DYNAlMIC PROPERTIES OF HUMAN MOTONEUBRONS

For limb muscles, a-motoneurons in the spinal cord are the last elements to integrate and modulate signals from other parts of the central nervous system. Animal research on the dynamic properties of a-motoneurons shows that a-motoneurons behave as differentiators or high pass filters to injected currents. In behaving animals or human subjects, one cannot control the input currents to a-motoneurons. Therefore, human studies have controlled the force output and examined the associated firing patterns of motoneurons or the population electromyography (EMG) activity of the muscle. A monotonic or linear relationship has been shown between the EMG activity and the force output. No studies have related the fiiing patterns of underlying single motor units to EMC and force output. The foEowing study examined the relationship between simultaneously recorded population E,MG and single motor units to the dynamic and the steady state phases of ramp-and-hold force trajectories. The subjects traced ramp-and-hold isometric force trajectories using wrist flexion. The contraction speeds and amplitudes of force output were controlled. Surface EMG from wrist flexors and single motor unit (SMU) action potentials from flexor carpi radialis muscle were recorded. The background firing rate of SMU activity was controlled. Averaged responses of recfrfied surface EMG and single motor unit activities were constructed for each condition. EMG and SMU responses were quantified during the dynamic and the steady state phases. The dynamic activity of EMG was linearly related to the contraction speed while the steady state activity of EMG was linearly related to the contraction amplitude. The dynamic activity of a SMU increased nonlineady with the speed of contraction. An increase in the background firing rate of a SMU resulted in a decrease in its dynamic activity. The steady state activity of a SMU was affected by contraction amplitude but not by contraction speed. The steady stare activity increased nodinear!y with contraction amplitude. An increase in backpund firing rae decreased the magnitude of the steady state response. The linear responses of EMG during the dynamic and the steady state phases cannot be completely explained by the nonlinear responses of tonically firing SMUs. The linearity of EMG is suggested to result from recruitment of additional SrvlUs contributing phasically during the ramp-and-hold contraction.