Effects of voluntary activation on neuromuscular endurance analyzed by surface electromyography.

The purpose was to examine the relation between voluntary muscle activation and neuromuscular endurance of individual subjects based on the pattern of surface electromyography (EMG). The voluntary muscle activation was estimated from the relation between voluntary force and tetanic force superimposed on the voluntary force (twitch interpolation technique). 14 male subjects (10 regular exercisers and 4 sedentary; 21-29 years old) were divided into a High Voluntary Activation group and a Low Voluntary Activation group. A significant positive correlation of .72 (p<.01) was found between maximum voluntary torque and voluntary activation. A fatigue test was conducted during isometric contractions of 60% and 20% maximum voluntary torque. The endurance time was significantly longer for the Low Voluntary Activation group than the High Voluntary Activation group. The mean power frequency of voluntary EMG obtained from the vastus lateralis muscle decreased consistently whereas the average rectified value increased. The final change of mean power frequency relative to the initial value was significantly greater in the 60% Fatigue task than in the 20% Fatigue task. For the 60% Fatigue task, the final change of mean power frequency and average rectified value relative to the initial value was significantly greater in the Low Voluntary Activation group than in the High Voluntary Activation group. These results suggest that the individual differences in voluntary activation determine the neuromuscular performance usually evaluated as maximum voluntary torque and endurance time and that the voluntary activation may depend on the daily exercise.