Influence of biological sex and training history on force production and its variability in humans

Maximal and rapid force production represent the principal metrics of neuromuscular performance, yet both show substantial inter-individual and day-to-day variability. Biological sex and training history are known to considerably affect the absolute magnitude of these measures; however, their influence on variability is unclear.

To address this gap, we examined twenty-two young adults (12 males, 10 females), balanced for strength-trained (ST) and recreationally active (RA) status, who performed maximal ballistic isometric elbow-flexion trials in two sessions separated by 48-72 h. We assessed maximal voluntary force (MVF), rate of force development (RFD), and impulse during the initial 250 ms from force onset, and estimated maximal muscle-fiber conduction velocity (MFCV_MAX) from high-density EMG over the same window. Day-to-day variability was computed for all these metrics.

Both sex and training history significantly affected absolute MVF, RFD, and impulse (p < 0.05). In contrast, MFCV_MAX differed only by training history, being 0.14 m·s^-1 higher in ST (p < 0.05). ST participants also showed lower between-day variability in RFD and MFCV_MAX than RA (p<0.05). However, no differences were found for sex. Maximal RFD and impulse strongly correlated with MFCV_MAX across all groups (R²>0.6, p < 0.01).

Because MFCV_MAX is considered an indirect marker of the recruitment of larger motor units, our results suggest that ST participants may recruit relatively larger motor units during the initial contraction phase compared with RA individuals. In addition, more stable RFD across days is associated with comparable contribution from similarly sized motor units. Thus, within the present sample, day-to-day stability in rapid force appeared to be more strongly associated with training-related consistency than with biological sex.