Priming the plantarflexors with electrical stimulation induces more economical walking

Compared with other feasible gaits, the preferred mechanics of 'normal' (young, healthy, unimpaired) human walking appear to minimize whole-body metabolic cost. However, walking does not maximally excite the ankle plantarflexor muscles, indicating it is possible to increase their excitation and perhaps concomitantly reduce excitation of less economical hip muscles. Thus, increasing ankle plantarflexor mechanical output, and decreasing the corresponding hip extensor mechanical output, may reduce the metabolic cost of walking, even without assistive devices. Here, we used optimal control simulations of walking to suggest that increasing ankle plantarflexor excitation may reduce hip mechanical kinetics and whole-body metabolic cost. We then confirmed these simulations in an experiment involving 14 human participants where we primed participant ankle plantarflexors with 10 min of functional electrical stimulation. After removing stimulation electrodes from the body (not during stimulation), participants naturally walked with (i) increased voluntary soleus muscle excitation and ankle kinetics, (ii) modest decrements in hip extension kinetics, and (iii) a 2.3% decreased gross metabolic cost (p = 0.034) versus pre-stimulation baseline walking. The present findings demonstrate the first unassisted, preferred, mechanically feasible gait with a lower metabolic cost than that of 'normal' level-ground walking. 'Normal' human walking does not absolutely minimize metabolic cost.