Effect of knee joint position on soleus muscle function during isokinetic plantarflexion

Contribution of the gastrocnemii muscles to ankle moment is influenced by the knee joint position because they span the knee and the ankle joint as well. However, limited information is available on the effect of knee joint position on soleus activation under dynamic plantarflexion, hence the aim of this study was to investigate if soleus have a compensatory strategy in fascicle behavior or EMG activity during knee flexed plantarflexion in order to reduce the magnitude of the decrement in ankle moment.

Isokinetic dynamometry with EMG and ultrasound measurements was used to estimate medial gastrocnemius and soleus behavior during knee flexed and extended plantarflexions using three angular velocities. Seventeen healthy males were participated in this study.

Flexed knee plantarflexions resulted in lower peak ankle moments at all ankle angular velocities by 18% (P = 0.1062) at 30°∙s-1, 44% (P < 0.001) at 60°∙s-1 and by 18% (P = 0.0001) at 120°∙s-1. Soleus showed significantly higher EMG activity during knee flexed plantarflexion at 30°∙s-1 (P = 0.0094) and 60°∙s-1 (P = 0.0142). The magnitude of mean shortening of the medial gastrocnemius and soleus show statistically significant difference between knee flexed and knee extended plantarflexion at any contraction velocity.

Soleus may perform a compensatory EMG activity in knee flexed plantarflexions possibly to counteract the reduced contribution of gastrocnemius to ankle moment at low angular velocity contractions.