Discharge Properties of Motor Units in Relation to Recruitment Order in Voluntary Contraction

Abstract The discharge pattern and recruitment order of single motor units in voluntary contraction of the normal human anterior tibial muscle was studied with an electrode having a high selectivity and a high positional stability in the muscle. In sustained contractions each motor unit was activated at a characteristic level of tension. The higher the threshold of the motor unit in sustained contraction, the higher was the frequency when the unit attained a discharge at regular intervals and the higher tended the “maximum frequency‘ ‐ of the unit to be. Motor units with low thresholds in sustained contractions exhibited a continuous or tonic discharge in strong sustained contractions, whereas units with high thresholds tended to exhibit a discontinuous or phasic discharge. For some units the threshold of activation remained stable, for others it increased during activity. In twitch contractions the recruitment order of motor units differed considerably from that in sustained contractions.