The modulation of forward propulsion, vertical support, and center of pressure by the plantarflexors during human walking.

The gastrocnemius and soleus both contribute to the ankle plantarflexor moment during the mid- and terminal stance phases of gait. The gastrocnemius also generates a knee flexion moment that may lead to dynamic function that is unique from the soleus. This study used a muscle stimulation protocol to experimentally compare the contributions of individual plantarflexors to vertical support, forward propulsion and center of pressure (CoP) movement during normal gait. Twenty subjects walked on an instrumented treadmill at self-selected speeds with stimulating surface electrodes affixed over the medial gastrocnemius and soleus muscles. Short duration pulse trains (90 ms) were used to stimulate either the gastrocnemius or soleus at 20% or 30% of the gait cycle (GC) of random strides. Changes in ground reactions between stimulated and non-stimulated strides were evaluated to characterize the influence of each muscle on whole body movement during mid- (stimulation onset at 20% GC) and late (30% GC) stance. The gastrocnemius and soleus each induced an increase in vertical support and anterior progression of the CoP in mid-stance. However, late stance gastrocnemius activity induced forward acceleration, while both mid- and terminal stance soleus activity induced braking of forward velocity. The results suggested that the individual plantarflexors exhibit unique functions during normal gait, with the two muscles having opposite effects on forward propulsion. These empirical results are important both for enhancing the veracity of models used to predict muscle function in gait and also clinically as physicians seek to normalize gait in patients with plantarflexor dysfunction.