Human cardiovascular and humoral responses to moderate muscle activation during dynamic exercise

We examined the hypothesis that activation of the muscle metaboreflex durin g dynamic exercise would augment influences tending to cause a rise in argi nine vasopressin, plasma renin activity, and catecholamines during dynamic exercise in humans. Ten healthy adults performed 30 min of supine cycle erg ometer exercise at similar to 50% of peak oxygen consumption with or withou t moderate muscle metaboreflex activation by application of 35 mmHg lower b ody positive pressure (LBPP). Application of LBPP during the first 15 or la st 15 min of exercise increased mean arterial blood pressure, plasma lactat e concentration, and minute ventilation, indicating an activation of the mu scle metaboreflex. These changes were rapidly reversed when LBPP was remove d. During exercise at this intensity, LBPP augmented the release of arginin e vasopressin and catecholamines but not of plasma renin activity. These re sults suggest that, although in humans hormonal. responses are induced by m oderate activation of the muscle metaboreflex during dynamic exercise, the thresholds for these responses may not be uniform among the various glands and hormones.