Heat Acclimation with Controlled Heart Rate: Influence of Hydration Status

This study aimed 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 maximal oxygen uptake (V˙O2max) in cool conditions and self-paced exercise in the heat was also determined.

Eight men (V˙O2max, 55 ± 7 mL·kg·min) completed two 10-d interventions in a counterbalanced crossover 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% relative humidity. Initial workload (172 ± 22 W) was adjusted over the last 75 min to maintain exercising HR equivalent to 65% V˙O2max. A V˙O2max test in cool conditions and 30-min time trial in hot-humid conditions were completed before and after HA.

HR at the end of the initial 15 min workload was 10 ± 5 bpm lower on day 10 in both interventions (P < 0.001). The workload necessary to maintain exercising HR (145 ± 7 bpm) 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, P = 0.06), but not HA-DEH (P = 0.12). Skin temperature decreased during HA-EUH (0.6°C ± 0.5°C, P = 0.03), but not HA-DEH (P = 0.30). There was a main effect of HA on V˙O2max (~3 mL·kg·min, P = 0.02); however, neither intervention independently increased V˙O2max (both, P = 0.08). Time-trial performance increased after HA-EUH (19 ± 16 W, P = 0.02), but not HA-DEH (P = 0.21).

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 seem to significantly increase V˙O2max in cool conditions.