The menopausal transition marks the cessation of the female reproductive cycle and is characterised by a rapid decline in oestrogen and progesterone. These primary female sex hormones also have excitatory and inhibitory properties and may modulate motor function to an ill-defined extent (Piasecki et al., 2024). Many detrimental symptoms of the menopause may be alleviated with hormone replacement therapy (HRT), which has beneficial effects on skeletal muscle function (Ronkainen et al., 2009). Non-pharmacological interventions such as physical exercise can also alleviate symptoms (Bondarev et al., 2018), and as such, highly active post-menopausal females present an excellent research model in this field. The purpose of this study was to explore motor unit discharge rates (MUDR) in three groups of post-menopausal females; those with and without exogenous hormone intake, and competitive masters athletes.

Fifteen post-menopausal females aged 61.2 yrs (±8.91) were recruited according to NICE guidelines. Post-menopausal females (PO; n=5) were not using any form of hormone therapy, and HRT users (HRT; n=5) were taking combined hormone therapy for a minimum of 6 months. Post-menopausal Masters athletes (MA; n=5) were competing at national level athletics competitions and were not taking any exogenous hormones. High-density surface electromyography (HD-sEMG) signals were sampled from the tibialis anterior (TA) during ramped contractions peaking at 30% of maximum dorsiflexion force. Signals were decomposed into individual MU spike trains, and the discharge rate was calculated at MU recruitment and de-recruitment. The ratio of MUDR at these phases is also reported, with values below 1 indicating higher DR at de-recruitment. Linear mixed effects regression models were used with group as a fixed factor and subject as a random intercept, and estimated marginal means from model outputs are presented. Statistical significance was accepted at p<0.05.

The mean number of TA MUs recorded from the females was 21.4±3.78 for post-menopause, 14.4±5.41 HRT users and 21±8.4 for the masters athletes. MUDR at recruitment did not differ across groups (PO = 8.09pps, HRT =8.74pps, MA = 8.82pps; p>0.4). Similarly at de-recruitment, MUDR did not differ across groups (PO = 7.15pps, HRT =6.41pps, MA = 8.82pps; p>0.1). Comparing the ratio of ratio of MUDR at recruitment to de-recruitment also showed no effect of group (PO = 1.24, HRT =1.42, MA = 1.27; p>0.1).

Data from this pilot study identify minimal effects of exercise and HRT use on tibialis anterior MUDR. However, the proposed excitatory and inhibitory effects of oestrogen and progesterone may be more evident in measures beyond DR alone. This work highlights the ability to investigate these parameters in functionally and pharmacologically distinct humans and presents an interesting model for future research in this area with clear translational benefit to health and performance.