Sex-related differences in submaximal but not maximal motor unit firing rates of the human vastus intermedius and vastus lateralis

Sex-related differences in motor unit (MU) behaviour are sparse, particularly across a full range of contraction intensities, and contradictory, with various mechanisms proposed to address potential differences. Thus, the purpose was to investigate MU firing rates and electrically evoked contractile properties of the quadriceps femoris in males and females, across the full force range. In 16 participants (8 females:8 males) ultrasound-guided intramuscular tungsten microelectrodes were used to record MU firing rates from the vastus intermedius and lateralis during isometric knee extension contractions upwards to maximum. Electrically evoked twitch and doublet responses assessed contractile properties of the quadriceps. Strength in females was ~60% of males (P<0.001), with ~95% voluntary activation during maximal contractions in both groups. A total of 4122 MU trains were analyzed (balanced sample across sex). Grouped firing rates in females were 1.6 to 2.8 Hz higher (P<0.001) at the submaximal intensities, with no difference in MU firing rates at MVC (P=0.694). No difference in twitch or doublet parameters of contractile speed were found (P>0.05). These differences in absolute firing rates may have functional implications particularly at submaximal intensities in which small alterations in firing rate induce large changes in force. Force modulation through rate coding during submaximal contractions is different between males and females in these two muscles of the quadriceps. However, maximal rates were not different and thus the absolute rate-coding capacity of both sexes is the same. The difference at submaximal contractions may be due to intrinsic motor neuron excitability rather than intrinsic muscle contractile speed.