Author(s): AMORUSO, P., LECCE, E., MARTIRE, F., SCOTTO DI PALUMBO, A., SACCHETTI, M., BAZZUCCHI, I., Institution: UNIVERSITY OF ROME , Country: ITALY, Abstract-ID: 1283

INTRODUCTION:
Maximal voluntary force (MVF) and rate of force development (RFD) are commonly used to quantify force-producing capacity. Day-to-day fluctuations in these measures are well documented[1,2], with greater variability in RFD. Although biological sex and training history influence MVF and RFD[3,4], their effects on day-to-day variability remain unclear. This study evaluated the effects of biological sex and training history on force production and its day-to-day variability, while characterizing the underlying neuromuscular mechanisms.
METHODS:
Twenty-two healthy young adults (females, n=10; males, n=12) were classified as strength-trained (=3 years; ST) or recreationally active (RA). Participants performed three maximal isometric elbow-flexor contractions as hard and as fast as possible, while biceps brachii activity was recorded using high-density surface electromyography (HDsEMG). MVF and RFD were derived from filtered force signals over fixed and time-varying windows (50ms) up to 250 ms from force onset. Given the role of larger motor unit recruitment in determining MVF and RFD, maximal muscle fiber conduction velocity (MFCVMAX) was estimated from HDsEMG as an index of motor unit recruitment. Day-to-day variability was calculated as the absolute percent difference between sessions. Two-way ANOVAs tested group (sex; training history) effects; Pearson correlations assessed associations with MFCVMAX (R²).
RESULTS:
In absolute terms, males exhibited greater MVF (p<0.001), RFD0-50 (p=0.002), and RFDMAX (p=0.003) than females, but not when normalized to the available force ( %MVF; p>0.05). A similar pattern was observed for training history, with greater absolute values in ST (p<0.05) than RA, and no relative differences. MFCVMAX was not influenced by biological sex (p>0.05), but was higher in ST than RA (p=0.04). Training history, but not sex, affected day-to-day variability in RFDMAX (p=0.02) and MFCVMAX (p=0.03), both lower in ST. Early RFD was associated with MFCVMAX across sexes (M: RFD0-50, R²=0.45; RFDMAX, R²=0.68; F: RFD0-50, R²=0.26; RFDMAX, R²=0.72) and training groups (ST: RFD0-50, R²=0.49; RFDMAX, R²=0.67; RA: RFD0-50, R²=0.68; RFDMAX, R²=0.22). Day-to-day variability in RFDMAX correlated with variability in MFCVMAX across sexes (M: R²=0.60; F: R²=0.96) and training groups (ST: R²=0.59; RA: R²=0.87).
CONCLUSION:
Sex-related differences in RFD reflect baseline differences in absolute muscle force rather than motor-unit recruitment patterns, whereas training history modulates RFD via both greater force and enhanced recruitment. Despite absolute differences, relative force production is comparable across sexes and training history. Importantly, trained individuals show lower day-to-day RFD variability, consistent with more stable motor unit recruitment across days.

References:
1. https://doi.org/10.1007/s00421-016-3346-6
2. https://doi.org/10.1113/JP277396
3. https://doi.org/10.1113/JP284198
4. https://doi.org/10.7717/peerj.17156