Activation patterns of the soleus and gastrocnemius muscles during different motor tasks.

The activation patterns of the two main ankle extensor muscles, the soleus, with a predominance of slow-twitch muscle fibers, and gastrocnemius, in general consisting of >50% fast-twitch muscle fibers, were studied in 10 normal men. They performed four different motor tasks on a force plate, standing on one leg (STND), toe-standing on one leg (TOE), hopping with two legs, either with maximal frequency (mean 4.5 Hz, height 1 cm, FAST) or maximum height (mean 29 cm, frequency 1.6 Hz, MAX). Electromyographic (EMG) signal was recorded with bipolar surface electrodes (identical interelectrode distance), sampled at 1 kHz, and was processed by an on-line computer system. The mean EMG amplitude increased successively for both muscles from standing to maximal hopping; average values increased 10.8 times for gastrocnemius nemius but only 6.3 times for soleus, respectively. The mean power frequency showed less marked but significant increases in both muscles. EMG amplitude ratios of the soleus to the gastrocnemius were 1.67 in STNG, 1.16 in TOE, 0.92 in FAST and 0.81 in MAX. Consequently, there was a shift in relative activation magnitude from the slow soleus to the fast gastrocnemius muscle with increasing demands of force and speed. These findings in humans are in line with earlier observations in cats, indicating a task-specific preferential reliance on either of the two functionally specialized muscles within the ankle extensor synergy.