Effect of muscle length on the recruitment property of single motor units in humans

In order to examine the effect of muscle length on the recruitment property of a single motor unit (MU), the action potentials of the MU in the human gastrocnernius muscle were recorded during voluntary isometric contraction at different knee joint angles. We found that the recruitment property of MU, which is determined by relationship of recruitment threshold torque (T-th) and ramp contraction speeds, changed as the knee joint angle was changed from 0degrees to 90degrees. The decrease of T-th with contraction speed occurred only when the muscle was held long. This result indicates that the MU recruitment property, which decrease of T-th with higher contraction speed, needs both central command and Ia inputs to the alpha-motoneuron. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.