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. A total of 11 participants (9 men; 23 ± 4 yr) completed two sessions on separate days: an experimental session with five sets of 1-min passive forearm stretching, each separated by 15 s of rest, and an optional time-matched control session. MSNA was recorded via peroneal microneurography, whereas cardiovascular variables were continuously monitored (ECG, finger arterial pressure, and brachial sphygmomanometry). Passive stretching evoked robust increases in burst frequency (Δ10 ± 2.2 bursts·min^-1, P < 0.05), burst incidence (Δ16 ± 4.1 bursts·100 hb^-1, P < 0.05), and mean blood pressure (Δ10 ± 2.2 mmHg, P < 0.001). Torque during stretch was positively correlated with the change in burst incidence (r = 0.652, P = 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 min 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.NEW & NOTEWORTHY This study provides the first direct evidence in humans, using microneurography, that passive forearm stretching elicits robust increases in sympathetic burst frequency and incidence. The positive association between torque and the magnitude of the sympathetic response highlights the load-dependent nature of muscle mechanoreceptor activation in shaping autonomic outflow. Together, these findings establish passive forearm stretching as a practical and physiologically robust model for investigating mechanoreflex function and human sympathetic regulation.