Fluctuations in motor unit recruitment threshold during slow isometric contractions of wrist extensor muscles in man.

Motor unit activity was recorded in the two extensor carpi radialis muscles with metal microelectrodes during isometric contractions. When tested at different 'free' increasing contraction velocities the recruitment thresholds (rt) systematically decreased, whereas when tested at different 'imposed' increasing velocities the rt exhibited great variability, with no observable overall tendency. Moreover, when measured at a given test velocity imposed periodically during MU investigation, rt also displayed great variability. These results suggest that the notion of rt tends to be oversimplified. The motoneurone pool excitability is highly dependent on both the experimental situation and the complexity of the motor task.