Mechanoreflex Activation via Passive Muscle Stretching Evokes Pressor Response without Altering Cardiac Baroreflex Sensitivity in Humans

Autonomic cardiovascular responses to mechanoreflex activation have been extensively documented, demonstrating significant adjustments such as cardiac vagal inhibition, sympathetic activation, pressor response, and baroreflex resetting. Notably, decreased cardiac baroreflex sensitivity has been observed during passive leg cycling, a well-established method of activating muscle mechanoreflexes in humans. However, the large blood volume mobilization associated with cycling confounds the understanding of muscle mechanoreflex's role in modulating cardiac baroreflex sensitivity. An alternative approach is passive stretching of small muscle groups, such as the forearm flexors, which remains unknown. Given that, the present study tested the hypothesis that muscle mechanoreflex activation via sustained passive muscle stretching attenuates cardiac baroreflex sensitivity. Nine healthy volunteers (8 men; age: 24±3 yr) participated in the study. After a 10-minute rest in the supine position with a neutral wrist joint angle, each volunteer underwent one protocol: a single set of 1-minute unilateral passive stretching of the forearm flexors. Stretch intensity was standardized to a Borg Rating of Perceived Exertion (RPE) of 7, and the force generated between the passively stretched hand and the experimenter’s hands was recorded using a force transducer. Cardiovascular measures included beat-to-beat heart rate (ECG) and blood pressure (BP, finger photoplethysmography), together with brachial arterial BP using an automated digital sphygmomanometer (Dixtal, DX2022). Results showed consistent increases in systolic (Δ12±3 mmHg, p<0.05), diastolic (Δ5±2 mmHg, p<0.05), and mean BP (Δ7±2 mmHg, p<0.05) during passive muscle stretching. Cardiac vagal modulation, indicated by RMSSD, significantly decreased (Δ24±8%, p<0.05), while heart rate increased (Δ5±3%, p<0.05) during stretching. Despite these pronounced adjustments, no significant changes were observed in cardiac baroreflex sensitivity in response to muscle mechanoreflex activation (rest: 23.7±11.14 ms.mmHg - 1 vs. stretching: 24.9±7.42 ms.mmHg - 1 , P=0.399). In conclusion, contrary to our hypothesis and existing literature, passive muscle stretching induces notable autonomic and cardiovascular responses without altering cardiac baroreflex sensitivity in healthy young subjects. CNPq 307764/2022-2; CAPES (001); FAPDF QUT South America Scholarship This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.