Sex-Related Differences in Motor Unit Firing Rates Between Muscles of the Upper and Lower Limb

Studies identifying sex-related differences in motor unit behavior are limited and show conflicting results. Differences across studies may depend on the muscle and contraction intensity being investigated. Therefore, it is necessary to identify sex-related differences in motor unit firing rates (MUFR) across muscles of the upper and lower limb and at various contraction intensities. The objective of this study was to compare MUFR between healthy males and females in the first dorsal interosseous (FDI) and tibialis anterior (TA) muscle. It was hypothesized that females would have higher MUFR during maximal contractions of the FDI, and submaximal contractions of the TA, while males would have higher MUFR during maximal contractions of the TA and submaximal contractions of the FDI. Twenty healthy males (age: 23.2 ± 2.7 years) and 20 healthy females (age: 22.7 ± 2.2 years) completed two testing sessions, one for the FDI and one for the TA. During each session, isometric maximal voluntary force (MVC) was recorded and participants completed three isometric contractions of index finger abduction or ankle dorsiflexion at 20% MVC, 50% MVC, and 100% MVC. A four-pin surface EMG electrode (Galileo wireless EMG, Delsys Inc.) was placed on the skin over the muscle to record the electrical activity of the muscle during each contraction. The EMG signal was decomposed into individual motor units and MUFR were calculated. Males had significantly greater MVC force than females in both muscles (p < 0.01). There were no main effects of sex on motor unit firing rates (p = 0.35, η 2 = 0.02), however, there was a sex by muscle interaction, such that males had significantly higher MUFR than females within the TA (p = 0.03, η 2 = 0.12). More specifically within the TA, males had higher firing rates than females at 20% MVC (p = 0.03) and 50% MVC (p = 0.02), but not 100% MVC (p = 0.25). There was a main effect of contraction intensity on MUFR (p < 0.01, η 2 = 0.96), such that MUFR were higher during the 50% MVC contraction compared to 20% MVC (p < 0.01), and higher during 100% MVC contraction compared to the 50% MVC contraction (p < 0.01). This pattern was observed within both males and females (p < 0.01, η 2 > 0.93), and within both muscles (p < 0.01, η 2 > 0.92). Sex-related differences in motor unit firing rates in healthy, young adults may be muscle and contraction intensity dependent. This work was supported by the Natural Sciences and Engineering Research Council of Canada Discovery Grant (RGPIN-2019-05127; Christie). This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.