Gearing Up the Human Ankle-Foot System to Reduce Energy Cost of Fast Walking

Abstract

During locomotion, the human ankle-foot system dynamically alters its gearing, or leverage of the ankle joint on the ground. Shifting ankle-foot gearing regulates speed of plantarflexor (i.e., calf muscle) contraction, which influences economy of force production. Here, we tested the hypothesis that manipulating ankle-foot gearing via stiff-insoled shoes will change the force-velocity operation of plantarflexor muscles and influence whole-body energy cost differently across walking speeds. We used in vivo ultrasound imaging to analyze fascicle contraction mechanics and whole-body energy expenditure across three walking speeds (1.25, 1.75, and 2.0 m/s) and three levels of foot stiffness. Stiff insoles increased leverage of the foot upon the ground (p < 0.001), and increased dorsiflexion rangeof-motion (p < 0.001). Furthermore, stiff insoles resulted in a 15.9% increase in average force output (p < 0.001) and 19.3% slower fascicle contraction speed (p = 0.002) of the major plantarflexor (Soleus) muscle, indicating a shift in its force-velocity operating region. Metabolically, the stiffest insoles increased energy cost by 9.6% at a typical walking speed (1.25 m/s, p = 0.026), but reduced energy cost by 7.1% at a fast speed (2.0 m/s, p = 0.040). Stiff insoles appear to add an extra gear unavailable to the human foot, which can enhance muscular performance in a specific locomotion task. Humans take advantage of the functional interplay between the ankle joint and distal structures in the foot to walk and run effectively. Moreover, the plantarflexor muscle-tendon structures generate forces that help the body remain upright 1-3 and move the body forward from one step to the next 4,5. During walking or running, these muscles operate within favorable regions of force-length 6 and force-velocity (i.e., near isometric or low speeds) relationships 7-9 to produce force economically. Force production of the plantarflexors is facilitated by a gearing or lever-like function of the distal structures in the foot 10. In particular, structures like the toes, arch, and intrinsic muscles 11 can influence how the ground reaction force propagates underneath the foot, which in turn alters the force requirement of the ankle plantarflexor muscle-tendon unit. The ratio of the lever arms of the output ground reaction force and the input plantarflexor muscle-tendon force about the ankle 10 , termed gear ratio, can influence action of the plantarflexor muscles (Fig. 1). A high gear ratio can facilitate slower shortening of the plantarflexor muscles 12,13 , which could enhance force production, owing to the force-velocity relationship 14,15. The ability to modulate ankle-foot gear ratio may then help to maintain optimal function in different locomotor tasks, including steady-state walking, running 8,16,17 , and maximal-acceleration push-off 10,16. Common, everyday devices like footwear can alter the gearing-like function of the foot, consequently altering locomotor performance 13,16,18. For example, insole materials that minimize deflection aro...

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

“…How does adding carbon fiber plates to shoe soles affect athlete plantar flexor dynamics during running? Adding carbon fiber plates to shoe soles increases the footwear’s 3-point bending stiffness 12 , 13 , 15 , 17 , 24 , 25 , 28 and typically shifts the athlete’s center of pressure more anterior along the foot during ground contact 24 , 25 , 28 , 29 . These altered biomechanics generally yield a longer moment arm between the ground reaction force ( F GRF ) and the ankle-joint center ( ) 13 , 24 .…”

“…In particular, we sought to investigate how footwear 3-point bending stiffness affects soleus fascicle dynamics and running economy. Based on the reported interactions between adding carbon fiber plates to shoe soles, footwear 3-point bending stiffness 12 – 15 , 17 , 24 , 25 , 28 , 29 , and ankle-joint dynamics 13 , 14 , 24 , 29 , we hypothesized that running with shoes that have stiffer carbon fiber plates would increase soleus fascicle force generation while decreasing its operating length and shortening velocity during the ground contact. We also hypothesized that an optimal footwear bending stiffness would minimize soleus active muscle volume and aerobic energy expenditure.…”

Increasing the midsole bending stiffness of shoes alters gastrocnemius medialis muscle function during running

“…Interestingly, the metabolic cost was increased when walking in the stiff compared to a control condition. Ray and Takahashi 21 , however, demonstrated that increasing the stiffness of shoes when walking at faster speeds reduced the metabolic cost. This suggested that stiffening elements in shoes had a speeddependent effect during walking.…”

Achilles tendon moment arm changes with total ankle arthroplasty

“…Along with the lever arm of the ground reaction force, ATma is a critical factor in determining the mechanical advantage of the triceps surae as they actively plantarflex the ankle during locomotor activities. Associations have been found between ATma and ankle plantarflexor moment generation, 12,13 and ankle joint “gearing” has recently been found to influence locomotor energetics 14 . In addition, a strong, positive correlation has been found between the walking speed of slower‐walking elderly adults and ATma 15 .…”