Reductions in recruitment force thresholds in human single motor units by successive voluntary contractions.

Recruitment force thresholds of biceps brachii single motor units were studied in 4 male subjects before and after an isometric muscle contraction, passive muscle stretch, or following successive muscle contractions, muscle stretches or during alternations between muscle stretches and muscle contractions. Isometric muscle contractions of 5 s duration decreased subsequent single motor unit force thresholds. These force thresholds could usually be reset at or near precontraction force threshold values by passive muscle stretch induced by elbow extension. Single motor units showing reduced force thresholds following contraction were momentarily derecruited during and/or after muscle stretch. Successive muscle stretches alone did not significantly alter single motor unit force thresholds. In contrast, single motor unit recruitment force thresholds during successive weaker contractions were progressively lowered. Intercontraction muscle stretches maintained the single motor unit force thresholds at or near the initial force threshold level. The mechanism(s) underlying a muscle contraction-induced lowering of single motor unit force thresholds may reside in stretch reflex pathways.