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 (V̇O₂) and heart rate (HR) response to arm cranking. Seven men performed a 6-min arm-cranking exercise at an intensity requiring a V̇O₂ 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 V̇O₂ (phase 2) and HR responses to exercise were obtained from a mono-exponential fit [f(t) = baseline + gain·(1 - e(-(t-TD)/Υ))]. The response was evaluated by the mean response time [MRT; sum of time constant (Υ) and time delay (TD)]. No significant difference in V̇O₂ and HR gains between the C and WI trials was observed [V̇O₂ 0.78 (SD 0.1) vs 0.80 (SD 0.2) 1·min^-1, HR 36 (SD 7) vs 37 (SD 8) beats·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), V̇O₂ 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 V̇O₂ 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 V̇O₂ response to exercise.