MANUFACTURING & MATERIALS INNOVATION

In the human skeleton, an MTP joint is one that connectsa toe bone to a metatarsal bone and allows the toe tobend away from the foot. It is an extremely importantjoint as it allows us to walk with a balanced gait. Among

the many ways that footwear can affect the human body, isthat of ‘gearing’ at the ankle and knee joints during running.This gearing alters MTP joint bending as a function offorefoot bending stiffness and has been suggested as a wayto improve athletic performance and reduce the risk of injuryin a number of sports, especially football or soccer. The theory is that as the shoe or boot moves through

greater amounts of bending, forefoot stiffness should increasein a non-linear manner to shift the centre of pressure forwardto improve performance, while still restricting forefootbending in areas where turf-toe injury might result. The latterrefers to a sprain of the big toe joint resulting from excessiveupward bending caused by either stubbing the toe on theground or from pushing off repeatedly when running orjumping and is a common sporting and athletic injury. Theproblem is that with most materials used in footwear, stiffnessincreases in a linear rather than a non-linear fashion.

TUNEABLE CARBON FIBREAlthough carbon fibre has previously been tried in

footwear, due to the stiff nature of traditional composites, itsusage was limited and somewhat short-lived. Things arechanging however. American company Carbitex produces arange of proprietary carbon fibre materials which allow forstiffness modifications as a function of flexion angle. Insimple terms, it has developed a way to ‘tune’ the degree offlexibility of carbon fibre to suit different requirements. Thecompany claims that this exclusive technology is well-suitedto the function of many aspects of athletic footwear,including footwear gearing.Furthermore, it can also produce it with variable

directional stiffness which it says is particularly relevant tofootball boots as it provides support for the foot duringkicking by preventing excessive MTP flexion. When kickingthe ball, excessive joint flexion can cause injury or,alternatively, reduce kicking performance as it causes a lossof energy at the MTP joint. As kicking the ball is an integralpart of the game of football, controlling MTP flexion couldtherefore have potential advantages.Carbitex in fact produces three main types of material all

aimed, in the words of the company’s founder and Chairman,Junus Khan, to “allow the foot to do more of what it wants todo and to help prevent it from movements that can causeissues”. Its AFX is designed to provide a high level of stabilityand protection without limiting forefoot flexibility. Forexample, trail runners need a combination of comfort,support and flexibility to rapidly traverse uneven terrain.Minimalist shoes do not offer enough support, while‘maximalist’ shoes can be bulky and heavy. The companysays that AFX eliminates this compromise. The CX6 is used inthe upper to control stretch in order to enhance powertransfer, enable more efficient use of energy and more

precise control.The DFX insole and midsole plates are able to

exponentially increase stiffness to optimise performance inchanging scenarios. For instance, they are said to offer theflexibility of a sneaker when walking and the stiffness of atrack spike when sprinting. Carbitex says that “put simply,DFX enables shoes to have a truly dynamic midsole for thefirst time”. To prove the point, it asked the University ofCalgary, Canada, to carry out tests. Specifically, it wanted tosee if the changes in bending stiffness that the materialprovided were sufficient to limit excessive forefoot bending toreduce injury and shift the centre of pressure to alter the leverarm and so improve performance. For instance, what was theinfluence of gearing technology in the form of variable anddirectional forefoot bending stiffness in a shoe; first on theamount of forefoot bending and the position of the centre ofpressure under the foot during locomotion and, second, onthe amount of forefoot bending taking place when kicking?

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Gearing for sports shoes

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GEARING FOR SPORTS SHOES

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SEEKING PROOFTrials were carried out using a

standard adidas football boot withvarious inserted Carbitex DFX carbonfibre insoles to provide different degreesof gearing, including a special kickinginsole designed to resist hyper-flexion ofthe toes. Participants were made toperform linear locomotion at fivedifferent speeds: walking, slow run,medium run, fast run and sprintmeasured against a control boot with noinsert. Additional data was also collectedduring maximum effort kicking.From the information gathered, it

became clear that at the MTP joint, asboth running speed and normal MTPbending range of motion increased,the stiff insoles reduced the amount ofbending and shifted the centre ofpressure more anteriorly, while alsoreducing medial-lateral movement ofthe centre of pressure. Participants’movement patterns also changed sothat they increased their body leanand so improved their ability togenerate higher knee joint moment.The inserts also reduced keybiomechanical injury risk variablessuch as non-sagittal plane jointloading at the knee and ankle joints.As body lean increased, sagittal planejoint loading at the knee joint alsoincreased while that at the ankleremained constant, regardless ofrunning speed. As sagittal plane jointloading is thought to influence theperformance of linear movement, thisincreased loading at the knee couldassist in flexing and extending the leg,and so enhance performance.In addition to analysing performance

variables, biomechanical onesassociated with injury risk were alsoinvestigated. It has been suggestedthat increased non-sagittal plane jointloading can lead to joint injury. Jointloading is estimated by calculatingresultant joint moments, whichrepresent net torque or twisting loadson the joint and joint angular impulse.This in turn represents the cumulativeloading experienced by the jointthroughout the stance phase of amovement. While joint moments andangular impulse calculated frominverse dynamics cannot determinethe exact loading on actual jointstructures, these measures cannevertheless be used to predict totalload across a given joint.

