Physiological responses to 10 days of heart rate-clamped heat acclimation in hot-dry and hot-humid environments.

To examine changes in power output, thermal, physiological, and perceptual responses during a 10-day heart‑rate-clamped training protocol in hot-dry (HD), hot-humid (HH), and thermoneutral (TN) conditions. Thirty trained participants were allocated to TN (20°C, 40% RH), HD (40°C, 35% RH), or HH (31.5°C, 95% RH) (n = 10 per group), with HD and HH matched for WBGT (~31°C). Participants completed 10 days of 60-min cycling at a heart‑rate-clamped intensity (~65% VO2max in HD and HH; ~60% VO2max in TN). Power output and physiological responses were assessed on Days 1, 5, and 10. Mean power output increased from Day 1 to Day 5 (~27%, p = 0.020), with no further change thereafter. Increases were similar in HD and HH, with minimal change in TN. Rectal temperature rose similarly across conditions (~0.7-0.8°C), despite higher absolute values in HD. Skin temperature was higher in HD, plateauing by Day 5, but increased progressively in HH to Day 10. Perceptual strain was higher in HD and HH than TN (p < 0.001), with thermal comfort improving by Day 10. Power output improved rapidly within 5 days. Despite similar rectal temperature, responses stabilized earlier in HD, whereas HH required a full 10-day period.