Calculation of FSA method

A method for measuring frontal surface area

Developed by

Andrew D. Moss for use by British Cycling

Project documentation: part of the 2002/3 World Class Sport Science

Development Programme – a BASES/UKSI project collaboration.

List of contents

Title page...................................................................................................................................1

List of contents..........................................................................................................................2

1. Introduction...........................................................................................................................3

1.1. Pilot work experimental setup..........................................................................................4

1.1.2. Camera position............................................................................................................4

1.1.3. Calibration board position...........................................................................................4

1.2. Data collection experimental setup..................................................................................6

1.2.1. Camera position............................................................................................................6

1.2.2. Subject position.............................................................................................................6

1.2.3. Camera Setup................................................................................................................7

1.3. Pilot data collection............................................................................................................9

1.3.1. Measurement validity...............................................................................................91.3.2. Perspective error.......................................................................................................91.3.3. Measurement reliability..........................................................................................10

1.4. Results of pilot data collection.......................................................................................101.4.1. Measurement validity.............................................................................................101.4.2. Perspective error.....................................................................................................111.4.3. Measurement reliability..........................................................................................12

1.5. Sport specific pilot data collection................................................................................121.5.1. Assessment of three riding positions......................................................................121.5.2. Measurement reliability..........................................................................................131.5.3. Isolated assessment of subject and cycle................................................................13

1.6. Results of sport specific pilot data collection................................................................141.6.1. Assessment of three riding positions......................................................................141.6.2. Measurement reliability..........................................................................................141.6.3. Isolated assessment of subject and cycle................................................................14

2. Data collection with Great Britain Endurance Squad....................................................15

3. Guidelines for calculation of FSA from digital images........................................................15

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1. Introduction

Collaboration between BASES (British Association of Exercise and Sport Sciences) and the

UKSI (United Kingdom Sports Institute) resulted in nine positions for developing sport

scientists to work, through placement, with specific sports. Each position required the

associated placement individual to complete a specified number of contact hours and a project

mutually agreed by the sport supervisor, BASES supervisor and the placement individual in

the period May 2002 to May 2003.

This method document serves as proof of the completion of a project identified by Andrea

Wooles (sport supervisor: British Cycling) and agreed by Dr Andrew Jones (Bases

Supervisor) and Andrew D. Moss (placement individual). The agreed project was to develop a

method of measuring frontal surface area (FSA) to be used by British Cycling with the

ultimate aim of improving the performance of British cyclists.

The results of the pilot work are presented first. Data collected from the 2003 Great Britain

Endurance Squad using the developed method for measuring FSA is then presented. Finally,

detailed guidelines for equipment setup, image manipulation and measurement are provided.

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1.1. Pilot work experimental setup

The experimental setup is shown in figure 1.

1.1.2. Camera position

Height – the centre of the lens should be positioned at the same height as the centre of

the calibration board.

Distance from calibration board – minimum horizontal distance of 8 m was observed

between the camera and the calibration board. The scaling object was aligned

vertically with the frontal plane of the calibration board. The camera zoom facility was

used to select an appropriate image size. This process was performed in order to

minimise perspective error.

1.1.3. Calibration board position

The calibration board was positioned vertically. The bottom of the calibration board

was at a height of approximately 1 m. The frontal plane of the calibration board was

perpendicular to the optical axis of the camera.

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Figure 1. Pilot Work Experimental setup diagram.

Minimum camera to subject distance 10 m

Optical Axis of camera aligned with centre of squareWhite background

Camera & tripod

White groundsheet

Scaling object (1m Rule)

1m Approx

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1.2. Data collection experimental setup

The experimental setup is shown in figure 2.

1.2.1. Camera position

Height – the centre of the lens was positioned at the same height as the top of the cycle

head tube.

Distance from subject – minimum horizontal distance of 8 m was observed between

the camera and the top of the cycle head tube. The scaling object was vertically

aligned with the top of the cycle head tube. The camera zoom facility was then be used

to select an appropriate image size. This process was performed in order to minimise

perspective error.

1.2.2. Subject position

The cycle was positioned first, separately to the subject, to allow proper camera/cycle

alignment. The camera was positioned in such a way that only the cycle in frontal

profile was observed with the front wheel totally obscuring the rear. Once the cycle

was correctly positioned, the subject positioned himself on the cycle in the desired

position.

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1.2.3. Camera Setup

Camera used: Sony DSC-P9 4.0 Mega Pixel Digital Stills Camera.

Settings:

EV: OEV

Focus: Multi AF

White balance: Auto

Spot meter: On

ISO: Auto

Image size: 640 x 480

Picture quality: Fine

Record mode: Normal

Flash level: Normal

Picture effect: Off

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Optical Axis of camera aligned with top of head tubeWhite background

White groundsheet

Camera & tripod

Minimum camera to subject distance 10 m

Figure 2. Data Collection Experimental setup diagram.

