Modulation of spontaneous baroreflex control of heart rate and indexes of vagal tone by passive calf muscle stretch during graded metaboreflex activation in humans.

We examined whether spontaneous baroreflex modulation of heart rate and other indexes of cardiac vagal tone could be altered by passive stretch of the human calf muscle during graded concurrent activation of the muscle metaboreflex. Ten healthy subjects performed four trials: a control trial, resting for 1.5 min (0% trial); or 1.5 min of one-legged isometric plantar flexor exercise at 30, 50, and 70% maximal voluntary contraction. The incremental increases in blood pressure (BP) caused were then partially sustained by subsequent local circulatory occlusion (CO). After 3.5 min of CO alone, sustained calf stretch and CO were applied for 3 min. Spontaneous baroreflex sensitivity (SBRS) was progressively decreased with increasing exercise intensity (P <0.05). During CO, stretch decreased SBRS and increased BP similarly in all trials (P <0.05). Within 15 s of stretch onset, heart rate (HR) increased by 6 +/- 1, 6 +/- 1, 8 +/- 1, and 6 +/- 2 beats/min in the 0, 30, 50, and 70% trials, respectively (P <0.05), and root mean square of successive differences was decreased from CO-alone levels (P <0.05). During the second and third minutes of stretch, HR fell back but remained significantly above CO levels, and common coefficient of variance of R-R interval decreased progressively with increasing prior exercise intensity (P <0.05; 70% trial). This suggests that passive stretch of the human calf muscles decreases cardiac vagal outflow irrespective of the levels of BP increase caused by muscle metaboreflex activation and implies that central modulation of baroreceptor input, mediated by the actions of stretch-activated mechanoreceptive muscle afferent fibers, continues.