POSITIVE RESULTThe fact that the Carbitex inserts had

a positive effect in terms of reducingpotential injury to the ankle isencouraging as it has been shown that90% of all ligament injuries at the ankleare caused by internal rotation(transverse plane movement) andinversion (frontal plane movement) ofthe foot. The similar result seen at theknee joint is also a positive as increasesin cumulative loading have beendirectly linked to chronic types of injurysuch as painful ‘runners knee’ and evenosteoarthritis. While there were nosignificant differences in terms ofkicking performance, the special stiffinsole did nevertheless have the effect

of reducing the peak amount of MTPbending during the kick, suggesting thatthis could help to reduce kicking toeflexion injuries for footballers.These positive results clearly indicate

the potential of this type of carbon fibrematerial for enhancing performance andreducing injuries in sporting activities.The fact that these materials can be finetuned to offer so many combinations offlexing and rigidity within a singlecomponent is their most remarkablefeature and only goes to show howtechnically advanced some modernshoemaking materials have become. Itis therefore perhaps worthwhile to lookat the three Carbitex productsmentioned earlier in more detail.

Torq

ue (N

m)

Angle (Degrees)

Ankle Dorsiflexion

Shank Lean

Centre of pressure liftedShifted anterior

Shifted toward midline

Torque/angle graphs for DFX D147, D148 and D149 materials.

Overview of movement pattern changes of participants.

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CARBITEX AFXAFX stands for asymmetric flexibility,

so the material is flexible in onedirection and stiff in the other. It is alsomulti-directional in the sense that it canbe tuned to provide longitudinal,lateral-medial and torsional flexibility ina variety of flex ratios. While a numberof standard materials are produced, aninfinite variety of individual config -urations can be created depending onwhat the customer wants. It has alsobeen shown to be able to sustainrepeated flexing for excessive periods oftime without any loss of performance.Furthermore, whereas traditionalcarbon fibre insole plates can crackafter extensive flexing, Carbitex saysplates cut from AFX will not. Beingcarbon fibre, it is of course extremelylightweight.It is available in four substances,

C120 (1.3mm), C127 (2.3mm), C135(1.6mm) and DO92 (2.6mm), and canbe used in a variety of footwear types.

Cleated sports (football boots, etc.) –reduces stud pressure while stillproviding forefoot flexibility.

Work boots – increases stability andprotection without limiting forefootflexibility. Also helps reduce the sizeand weight of midsoles thus reducingwearer fatigue.

Snowboarding – provides stiffnesson the board but flexibility for walk -ing off board.

Motorcycle boots – top performanceon bike combined with flexibilitywhen walking.

Fashion – improved performancewith out adding bulk.

CARBITEX DFXTraditional materials increase

stiffness in a linear manner, so it isnecessary to choose a material that hasthe right degree of flexibility that isneeded. DFX plates are tuned to changestiffness so that it is possible to producea midsole that is either stiff or flexibleas required. This is because thematerial has the ability to increase itsnon-linear stiffness when flexed and,the degree to which it does this, can betuned for almost any category ofperformance footwear. As in the caseof AFX, the range of options for tuningis virtually limitless. At the same time, italso has all the properties in regard torepeated flexing, resistance to cracking

and lightweight.It is available in three substances,

D147 (1.3mm), D148 (1.2mm) and D149(1.9mm), and again has a wide range offootwear applications.

Football boots – helps protect againstturf toe injuries.

Running – energy conservation andhelps reduce fatigue.

Basketball – increased spring whenjumping.

Training – again increased spring plusgearing for flexibility when walking andstiffness when sprinting.

Both AFX and DFX can be cut bywater jet (preferred method), CNCrouter or conventional die-cutting. Inthe latter instance, it is important tokeep cutting dies sharp as they willwear more quickly than normal due tothe tough nature of the materials. Whenbonding these materials, both surfacesneed to be clean and dry; a light wipewith isopropyl alcohol (IPA) is effective.Aggressive scrubbing and/or moreaggressive cleaning agents should beavoided as they can damage thematerial. If found necessary, a mildabrasive can be used to lightly scratchthe surface to improve bonding.

CARBITEX CX6Carbon fibre has the highest tensile

strength of any commercially availablefibre and, within its operating limits, it hasvirtually zero elongation. CX6 harnessesthese key characteristics in anexceptionally flexible form enabling it tobe used in footwear uppers where it willcontrol stretch, increase power transferand ensure firm support to the foot. Atonly 0.43mm in substance for the flatfinish version and 0.53mm for thetextured version, it adds little bulk but stillhas has exceptional tensile strength. It isalso highly resistant to abrasion, tonguetear and the effects of UV light.Furthermore, it is fully compatible withtraditional upper construction techn iquessuch as cutting and stitching, and requiresno special equipment to process it.The fact that CX6 is available in over

30 combinations of weave, weight,colour and finish means it again has aconsiderable number of potentialfootwear applications.

Cycling shoes – tight fit on the foot andzero energy loss to the pedals.

Water ski – highly durable and providesrapid and precise control on demand.

Fashion – lightweight durability withoutlosing flexibility.

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CX6 provides a glove like fit and zero loss of energy to the pedals on cycling shoes.

AFX plates incorporated in the outsole reduce stud pressure without reducing forefootflexibility on football boots.

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