Sex-related differences in motor unit firing rate and pennation angle

Motor unit firing rate (MUFR) and pennation angle were measured concurrently in males and females from submaximal to maximal intensities. Thirty participants, (16 female, 14 male) performed isometric dorsiflexion contractions at 20%, 40%, 60%, 80%, and 100% of maximal voluntary contraction (MVC). During each contraction, measures of MUFR were obtained via surface electromyography decomposition, and muscle fiber pennation angle and fascicle length were obtained via ultrasound. There was no significant interaction effect of sex and contraction intensity present for mean MUFR (p = 0.24), pennation angle (p = 0.98), or fascicle length (p = 0.81). Males had greater mean MUFR (p < 0.001), pennation angle (p = 0.02), and fascicle length (p = 0.03) compared to females. In general, mean MUFR (p < 0.001) and pennation angle (p < 0.02) increased with increasing contraction intensity; however, fascicle length (p = 0.30) was similar across contraction intensities. There were no significant relationships between mean MUFR and pennation angle for males (r = 0.18, p = 0.13) or females (r = 0.20, p = 0.09), nor between mean MUFR and fascicle length for males (r = 0.20, p = 0.10) or females (r = 0.21, p = 0.07). Although sex-related differences in MUFR, pennation angle, and fascicle length were present, there were no relationships between MUFR and the muscle properties. These results suggest that sex-related differences in mean MUFR may not be associated with the sex-related differences in the muscle architectural properties currently investigated.