Behavior of single motor units of human tibialis anterior muscle during voluntary shortening contraction under constant load torque

Abstract

The recruitment and discharge frequency of motor units during voluntary shortening contraction with constant load torque were studied in tibialis anterior muscle of 10 human subjects trained to dorsiflex the tibiopedal joint as linearly as possible from 30° plantar flexion from the neutral position (tibiopedal joint angle, 90°) by 20° in 1, 2, 3, 5, 8, and 10 s. A constant load torque of 0, 5, 10, 20, and 25% of maximum voluntary torque of the muscle was applied. When speed of shortening contraction was constant there was considerable degree of constancy in the muscle length at which individual motor units were recruited. However, the recruitment order was not rigidly fixed among the units whose length range of recruitment overlapped. When the contraction was performed faster the units were recruited earlier. The discharge frequencies of the motor units changed little with a given ramp shortening of the muscle. There were also few changes in frequency among different contraction speeds. With increasing load torque to the muscle, the discharge frequency increased slightly at a rate of 1 to 2 Hz by increase of the load. The results suggest that the speed of shortening was coded by recruitment of motor units and that discharge frequency did not play any major role when a muscle was shortened under constant load torque.