Adding Stiffness to the Foot Modulates Soleus Force-Velocity Behaviour during Human Walking

Abstract

Previous studies of human locomotion indicate that foot and ankle structures can interact in complex ways. The structure of the foot defines the input and output lever arms that influences the force-generating capacity of the ankle plantar flexors during push-off. At the same time, deformation of the foot may dissipate some of the mechanical energy generated by the plantar flexors during push-off. We investigated this foot-ankle interplay during walking by adding stiffness to the foot through shoes and insoles, and characterized the resulting changes in in vivo soleus muscle-tendon mechanics using ultrasonography. Added stiffness decreased energy dissipation at the foot (p < 0.001) and increased the gear ratio (i.e., ratio of ground reaction force and plantar flexor muscle lever arms) (p < 0.001). Added foot stiffness also altered soleus muscle behaviour, leading to greater peak force (p < 0.001) and reduced fascicle shortening speed (p < 0.001). Despite this shift in force-velocity behaviour, the whole-body metabolic cost during walking increased with added foot stiffness (p < 0.001). This increased metabolic cost is likely due to the added force demand on the plantar flexors, as walking on a more rigid foot/shoe surface compromises the plantar flexors’ mechanical advantage.

Adding carbon fiber to shoe soles does not improve running economy: a muscle-level explanation

“…Adding carbon fiber plates to shoe soles increases the footwear’s 3-point bending stiffness 12, 13, 15, 23, 24 and typically shifts the athlete’s center of pressure more anterior along the foot during ground contact. 23, 24, 27 These altered footwear biomechanics theoretically increase plantar flexor force generation and decrease the respective muscle fiber’s operating lengths, such changes require more metabolic energy expenditure per contraction. 28, 29 Additionally, running with stiffer footwear may decrease plantar flexor shortening velocity, reducing muscle metabolic energy expenditure per contraction.…”

“…13, 23 Consequently, this longer moment arm leads to a greater GRF-induced ankle-joint flexion moment. 12, 13, 23, 27 To prevent the ankle-joint from collapsing, plantar flexor muscle-tendons (MTs) generate force ( F MTs ) and apply an opposite moment about the joint. The moment arm between the plantar flexor MTs and ankle-joint center is indicated by r MT .…”

“…33 Hence, deeming that muscle pennation changes are relatively small, increased MT force may further stretch spring-like tendons (tendon length: L T ) and yield shorter in-series operating muscles ( L M ). Lastly, adding carbon fiber plates to shoe soles may decrease plantar flexor muscle fascicle shortening velocity during ground contact, 14, 27 eliciting more economical muscle contractions. 28, 29 Absent of meaningful changes in ankle-joint mechanical power ( P ank ) and plantar flexor MT moment arms ( r MTs ), increasing plantar flexor MTs force ( F MTs ) decreases ankle-joint angular velocity ( ω ank ).…”

“…In particular, we sought to investigate how footwear 3-point bending stiffness affects muscle dynamics and running economy. Based on the reported interactions between adding carbon fiber plates to shoe soles, footwear 3-point bending stiffness, 12–15, 23, 24, 27 and ankle-joint dynamics, 13, 14, 23, 27 we hypothesized that running with shoes that have stiffer carbon fiber plates would increase soleus (plantar flexor muscle) force generation while decreasing soleus operating length and shortening velocity during the ground contact. We also hypothesized that an optimal footwear bending stiffness would minimize active soleus volume and aerobic energy expenditure.…”

Technical Aspects and Validation of a New Biofeedback System for Measuring Lower Limb Loading in the Dynamic Situation

“…Improvements can also be made in the design of the custom-made sandals. Previous studies have shown that rigid shoes and insoles have a significant influence on gait parameters [16]. In the ongoing development of the biofeedback system, adjustments in the sandal design are envisaged to overcome, at least in part, these issues.…”