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 product ion at varying knee angles, while investigating the activation of the gastrocnemius muscle at standardised non-optimal lengths. Voluntary pl antar flexor torque, supramaximally stimulated twitch torque and myoel ectric activity (EMG) from the triceps surae were measured at differen t knee angles. Surface and intra-muscular EMG were recorded from the s oleus muscle and the medial and lateral heads of the gastrocnemius mus cle in 10 male subjects. With the ankle angle held constant, knee angl e was changed in steps of 30 degrees ranging from 180 degrees (extende d) 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 effor t, supramaximal twitches were superimposed on all voluntary contractio ns, and additionally during rest. Maximal plantar flexor torque and re sting twitch torque decreased significantly in a sigmoidal fashion wit h 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, wh ereas 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 decrea se of gastrocnemius EMG RMS with decreasing muscle length may be broug ht about by a decrease in the number of fibres within the EMG electrod e recording volume and/or impaired neuromuscular transmission.