Propulsion without penalty: greater soleus force with stiffer footwear does not necessarily increase estimated soleus metabolic cost across walking speeds.

Numerous assistive devices have been designed to improve mobility by improving propulsion and reducing the metabolic cost of walking. Stiff carbon fiber insoles integrated into footwear have emerged as a potentially viable option by increasing longitudinal bending stiffness, providing additional leverage for the ankle joint musculature, and increasing soleus force output. However, it remains unknown whether this increased leverage comes with a metabolic penalty at the individual muscle level, which would create a translational barrier for prescribing carbon fiber insoles as targeted interventions. We incorporated motion capture, cine B-mode ultrasound, and electromyography data ( = 14) into a bioenergetic model to estimate soleus metabolic cost. Participants walked on an instrumented treadmill at 1.25, 1.75, and 2.0 m/s wearing standardized shoes containing either no carbon fiber insole (low stiffness), a 1.6-mm-thick insole (medium stiffness), or a 3.2-mm-thick insole (high stiffness). We found a significant main effect ( < 0.001) of walking speed, but not stiffness, for estimated soleus average metabolic power. These results indicate that increases in soleus force output while walking due to increased footwear bending stiffness do not statistically significantly alter muscle-specific metabolic cost, likely due to concomitant reductions in fascicle shortening velocity. As such, carbon fiber insoles may be a particularly useful assistive device for walking in those with ankle plantarflexion deficits. Increasing footwear bending stiffness via carbon fiber insoles has been shown to reduce soleus fascicle shortening velocity and increase force output. Here, we used a bioenergetic model to estimate the metabolic energy consumed by the soleus muscle with increasing footwear stiffness across walking speeds. Footwear stiffness did not statistically significantly alter estimated soleus muscle energy consumption at any speed, highlighting carbon fiber insoles' capacity to increase muscle force without a clear metabolic penalty.