Stiffening the human foot with a biomimetic exotendon

Ryan C Riddick,Luke A Kelly,Nicholas A T Brown,Dominic J Farris

doi:10.1038/s41598-021-02059-8

Ryan C Riddick, Luke A Kelly + Show 2 more

Open Access

https://doi.org/10.1038/s41598-021-02059-8

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Abstract

Shoes are generally designed protect the feet against repetitive collisions with the ground, often using thick viscoelastic midsoles to add in-series compliance under the human. Recent footwear design developments have shown that this approach may also produce metabolic energy savings. Here we test an alternative approach to modify the foot–ground interface by adding additional stiffness in parallel to the plantar aponeurosis, targeting the windlass mechanism. Stiffening the windlass mechanism by about 9% led to decreases in peak activation of the ankle plantarflexors soleus (~ 5%, p < 0.001) and medial gastrocnemius (~ 4%, p < 0.001), as well as a ~ 6% decrease in positive ankle work (p < 0.001) during fixed-frequency bilateral hopping (2.33 Hz). These results suggest that stiffening the foot may reduce cost in dynamic tasks primarily by reducing the effort required to plantarflex the ankle, since peak activation of the intrinsic foot muscle abductor hallucis was unchanged (p = 0.31). Because the novel exotendon design does not operate via the compression or bending of a bulky midsole, the device is light (55 g) and its profile is low enough that it can be worn within an existing shoe.

Highlights

Shoes are generally designed protect the feet against repetitive collisions with the ground, often using thick viscoelastic midsoles to add in-series compliance under the human
We developed and implemented a biomimetic exotendon to act in parallel with the plantar aponeurosis and intrinsic foot muscles
The exotendon increased the stiffness of the plantar Muscle Tendon Unit (MTU), by approximately 9%, during a bilateral hopping task

Summary

Introduction

Shoes are generally designed protect the feet against repetitive collisions with the ground, often using thick viscoelastic midsoles to add in-series compliance under the human. Because shoes generally have minimal capacity to constrain the movement of the foot inside the shoes, one performance enhancing strategy is to tune the compressive elasticity of a shoe midsole, which can lead to improvements in running e conomy[10,11] Another approach is to incorporate stiff carbon fiber plates into a shoe to increase the longitudinal bending stiffness of s hoes[13]. To the best of our knowledge, there are currently no devices which target these two major joints synchronously to increase the overall elastic work of the foot without restricting the motion of the arch Part of this absence is likely due to the pervasive methodology in biomechanics of interpreting joint dynamics without considering structures which span multiple joints. Analyzing the action of these two joints together or modelling the plantar fascia directly show that the plantar fascia operates nearly elastically when stepping on level ground[4,6,24]

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