Fatigue-induced decline and recovery of maximal motor unit firing rates did not differ between males and females in two muscles of the quadriceps femoris

Studies on sex-related differences in motor unit (MU) behaviour with muscle fatiguing tasks are limited, particularly during contractions of maximal intensity and usually to one muscle within a synergistic group. Therefore, the purpose was to investigate the influence of sex and muscle on maximal MU firing rates during and following a sustained maximal contraction. Ultrasound-guided intramuscular tungsten microelectrode MU recordings were obtained from the vastus intermedius (VI) and lateralis (VL) of 16 participants (8 females, 8 males) during a sustained fatiguing one-minute maximal voluntary isometric knee extension task and throughout a subsequent 5-minute recovery period. Contractile properties of the quadriceps were assessed via electrically evoked doublet (100 Hz) responses. Despite a decline of force and MU firing rates (both by ~50%; P<0.05) at task termination, there were no differences in the reduction and subsequent recovery of maximal firing rates in both muscles (VI vs VL) and between males and females. Additionally, there were no sex-related differences in parameters of electrically evoked contractile speed immediately following the fatiguing task. Instantaneous force measurement at end of task was better maintained in the females (P<0.01), however there were no significant differences in the area of the force -time curve (i.e. fatigue index; P>0.05). These findings indicate that firing rate adaptations are not different between males and females in either muscle when the neuromuscular system is stressed with sustained maximal contractions.