testing.

55

Chapter 6

CONCLUSION

6.1 Overview The purpose of this study was to investigate the effects different stud types have on

traction when being rotated over an artificial surface. The primary aim was to analyse

the quantitative data in order to obtain a greater understanding of how stud variables

effect traction at the boot-surface level. With this information, conclusions can then

be drawn to see how the testing procedure and apparatus used could be improved

for future studies. Even though these quantitative results can shed light on traction

properties for each stud type, they are not representative of how athletes interact

with the playing surface. The initial objectives of the study were:

To obtain a greater understanding of the traction that occurs at boot – surface level

on artificial surfaces by conducting FIFA testing on different stud types and analyse

the quantitative data that is outputted.

The second objective was to gain a further insight into the mechanical testing

procedure and how different variables affect peak torque and stiffness.

The final purpose of the study was to conclude on what stud type has the best

traction properties out of the studs selected.

In order to complete the first objective, a Literature Review was conducted into

previous studies that had related topics with this study. Existing experiments were

read and reviewed that covered information on; the physics of traction, artificial

surfaces, studs and stud types and the history of the football boot. All of these

sources gave a great insight into boot-surface interactions which then helped in

attaining the second aim of conducting mechanical testing.

The second aim was achieved with hands on interaction with the FIFA testing device

as well as guides from literature reviews. The further insight was ascertained early

on during the study as sensors were added to the rotational traction instrumentation

which helped highlight the torque and velocity values whilst also increasing

understanding. When learning about the quantitative data outputted from the testing,

56

it was then easier to figure out the plan of action and how to move forward in order to

complete the task and achieve the values needed.

6.2 Summary of Findings The key findings from this experiment are highlighted below:

1) Stiffness is just as important, if not more, than peak torque when determining

rotational traction properties. Stiffness takes into account the torque required

to rotate the device a certain amount, not just the torque.

2) The literature review highlighted that there are many variables that can affect

traction at the boot – surface level. Stud type, stud length, stud material, stud

configuration, the surface type, surface infill and the load applied.

3) The initial stiffness (‘Stiffness A’) is the most appropriate when comparing

values with human movement. As this stiffness value occurs between 0-5°,

comparisons can then been drawn with the athlete’s foot and ankle as this is

close to human’s range of movement.

4) The velocity of rotation can be made more consistent by having an

autonomous turning mechanism, which would make peak torque and stiffness

values more reliable throughout. Also, angular velocity could be matched with

the speed of elite athletes’ movements, making the device more sport-specific.

5) Lab testing allows confounding variables such as moisture and temperature

to be eliminated.

6) Artificial surfaces need regular maintenance, not only to increase the longevity,

but to improve traction properties as well. It was found during testing that once

the rubber infill on the test piece had been compacted, it was difficult for the

studs to penetrate and gain traction. Whereas, if the artificial surface was

regularly raked, then the rubber infill will loosen and migrate so there is an

even surface.

6.3 Future Research Future studies into traction at the boot-surface level should look at the results and

findings of this paper so that more can be understood and advancements can be

made. Identical studs should be incorporated so only one variable differs between

trials which would allow closer comparisons to be made.

57

A future progression in angular velocity that should be adapted in the study is a

monitor that tells the user a live speed of the current rotation. This would enable the

user to be more consistent with the torque wrench and they would be able to slow

down or speed up depending on what the monitor says. This would give more

consistent peak torque and stiffness values, making the reliability of the study higher.

FIFA should design a test foot that is not just circular but allows an outsole or a sole

plate to be attached. This would mean that not only different stud types can be

analysed, but also different stud configurations.

Some of the literature reviewed in Chapter 2, highlighted that SBR granule size of

the rubber infill had an effect on traction. If a study was designed to find out if traction

is better with smaller granules or larger granules than this could helped be adopted

into the construction of artificial surface sin the future, in turn, improving performance

and reducing injuries.

