Heat Acclimation with Controlled Heart Rate

Abstract

Purpose To characterize the adaptive responses to heat acclimation (HA) with controlled heart rate (HR) and determine whether hydration strategy alters adaptations. The influence of HA on V[Combining Dot Above]O2max in cool conditions and self-paced exercise in the heat was also determined. Methods Eight males (V[Combining Dot Above]O2max: 55±7 ml.kg-1.min-1) completed two 10-day interventions in a counterbalanced cross-over design. Fluid intakes differed between interventions to either maintain euhydration (HA-EUH) or elicit similar daily body mass deficits (2.85±0.26%; HA-DEH). HA consisted of 90 min of cycling in 40°C and 40% RH. Initial workload (172±22 W) was adjusted over the last 75 min to maintain exercising HR equivalent to 65% V[Combining Dot Above]O2max. A V[Combining Dot Above]O2max test in cool conditions and 30 min time-trial in hot-humid conditions were completed before and after HA. Results HR at the end of the initial 15 min workload was 10±5 beats.min-1 lower on day 10 in both interventions (P<0.001). The workload necessary to maintain exercising HR (145±7 beats.min-1) increased throughout HA-EUH (25±10 W, P=0.001) and HA-DEH (16±18 W, P=0.02). There was a main effect of HA on sweat rate (P=0.014), which tended to increase with HA-EUH (0.19±0.18 L.h-1, P=0.06) but not HA-DEH (P=0.12). Skin temperature decreased during HA-EUH (0.6 ± 0.5°C, P=0.03), but not HA-DEH (P=0.30). There was a main effect of HA on V[Combining Dot Above]O2max (~3 ml.kg-1.min-1, P=0.02); however, neither intervention independently increased V[Combining Dot Above]O2max (both P=0.08). Time-trial performance increased following HA-EUH (19±16 W, P=0.02), but not HA-DEH (P=0.21). Conclusions Controlled HR exercise in the heat induces several HA adaptations, which may be optimized by maintaining euhydration. HA-EUH also improves self-paced exercise performance in the heat. However, HA does not appear to significantly increase V[Combining Dot Above]O2max in cool conditions.