Scaling object (1m Rule)

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1.3. Pilot data collection

1.3.1. Measurement validity

By assessing measurement validity, we are answering the question “are we measuring what

we think we are measuring”, which in this case is FSA. In order to answer this question the

following pilot work was performed.

Using the camera settings and experimental setup detailed in figure 1, three square pieces of

matt black fibreboard were each photographed three times (figure 3). The three pieces of

fibreboard had surface areas of 0.25 ± 0.01, 0.56 ± 0.01 and 1.00 ± 0.01 m2. FSA was

calculated for each piece of fibreboard. The percentage variation between the actual and

calculated FSA is reported in the results section.

Figure 3. Fibreboard squares of known FSA used in the assessment of measurement

validity and reliability.

1.3.2. Perspective error

Perspective error occurs with deviation of the subject from the optical axis or focal plane of

the camera. The effect is incorrect information about the dimensions of objects or the special

relationship of objects i.e. objects closer to the camera can seem larger than they really are.

Using the camera settings and position detailed in figure 1, the 1 m2 fibreboard square was

photographed 0.5 m in front of and behind the scaling object. The variation of ± 0.5 m from

the scaling object was chosen as an approximation of the distances between the leading edge

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of the front tyre and the scaling object and, the scaling object and the rider’s abdominal area.

The percentage variation between the actual and calculated FSA will be reported.

1.3.3. Measurement reliability

Measurement reliability concerns the accuracy of subsequent measurements using the same

method. In order to assess measurement reliability the following pilot work was performed.

Using the camera settings and position detailed in figure 1, the 1 m2 fibreboard shape was

photographed 10 times. Each time the setup was repeated as if a separate data collection

session were being performed. The FSA values were compared using the coefficient of

variation.

1.4. Results of pilot data collection

1.4.1. Measurement validity

Each of the fibreboard squares was measured with a metal rule (1 m), graded in 0.01 m

increments. The measured FSA area of the three squares was 0.25 ± 0.01, 0.56 ± 0.01 and 1 ±

0.01 m2. The mean FSA of the three fibreboard shapes calculated from the photographic

images analysed in the NIH image software was 0.25 ± 0.00, 0.56 ± 0.00 and 1 ± 0.00 m2. The

agreement between these vales suggests this is a valid method of assessing FSA. These results

are shown graphically in figure 4.

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Figure 4. Comparison between the FSA of the three fibreboard squares measured with a 1 m

metal rule and FSA calculated using the method under validation.

1.4.2. Perspective error

The FSA of the 1 m2 fibreboard square was calculated to be 0.91 m with the board 0.5 m

behind the scaling object. With the 1 m2 fibreboard square 0.5 m in front of the scaling object,

the FSA was calculated to be 1.19 m. These values indicate a considerable variation in FSA

caused by variation in the position of the scaling object. The relationship between the position

of the scale object and the FSA of the 1 m2 fibreboard square can be explained by equations y

= 0.095x + 0.905 and y = -0.09x + 0.905 These equations relate to the scale object positioned

behind and in front of the 1 m2 fibreboard square respectfully.

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The mean FSA of the 1 m2 fibreboard square photographed ten times was 0.99 ± 0.01 m2 The

coefficient of variation was 0.01 %. The agreement between the vales suggests this is a

reliable method of assessing FSA.

1.5. Sport specific pilot data collection

The pilot work detailed above provides an indication of the validity and reliability of this

method of calculating frontal surface area. In order to be sure that this method will produce

accurate and reliable FSA measurements when applied to the study of cyclists the following

sport specific pilot work was performed.

1.5.1. Assessment of three riding positions

A full data collection session was performed with a subject adopting three different riding

positions (figure 5): Bars – hands on the handlebars next to the stem, hoods – hands

positioned as if on the brake hoods, and drops – hands on the drop section of the handlebars.

The experimental and camera setup is shown in figure 2. Three photographs were taken of

each position

Figure 5. Three riding positions: Bars, Hoods and Drops.

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Using the experimental setup detailed in figure 2, the subject was photographed 10 times.

Each time the setup was repeated as if a separate data collection session was being performed.

The subject was instructed to adopt the same position with hands on the handlebars next to the

stem. The FSA values were compared using the coefficient of variation.

1.5.3. Isolated assessment of subject and cycle

The FSA of the cyclist in standard anatomical position (figure 6) may need to be measured in

order to quantify the reduction in the cyclists FSA when positioned on the cycle.

In order to provide a comparison between the FSA of the subject and the cycle in isolation

and the subject in position on his cycle a full data collection session was performed with a

subject in the standard anatomical position next to the scale object and cycle.

Figure 6. Subject in anatomical position next to cycle.

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1.6. Results of sport specific pilot data collection

1.6.1. Assessment of three riding positions

The mean FSA of a subject photographed in three positions (Bars, Hoods and Drops) was

0.40 ± 0.00, 0.42 ± 0.01 and 0.40 ± 0.00 m2 respectively. The coefficient of variation for each

of the three positions was 0.00, 0.23 and 0.00 % respectively.


