Acute caffeine ingestion differentially alters motor unit behaviour during submaximal and fatiguing contractions

Background: Caffeine is widely used as an ergogenic aid, yet its effect on neuromuscular function remains heterogeneous across exercise conditions. Although caffeine is known to enhance central nervous system excitability through adenosine receptor antagonism, the extent to which these effects modify spinal motor output remains unclear. Therefore, the present study aimed to characterise acute caffeine-induced alterations in neuromuscular properties during submaximal and sustained fatiguing contractions of the knee extensors. Methods: Seventeen healthy adults completed a randomised, placebo-controlled crossover study (3 mg/kg caffeine). Circulating serum caffeine concentrations were periodically quantified, while motor unit (MU) discharge behaviour, and delta F were derived from high-density surface electromyography. Statistical significance was set at p = 0.05. Results: No intervention effects were observed for MVC, force steadiness, or time to task failure (all p > 0.05). During graded contractions, vastus lateralis MU discharge rate was higher under caffeine (p < 0.001), and discharge variability was lower (p = 0.016). Recruitment and derecruitment thresholds were significantly greater under caffeine at 50% MVC (both p < 0.012), whereas paired MU analyses of delta F was unchanged (p = 0.083). During sustained contractions, caffeine altered fatigue-related trajectories of both MU discharge and discharge variability (intervention * phase interaction, p < 0.011). Conclusions: Acute caffeine ingestion alters motor unit behaviour without enhancing maximal force or fatigue resistance. These effects vary with contraction intensity and fatigue state but are not accompanied by a detectable change in discharge rate hysteresis in the vastus lateralis.