58

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Cawley PW, Heidt RS, Scranton PE, Losse GM, Howard ME. Physiologic axial load,

frictional resistance, and the football shoe-surface interface. Foot ankle Int / Am

Orthop Foot Ankle Soc [and] Swiss Foot Ankle Soc. 2003;24(7):551–6.

Chaudhary, Vivek. "Who's The Fat Bloke In The Number Eight Shirt?". the Guardian.

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Clarke JD, Carré MJ. Improving the performance of soccer boots on artificial and

natural soccer surfaces. Procedia Engineering. 2010. p. 2775–81.

Desso 2007

"Football Boots. Late 19Th Century - NFM". NFM. N.p., 2017. Web. 8 May 2017.

Garcia AC, Martinez A, Solaz JS, Montero J, Dura JV, Ramiro JF. Development of a

method for measuring horizontal forces in soccer boots studs during skills

performance. Proceedings of the 7th Symposium on Footwear Biomechanics. 2001

Jan-Kieft G. Quality Monitoring of 50 Artificial Turf Fields. Sport 7th Work

“Maintaining Perform Synth Surfaces.” 2009;(Loughborough University).

Garside Aggregate. Silica Sand 2EW. Garside Aggregate Sands. [ONLINE]

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2ew-tds.pdf [Accessed 12th April 2017]

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J.V. Gulminea, , P.R. Janissekb, , H.M. Heisec, , L. Akcelrudd. Degradation profile of

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performance. Footwear Science. 2011. p. 127–38.

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Villwock MR, Meyer EG, Powell JW, Fouty AJ, Haut RC. The effects of various infills,

fibre structures, and shoe designs on generating rotational traction on an artificial

surface. Proc Inst Mech Eng Part P J Sport Eng Technol. 2008;223(1):11-9

Villwock M.R, Eric G. Meyer, MS, John W. Powell, PhD, ATC, Amy J. Fouty, CSFM,

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41.

.

63

FIGURES

Figure 1.1 - Flow diagram highlighting how the chapters interact throughout the

report

Figure 2.1 - Cross-sectional view of the construction of an artificial surface

Figure 2.2 - Visual representation of different maintenance method

Source: http://www.technicalsurfaces.co.uk/why_maintenance

Figure 2.3 - A New Balance athlete applying a force at angle θ from the vertical.

(Figure Adapted from Shorten M (2003))

Figure 2.4 - Illustrating how much the athlete leans forward in an anterior direction

depending on how much available traction there is. (Adapted from Luo G (2011))

Figure 2.5 - Different components of the FIFA Rotational Traction Device

Figure 2.6- The Rotational Traction Device used in Livesay's study (2006)

Figure 2.7 - The Rotational Traction Device used in Ballal's study (2014)

Figure 2.8 - The Rotational traction Device used in Villwock's study (2008)

Figure 2.9 – A torque vs rotation plot taken from Severn’s 2010 study

Figure 2.10 - A football boot timeline of how technology has advanced from 1930 to

2002

Figure 3.1 - The components used on the rotational traction device during data

collection

Figure 3.2 - Dimensions of the studs used during traction testing

64

Figure 3.3 - How the triangular stud was orientated on the test foot to monitor the

difference in traction values

Figure 3.4 - The first user input that the MATLAB script requires

Figure 3.5 - Selecting the point on the graph at which peak torque starts being

recorded

Figure 3.6 - Selecting the stiffness regions in MATLAB

Figure 3.7 - MATLAB outputting the results for the selected trial into the command

window for the user to log

Figure 3.8- The instrumentation used in order to measure the depth of the infill

Figure 3.9 - The instrumentation used to recondition the surface in between trials

Figure 4.1 - Stud Type Vs Peak torque

Figure 4.2 - Stud Type Vs Stiffness

Figure 4.3 - Stud Length Vs average Peak Torque

Figure 4.4 - Angular Velocity Vs Peak Torque

65

TABLES

Table 2.1 - A list of alternative mechanical testing devices from previous literature.