The mean FSA of a subject photographed ten times was 0.41 ± 0.01 m 2. The coefficient of

variation was 0.24 %. The agreement between the vales suggests this is a reliable method of

assessing FSA.

1.6.3. Isolated assessment of subject and cycle.

The mean FSA of the subject in the standard anatomical position and the cycle photographed

10 times was 0.53 ± 0.01 and 0.11 ± 0.01 m2 respectively. The coefficient of variation for

each of the three positions was 0.18 and 0.09 % respectively.

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2. Data collection with Great Britain Endurance Squad

Photographs were taken of six members of the Great Britain endurance quad in the standard

anatomical position and in aerodynamic position on their cycles. The experimental setup

shown in figure 2 was used. Due to the colours of their clothing, the use of a white

aerodynamic helmet and white shoes, the photographs required blacking out manually before

thresholding (See Calculation of FSA) to distinguish the athletes from the white background.

Two measurements of FSA were made of each athlete in each of the two positions. The

results are shown in table 1. In order to quantify the repeatability of the measurements, the

FSA of a subject was calculated twelve times from one image. The mean FSA of the athlete

on his cycle in aerodynamic position was 0.33 ± 0.01 m2. The coefficient of variation for each

of the three positions was 0.03 % The agreement between the vales suggests this is a reliable

method of assessing FSA.

Table 1. Great Britain Endurance Squad mean FSA values

Athlete FSA values

Alteration to riding

positionAthlete on

bikeAthlete in

anatomical positionBike Athlete on bike Bike

(m2) (m2) (m2) (m2) (m2)

1 0.34 0 0.59 0.02 0.09 0.01

2 0.33 0.01 0.59 0 0.08 0.01

3 0.35 0 0.66 0.01 0.09 0.01

4 0.33 0 0.61 0.01 0.10 0.01

5 0.36 0.01 0.67 0.01 0.10 0

6 0.33 0.01 0.62 0.01 0.09 0 0.34 0.01 0.09 0

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3. Guidelines for calculation of FSA from digital images

The following are specific guidelines for the manipulation of digital images in Adobe

Photoshop version 7 and the measurement of FSA in NIH image for Mac and Scionimage for

PC.

Section 1.2. (Data collection experimental setup) details the setup and positioning of the

camera and the subject. Figure 2 provides a visual reference for this setup.

Import images to be used into Adobe Photoshop image software (version 7 is

recommended. The following guidelines are specific to this version), crop, and re-size

as appropriate using the Rectangular marquee and Crop tools in the Tools window

in the upper left hand corner of the screen. A cropped and re-sized image is shown in

figure 7.

Figure 7. Image cropped and re-sized in Adobe Photoshop version 7.

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Distinguish subject of analysis from background by performing the following. Click

on the Wand tool, located in the Tools window. Place on the sections of the subject to

be highlighted and highlight by pressing the left mouse button (multiple sections can

be highlighted by holding down the shift key while selecting with the left mouse

button) (figure 8).

Select Fill from the Edit menu and apply a colour to the subject that contrasts with the

background, in this case black (figure 9). Small areas of the subject or areas of the

subject that share a close colour match with the background may need to be

distinguished freehand using the various fill tools available. The method used here

will depend on the subject image. Figure 10 shows the subjects helmet being filled in

with the Brush tool.

Once the subject of analysis has been fully distinguished from the background (figure

11), save the image as an uncompressed tiff (tagged image format) image.

Figure 8. Multiple sections of subject image highlighted ready for filling.

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Figure 9. Colour determination using the fill window.

Figure 10. Continued “freehand” filling of subject image using the brush tool.

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Figure 11. Completed subject image filled to provide contrast to background colour.

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Import images into NIH image software if using a Mac (figure 12), or Scionimage

software if using a PC.

Figure 12. Image imported into NIH image.

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Ensure the LUT window is white at the top moving to black at the bottom. If this is

not the case, select Greyscale from the Options menu. Then select Invert from the

Edit menu.

Using the Straight-line tool located in the Tools window (5th tool from the top, right

hand side), draw a line across the scale object (figure 13). Go to the Analyse menu

and choose Set Scale. Choose the Units of measure first then enter the Known

Distance (if the scale object used is 1 m long, the Units would be Meter and the

Known Distance would be 1). Click the OK button to continue.

Figure 13. Setting the scale.

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Select Threshold from the Options menu. This should remove the background

leaving the subject clearly separate from the background. Adjust the threshold in the

LUT window to ensure the subject is fully separated from the background by placing

the mouse at the junction between black and white, holding down the left mouse

button and moving the threshold up or down.

Using the Wand tool, (9th tool from the top, left hand side), click to the left of the

subject. This should draw a border around the subject. Choose Measure from the

Analyse menu. The FSA value will appear in the Info window located in the bottom

left hand corner of the desktop (Figure 14).

Figure 14. Thresholding and FSA measurement.

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Calculation of FSA method

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