Increases in cardiac vagal modulation following muscle mechanoreflex activation via passive calf stretch: Impact of interindividual differences.

Muscle mechanoreflex is crucial to cardiac vagal modulation during exercise and can be activated during passive calf stretch. Herein, we aimed to determine whether cardiac vagal modulation following a single session of passive stretch is linked to interindividual cardiac vagal responses at the onset of passive calf muscle stretching in healthy young adults. Twenty-four volunteers (10 women) completed the experimental conditions in a randomised order over different days: a time-control condition and five sets of 1 min of unilateral passive stretching of the calf, with 15 s of rest between each stretching trial. Heart rate and systolic and diastolic blood pressure were continuously measured on a beat-to-beat basis before, immediately following, and at 15 and 30 min after the passive stretching intervention. Interindividual variations in cardiac vagal inhibition during the passive stretching session were identified, classifying volunteers into responder (n = 16) and non-responder (n = 8) groups. The onset of passive muscle stretching elicited an immediate reduction in cardiac vagal modulation (P = 0.026) and an increase in heart rate (P = 0.009) for the responders only. Cardiac vagal modulation significantly increased following 30 min of passive stretching (P = 0.010 vs. rest) for the responders only. During time control, all cardiac vagal variables were unchanged for both groups. In summary, our findings demonstrate that a single session of passive calf muscle stretching can enhance cardiac vagal modulation, but this effect is dependent on interindividual responses at the onset of stretching. These results highlight the role of muscle mechanoreflex activation in cardiac autonomic regulation and suggest that passive stretching may have potential cardiovascular benefits, particularly for individuals who exhibit a mechanoreflex-mediated response.