Table 3.1 - Dimensions of the studs during traction testing

Table 3.2 – A specification of the artificial surface used during testing

1

APPENDICES Appendix 1.1 – Gantt Chart

1

Appendix 1.2 – Personal Evaluation

Initially the project was very difficult to get into the rhythm and find a balance

between research and testing. Weekly project meetings with tutor Dr Steph Forrester

allowed a greater insight into the pace at which the project should be completed at

and the order to do tasks in.

A lot of the testing requirements and methodologies came from existing pieces of

literature or specifications from governing bodies such as FIFA which helped as a

guide and also meant that new testing procedures did not have to be created.

The Gantt chart in Appendix 1.1 enabled constructive project planning and was

utilised throughout in order to hit key milestones. It was difficult to assess at what

stage the project was at due to the study being heavily researched based. However,

as parts of the literature review were finished and testing commenced it was easier

to gauge what was left to do and the timeline to do it in.

During testing it was essential to log any raw data found in .csv files and then

present them to Dr Steph Forrester at the forthcoming meetings in order to track

progress and obtain advice on analysis. This helped save time towards the end of

project which then allowed time to be invested into the presentation of the report.

Organisation was a key factor throughout as there was such a wide range of

variables to test and maintain. During the testing and results phase a lot of work

entailed comparing results from various tests. This was difficult due to the sheer

amount of .csv files and data collection. There were variables that were tested within

this study however some were non-conclusive and more need to be done in future

proceedings to fully understand the effect stud types have on traction at the boot

surface level.

2

Appendix 1.3 – Objectives Form

3

Appendix 1.4 – MATLAB Script %% SCRIPT TO PROCESS ROTATIONAL TRACTION DATA clear all; close all; clc; addpath('my_functions'); %path = '../Grass traction/16-6-16/Holywell'; % path to data/results folders %disp('Files should be saved as P#T# corresponding to the position and trial number') % display command window message foldname='Testing/Long Metal Conical'; %XXXXXXXXXXX MIKE Variables = NaN; % predefine variables for loop L = 'y'; % A = y (yes) to enter loop first time while L=='y' % whilst answer = y (yes) stay in processing loop %% INPUTS %P = 'Position no: '; % display command wimdow message T = 'Trial no: '; p = 1;%input(P); % save inputs from command window t = input(T); clear P T %% READ IN DATA AND SORT data = csvread([foldname '/T',num2str(t),'.CSV'],23,0); % read in unfiltered data and zero vectors D(:,1)= data(:,1); % time vector D(:,2)= (data(:,2)-data(1,2)); % torque vector D(:,3)= data(:,4)-data(1,4); % displacement vector D(:,4)= data(:,6)-data(1,6); % rotation vector if min(D(:,2)) < -20% if torque is -ve(anticlockwise rotation) correct vector D(:,2) = D(:,2)*-1; end clear data %% FILTER DATA % Filter data with low pass butterworth (see my_functions folder D(:,2)= ButterFilter(250,35,D(:,2)); % torque D(:,3)= ButterFilter(250,8,D(:,3)); % displacement D(:,4)= ButterFilter(250,4,D(:,4)); % rotation

