Passive forearm stretching evokes robust and load-dependent muscle sympathetic nerve activity responses in humans

Muscle mechanoreflex has long been considered to make, at most, a minimal contribution to human sympathetic activation, as previous studies of passive exercise have typically reported absent, small, or transient responses. Whether passive stretching alone can elicit a sustained increase in muscle sympathetic nerve activity (MSNA), and whether such responses are dependent on mechanical load, remains unknown. We tested the hypothesis that passive forearm stretching is sufficient to activate the muscle mechanoreflex and provoke a load-dependent sympathetic response in healthy young adults. Eleven participants (9 men; 23±4 years) completed two sessions on separate days: an experimental session with five sets of 1-minute passive forearm stretching, each separated by 15-s of rest, and an optional time-matched control session. MSNA was recorded via peroneal microneurography, while cardiovascular variables were continuously monitored (ECG, finger arterial pressure, brachial sphygmomanometry). Passive stretching evoked robust increases in burst frequency (Δ10±2.2 bursts·min⁻¹, PPPP=0.029). Mechanoreflex-driven sympathoexcitation was accompanied by a rightward resetting of the spontaneous sympathetic baroreflex operating point, without alteration in reflex gain. All variables returned to baseline within 15-30 minutes of recovery. These findings provide the first direct evidence that passive forearm stretching elicits robust and sustained increases in MSNA in healthy humans, with responses exhibiting a clear load-dependent profile. Passive stretching thus emerges as a practical and physiologically robust model for probing mechanoreflex function and sympathetic regulation in humans.