Water immersion delays the oxygen uptake response to sitting arm-cranking in humans

We investigated the effect of central hypervolaemia during water immersion up to the xiphoid process on the oxygen uptake (VO2) and heart rate (HR) response to arm cranking. Seven men performed a 6-min arm-cranking exercise at an intensity requiring a VO2 at 80% ventilatory threshold both in air [C trial, 29 (SD 9) W] and immersed in water [WI trial, 29 (SD 11) W] after 6 min of sitting. The VO2 (phase 2) and HR responses to exercise were obtained from a mono-exponential fit [f(t) = baseline + gain x (1 - e(-(t-TD)/tau))]. The response was evaluated by the mean response time [MRT; sum of time constant (tau) and time delay (TD)]. No significant difference in VO2 and HR gains between the C and WI trials was observed [VO2 0.78 (SD 0.1) vs 0.80 (SD 0.2) l x min(-1), HR 36 (SD 7) vs 37 (SD 8) beats x min(-1), respectively]. Although the HR MRT was not significantly different between the C and WI trials [17 (SD 3), 19 (SD 8) s, respectively), VO2 MRT was greater in the WI trial than in the C trial [40 (SD 6), 45 (SD 6) s, respectively; P < 0.05]. Assuming no difference in VO2 in active muscle between the two trials, these results would indicate that an increased oxygen store and/or an altered response in muscle blood distribution delayed the VO2 response to exercise.