Knee position affects medial gastrocnemius and soleus activation during dynamic plantarflexion: no evidence for an inter-muscle compensation in healthy young adults

Knee joint position influences ankle torque, but it is unclear whether the soleus compensates to counteract the reductions in gastrocnemius output during knee-flexed versus knee-extended plantarflexions. Therefore, the purpose of this study was to determine the effects of knee joint position and plantarflexion contraction velocity on ankle plantarflexion torque and electromyography activity of the medial gastrocnemius and soleus in healthy young adults. Healthy male participants (n=30) performed concentric plantar flexions in a custom-built dynamometer from 15° dorsiflexion to 30° plantarflexion at gradually increasing velocities during each contraction at 30, 60, 120, 180, and 210° s-1 in a supine position with the knee fully extended and while kneeling with the knee fixed in 90° flexion. Two 16-channel linear electromyographic (EMG) arrays were placed over the medial gastrocnemius and soleus muscles. Plantarflexion torque during flexed-knee versus extended-knee plantarflexions was 31% lower (P=0.002) averaged across the five contraction velocities. The overall EMG activity of the medial gastrocnemius was 35% lower (P=0.002) during knee-flexed versus knee-extended plantarflexions. In the first half of plantarflexions at slower contractions, soleus EMG activity was 15% and 28% higher (both P=0.002) in knee-flexed versus knee-extended plantarflexion, respectively. We conclude that knee position affects medial gastrocnemius and soleus activation during dynamic plantarflexion, with plantarflexion torque being smaller in the knee-flexed versus knee-extended position. However, we found no evidence that changes in soleus activation would compensate for the decrease in medial gastrocnemius activation.