Motor units of medial gastrocnemius muscle in the rat during the fatigue test. I. Time course of unfused tetanus.

Time course of tetanus recorded during 40 Hz stimulation was examined on 70 motor units (MUs) of medial gastrocnemius muscle in 41 rats. Three types of MUs were distinguished: slow(S), fast resistant to fatigue (FR) and fast fatigable (FF). In fast MUs of either type, the stimulation induced unfused tetanus with noticeable sag. It was found that in cases of tetani in FF MUs, the sag developed earlier than in FR MUs. Timing of tetanic peak before the sag appeared correlated with contraction time. Some fast MUs demonstrated a distinct time course of tetanus: the weakly accentuated sag appeared early and it was followed by a relatively significant rise in tetanic tension. However, in slow MUs, 40 Hz stimulation induced fused or fused-like tetani. In these MUs the sag was never observed. The obtained results indicate the relation between the sag time course in tetanus and the extent of tetanus fusing.