Heat Acclimation with Controlled Heart Rate: Influence of Hydration Status

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 mlkgmin) 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 beatsmin lower on day 10 in both interventions (P<0.001). The workload necessary to maintain exercising HR (145±7 beatsmin) 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 Lh, 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 mlkgmin, 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.