Bikefit Science:
Foot/Shoe/Pedal Interface

The foundation of effective bike-fitting despite its complexities is the foot/shoe/pedal (FSP) interface. Often this area can be neglected, undervalued or misunderstood by cyclists, coaches and bike-fitters (1). The structural components of the FSP interface dictates how effectively pedal forces are conveyed down the cranks; and how abnormal stresses are dispersed at the interface rather than traveling up the kinetic chain which may potentially lead to overuse injuries. 

The FSP interface although complex, represents the mechanical link between rider and bike such as;

  • Leg lengths differences (true of functional)

  • Foot and limb alignment

  • Shoe insert

  • Shoe design    

  • Cleat and pedal design and cleat position in five directional planes (1,2)

Assessing - Leg Length Differences (LLD)

Leg length differences (LLD) can either be functional or anatomical (3,4). Anatomical LLD is identified when a true bone length difference (tibia or femur) exists. However a functional LLD is recognised when the shortening of one leg has occurred without a bone length difference. A functional LLD difference can often be caused by a number of factors for example:

 

  1. Pelvic dysfunction  

  2. Muscle weakness

  3. Muscle imbalance

  4. Unilateral foot pronation

Assessing LLD using a range of diagnostic tests

Functional LLD can often be mistaken for anatomical LLD. Using a reliable range of simple differential diagnostic tests may often deliver sound clinical judgment in the absence of X-Rays and scans (5,6,7,8,9).

Carbon insloes - balancing
power transfer with power loss

Carbon Technology

Over the past decade there have been advancements in rigid carbon fibre frames together with carbon wheels to improve stiffness and power transfer. However despite these advancements, research studies have shown that carbon fibre soled shoes to be 42% stiffer in longitudinal bending and 550% stiffer in three point bending in relation to plastic shoes (16).  This would suggest that although advancements have been made to provide greater power transfer this has also increased forefoot pressures. Furthermore previous studies have reported by increasing the power outputs potentially leads to greater forefoot pressures causing the foot to collapse and pronate resulting to further forefoot misalignment (17,18,19,20).


In summary, although carbon fibre insoles promotes the power transfer they may also cause the foot to collapse and roll inwards leading to potential power loss.

Foot Misalignment

Less than 10% of most cyclists have a neutral foot according to research results (10,11). Other research findings such as Garbalosa et al (12) found that 87% of cyclists have forefoot varus misalignment, 9% forefoot valgus misalignment and 4% have a neutral forefoot-rear-foot relationship. Consequently Millslagle et al (13) reported that the conventional pedal systems are often manufactured for the cyclist to connect to the pedal flat footed. This would suggest that the conventional pedal systems are potentially suitable to approximately 4% of the cycling population who do not suffer from forefoot misalignment.

Foot misalignment (tilt)

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The minimum amount of misalignment that occurs at the FSP interface potentially results in pedalling asymmetry which also leads to power loss and knee injuries (14, 15). Forefoot varus is the inverted position of the bones in the forefoot in relation to the heel, whereas a true forefoot varus is a structural fixed deformity. A flexible forefoot varus is often referred to as ‘supinatus’ which is considered an adaptive position of the forefoot in response to a greater degree of rear-foot eversion (over-pronation).

Varus Cleat Wedges

Forces applied to the pedal during the downstroke reach their maximum at approx 90° of crank angle (17,18) causing the foot to tilt (pronate) in the direction that allows the forefoot to become parallel with the pedal (19,21).  Inward tilting of the forefoot causes the knee to move inwards towards the top-tube – depicted by blue arrows in (fig. 1). The dotted-oval trace (fig.1) represents knee motion during a pedal revolution. Varus wedges support the forefoot in cyclists with forefoot tilt (fig. 2). Studies have demonstrated increased power output when using wedges compared without wedges (14,21,22).

Fig 1 & Fig 2

Research undertaken at Manchester Metropolitan University demonstrated an average increase in power output of 3.8% in favour of using wedges compared without wedges.  furthermore, the  findings demonstrated that varus wedges offer greater performance benefits to those riders with greater forefoot misalignment (14).

