Caffeine enhances soleus motoneuron output and preserves torque during repetitive wide-pulse high-frequency stimulation

The influence of caffeine on human motor output is debated. We tested whether acute caffeine ingestion (6 mg·kg-1) modulates motor unit behavior and preserves evoked torque during repeated tetanic contractions elicited by wide-pulse high-frequency neuromuscular electrical stimulation (WPHF). In a randomized, double-blind, crossover study, 24 adults completed caffeine and placebo sessions. High-density surface electromyography from soleus (SOL) and gastrocnemius medialis (GM) was decomposed to track motor units across time. We quantified indices linked to persistent inward currents (delta frequency, normalized delta frequency, brace height, and self-sustained firing duration), H reflex parameters, and responses to WPHF (torque-time integral, extra torque-time integral, sustained electromyography) and used linear mixed models (LMMs) to assess the effect of time, condition, and their interaction. Caffeine selectively increased delta frequency and self-sustained firing duration in SOL, but not in GM. Normalized delta frequency and brace height, as well as H-reflex parameters, were unchanged. During repeated WPHF trains, caffeine reduced the decrease of evoked torque; mean torque-time integral and extra torque-time integral were higher than placebo across trains, and sustained electromyography was also higher in SOL and GM. These findings indicate that caffeine enhances motoneuron output in SOL but not in GM, while it tempered fatigue development in response to repeated WPHF trains. The pattern is consistent with spinal adjustments in response to caffeine, without excluding other mechanisms and highlights a simple, low-cost strategy to support neuromuscular electrical stimulation in practice.NEW & NOTEWORTHY Acute caffeine ingestion selectively enhances motor unit excitability in the soleus (SOL) but not in gastrocnemius medialis (GM) and preserves the torque evoked via repeated wide-pulse high-frequency stimulation. These effects point to a preferential facilitation of spinal mechanisms sustaining motoneuron output during repetitive activation. The specific effect observed on SOL suggests a spinal contribution to support caffeine as a practical ergogenic substance to enhance the response to repeated contractions evoked via neuromuscular electrical stimulation.