Contribution of individual muscle activity to measurable net force fluctuations during isometric steady contractions

To estimate the contribution of individual muscles to steadily control measurable net force during isometric contractions in a multiple-muscle model, we calculated cross correlation function between isometric plantarflexion force and rectified surface EMG (rEMG) of the involved muscles in low frequency (< 3Hz). Among the plantarflexor muscles, temporal force profiles were selectively correlated with those of rEMG in the medial gastrocnemius and soleus muscles, but not the lateral gasctrocnemius, suggesting that these two muscles may predominantly determine the plantar force fluctuations.