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References:

  1. Dinsdale, N.J., and Dinsdale, N.J. (Miss) (2014) Modern-day Bikefitting can offer proactive therapists new opportunities, sportEX dynamics, 39:25-32

  2. Dinsdale, N.J., and Dinsdale, N.J. (Miss) (2011) The benefits of anatomical and biomechanical screening of competitive cyclists, sportEX dynamics, 28:17-20

  3. Knutson, G. (2005) Anatomic and functional leg-length inequality: A review (Part 1). Chiropractic & Osteopathy, 13(11):

  4. Knutson, G. (2005) Anatomic and functional leg-length inequality: A review and recommendation for clinical decision-making. Part 2, the functional or unloaded leg length asymmetry, Chiropractic & Osteopathy, 13;12

  5. Brady, R., Dean, J., Skinner, T. et al., (2003) Limb length inequality: Clinical implications for assessment and intervention. Journal of Orthop & Sports Physical Therapy, 33, 221-234.

  6. Cooperstein, R., Haneline, M., and Young, M. (2007) Mathematical modelling of the so called Allis test: a field study in orthopedic confusion. Chiropractic & Osteopathy, 15:3 doi: 10.1186/1746-1340-15-3.

  7. Krawiec, C., Denegar, C., Hertel, J., et al., (2003) Static innominate asymmetry and leg length discrepancy in asymptomatic athletes. Manual Therapy, 8(4), 207-213.

  8. Caselli, M. and Rzonca, E. (2002) Detecting and treating Leg-Length Discrepancies. Podiatry Today, 15(12), 65-68.

  9. Juhl, J. (2004) Prevalence of frontal plane pelvic postural asymmetry. J Am Acad Osteopath Assoc, 104(10), 411-21.

  10. Whitney, K.A. (2003). Foot deformities Part II, Journal of Clinics in Podiatric Medicine & Surgery, 20(3), 511-526.

  11. Cornwall, M. (2000) Common pathomechanics of the foot. Journal of Athletic Therapy Today. 5,10-16.

  12. Garbalosa, J., McClure, M., Catlin, P., et al., (1994) The frontal plane relationship of the forefoot to the rearfoot in an asymptomatic population, Journal of Orthopaedic and Sports Physical Therapy, 20, 200-206.

  13. Millslagle, D., Rubbelke, S., Mullin, T., et al., (2004) Effects of foot-pedal positions by inexperienced cyclists at the highest aerobic level, Perceptual and Motor Skills, 98, 1074-1080.

  14. Dinsdale, N.J., and Williams, A.G. (2010) Can forefoot varus wedges enhance anaerobic cycling performance in untrained males with forefoot varus? Journal of Sport Scientific and Practical Aspects, 7(2):5­-10

  15. Asplund, M., and St Pierre, P. (2004) Knee pain and bicycling, The Physician and Sports Medicine, 32:23-30

  16. Jarboe, N. and Quesada, P. (2003) The effects of cycling shoe stiffness on forefoot pressure. Foot Ankle Int., 24(7), 784-788.

  17. Farrell, K., Reisinger, K., and Tillman, M. (2003) Force and repetition in cycling: possible implications for Iliotibial band friction syndrome. The Knee, 10, 103-109.

  18. Davies, R. and Hull, M. (1981) Measurement of pedal loading in bicycling: II. Analysis and results. Journal of Biomechanics, 14, 857-872.

  19. Hennig, E., and Sanderson, D. (1995) In-shoe pressure distributions for cycling with two types of footwear at different mechanical loads. Journal of Applied Biomechanics, 11, 68-80.

  20. Sanner, W. and O’Halloran, W.D. (2000) The biomechanics, etiology, and treatment of cycling injuries. Journal of American Podiatric Medical Association, 90, 354-376.

  21. Hannaford, D., Moran, G.T., and Hlavac, A.M. (1986) Video analysis and treatment of overuse knee injury in cycling: a limited clinical study. Clinics in Podiatric Medicine and Surgery, 3, 671-678.

  22. Moran, G. and McGlinn, G. (1995) The effect of variations in the foot pedal interface on the efficiency of cycling as measured by aerobic energy cost and anaerobic power. Biomechanics in Sport, 12, 105-109.

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