Influence of gastrocnemius muscle length on triceps surae torque development and electromyographic activity in man

The present study was designed to determine the relative contribution of the gastrocnemius muscle to isometric plantar flexor torque production at varying knee angles, while investigating the activation of the gastrocnemius muscle at standardised non-optimal lengths. Voluntary plantar flexor torque, supramaximally stimulated twitch torque and myoelectric activity (EMG) from the triceps surae were measured at different knee angles. Surface and intra-muscular EMG were recorded from the soleus muscle and the medial and lateral heads of the gastrocnemius muscle in 10 male subjects. With the ankle angle held constant, knee angle was changed in steps of 30 degrees ranging from 180 degrees (extended) to 60 degrees (extreme flexion), while voluntary torque from a 5-s contraction was determined at 10 different levels of voluntary effort, ranging from 10% of maximal effort to maximal effort. To assess effort, supramaximal twitches were superimposed on all voluntary contractions, and additionally during rest. Maximal plantar flexor torque and resting twitch torque decreased significantly in a sigmoidal fashion with increasing knee flexion to 60% of the maximum torque at 180 degrees knee angle. For similar levels of voluntary effort, the EMG root mean square (RMS) of gastrocnemius was less with increased knee flexion, whereas soleus RMS remained unchanged. From these data, it is concluded that the contribution of gastrocnemius to plantar flexor torque is at least 40% of the total torque in the straight leg position. The decrease of gastrocnemius EMG RMS with decreasing muscle length may be brought about by a decrease in the number of fibres within the EMG electrode recording volume and/or impaired neuromuscular transmission.