Central command and metaboreflex cardiovascular responses to sustained handgrip during microgravity

Four subjects were studied before and during a 16-day space flight. The test included 2 min of rest, 2 min of sustained handgrip (SHG), and 2 min of post-exercise circulatory occlusion (PECO). Heart rate (HR) and mean arterial pressure (MAP) responses to central command and mechanoreceptor stimulation were determined from the difference between SHG and PECO. Responses to metaboreceptor stimulation were determined from the difference between PECO and rest. Late in-flight (days 12–14) the central command/mechanoreceptor component of the HR response was reduced by 5 bpm (P = 0.01) from its pre-flight value of 15 (±3) bpm (mean (±SEM)). At the same time the metaboreflex responses of HR and MAP were unchanged. The attenuated HR response to central command was likely of baroreflex origin. Together with a parallel study of PECO after dynamic leg exercise, our data indicate that central processing of metaboreflex inputs is unchanged in microgravity whereas metaboreflex inputs from weight-bearing muscles are enhanced.