4

%% CALCULATE ROTATIONAL VEL AND ACCEL [D(1:length(D),5), D(1:length(D),6)] = VelAcc(D(:,1),D(:,4)); % calculate velocity and acceleration of rotation %% SELECT CAPTURE START POINT figure(1); FigPos(1,1) % Plot torque and rotation on same graph plot(D(:,1),D(:,2),D(:,1),D(:,4)); ylim([min(D(:,2)-5) max(D(:,2)+5)]); legend('Torque','Rotation') Titles('Click start of recording','Time (s)','Torque (Nm)') [x1(1,1),~] = ginput(1); close gcf % promt to click on start of reading SOC = find(D(:,1)>=x1(1,1),1,'first'); % find cell no. of start of capture (SOC) EOC = find(D(:,2)==max(D(:,2)),1,'first')+10; % find cell no. of end of capture (EOC) D(:,2) = D(:,2)-D(SOC,2); % set chosen torque start point to 0 D(:,4) = D(:,4)-D(SOC,4); % set chosen rotation start point to 0 if max(D(:,4))>=300||min(D(:,4))<=-300% discard bad rotation data D(:,4)= NaN; end clear x1 %% CALCULATE TORQUE VALUES if t==1 && p==1; len = 0; else len = size(Variables,1); end % set array size based on number of files Variables(len+1,1) = max(D(SOC:EOC,2)); % fill array with data if isnan(D(1,4))==0 % if rotation data is good plot stiffness %% PLOT STIFFNESS figure(2); FigPos(1,1); % plot stiffness plot(D(SOC:EOC,4),D(SOC:EOC,2),'LineWidth',1.2) Titles('Stiffness','Rotation angle (deg)','Torque (Nm)') FormatAxis(0, D(EOC,4), 'off', 0, max(D(SOC:EOC,2)+1),'off') grid on; box on; %% SELECT STIFFNESS REGIONS x1=NaN(2,1); % pre-define array for ii=1:2 if (ii==1);set(gca,'FontSize',15); title('**CLICK STIFFNESS A END/B START**','FontWeight','bold');end if (ii==2);set(gca,'FontSize',15); title('**CLICK STIFFNESS B END**','FontWeight','bold');end [x1(ii),~]=ginput(1); % prompt click on graph end title(''); print('-r300','-dtiff',[foldname '/T' num2str(t) 'res.tiff']) %close gcf;

5

clear ii % close graph S_A = find(D(SOC:EOC,4)>=x1(1,1),1,'first')+SOC; % Find cell no. of top rotation limit SOC = SOC+find(D(SOC:EOC,2)>=(0.2*max(D(SOC:S_A,2))),1,'first'); S_A = SOC+find(D(SOC:EOC,2)<=(0.9*max(D(SOC:S_A,2))),1,'last'); S_B = find(D(SOC:EOC,4)>=x1(2,1),1,'first')+SOC; clear x1 Variables(len+1,2) = D(D(:,2)==max(D(SOC:EOC,2)),4);% angle of peak torque Variables(len+1,3) = (D(EOC,4)-D(SOC,4))/(D(EOC,1)-D(SOC,1)); % speed from start to peak torque poly = polyfit(D(SOC:S_A,4),D(SOC:S_A,2),1); Variables(len+1,4) = poly(1,1); % stiff A calculation poly = polyfit(D(S_A:S_B,4),D(S_A:S_B,2),1); Variables(len+1,5) = poly(1,1); % stiff B calculation else Variables(len+1,2:6) = NaN; % if rotation data is bad fill array with NaN's end Varnames(len+1,1:5)={'Tpk','angTpk','angvelMN','stiff_A','stiff_B'}; %Results array2table(Variables,'VariableNames',{'torque','angle','speed','stiff_A','stiff_B'}); % display results in command window disp(Varnames); disp(Variables); a = 'Process another trial? (y/n): '; % promt user to process another tiral A = input(a,'s'); if A~='y' % stop matlab crashing if input isnt 'y' or 'n' if A~='y' if A~='n' disp('Please use valid input') a = 'Process another trial? (y/n): '; A = input(a,'s'); else if A=='n'; L='n'; end end end %clc disp(['last trial processed: P',num2str(p),'T',num2str(t)]) % last trail processed clear poly D SOC EOC S_A S_B i outputs len a save([foldname '/T' num2str(t) 'res.mat']) % save results array

6

end % for i = 1:p % PosAv(i,1) = mean(Variables((i*t)-(t-1):i*t,1)); % find position averages % end %save([foldname '/PosAv.mat']) % save results array %clear A i p t Variables path L

7

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