Introduction: Deterioration in muscle strength and function is an inevitable consequence of the normal ageing process. The intrinsic foot muscles, which stabilise the medial longitudinal arch and contribute to balance and mobility, are particularly affected. Such declines increase the risk of falls and impair activities of daily living. This phenomenon reflects alterations in motor unit (MU) properties, which may include MU firing. Intrinsic foot muscle strength is important for foot function, with the flexor digitorum brevis (FDB) muscle contributing to postural control (Okai and Kohn, 2015, Fukuyama et al., 2024). To date, no studies have yet investigated the effect of age and sex on MU firing in the FDB muscle. This study aimed to compare the MU firing rate during submaximal isometric toe grips in young and older individuals of both sexes.
Methods: Following institutional ethical approval, fifteen young (6 females, aged 22.8 ± 2.8 years, BMI: 24.12 ± 4.15 kg/m2) and 14 older (5 females, aged 66.6 ± 3.8 years; BMI: 23.73 ± 2.76 kg/m2) participants volunteered for the study. Participants were tested for maximal isometric toe grip force (MTGF) and sustained isometric toe grip at 10, 20, 30 and 40%MTGF using a toe grip dynamometer. High-density surface EMG signals from the FDB muscle were simultaneously recorded using a 611 grid of 64 electrodes. Force steadiness was estimated as the coefficient of variation. MU firing rate and firing rate variability (i.e., CoV-ISI) were estimated from decomposed motor units.
Results: No age-related differences in height, weight or BMI were observed. Males exhibited a greater age-related decline in MTGF (170.7 ± 64.9 vs 97.8 ± 34 N; p < 0.001), whereas no difference was observed between young and older females (81.24 ± 22.3 vs 81.1 ± 27.4 N). No age-related differences were observed in MU firing rate or its variability during submaximal contractions. However, a main effect of contraction intensity was observed in both MU firing rate (p < 0.001) and CoV-ISI (p = 0.002). MU firing rate was lowest at 10%MTGF (all p < 0.05) when compared to other intensities and was lower at 20%MTGF than at 30 (p = 0.02) and 40%MTGF (p < 0.04). COV-ISI at 10%MTGF was lower than at 40%MTGF. Similarly, force steadiness was lowest at 10%MTGF when compared to the other intensities (all p < 0.001).
Conclusions: This study is the first study to examine the FDB motor unit firing behaviour in young and older individuals of both sexes. During submaximal isometric toe grips, no age-related differences in MU firing rate or variability were observed. However, the firing rate increased with contraction intensity, and CoV-ISI was lowest at 10%MTGF, consistent with the force steadiness pattern. Despite preserved MU firing, older males exhibited a more pronounced reduction in maximal toe grip force, but this was not observed in females. These preliminary results suggest an intensity-dependent modulation of MU behaviour in the FDB muscle and highlight the need for larger cohorts or longitudinal studies to elucidate the neuromuscular determinants of age-related loss of MTGF.