Isothermic heat acclimation requires lower exercise durations to elicit superior adaptation compared to a fixed intensity protocol: Training and Competing in the Heat Conference, Aspetar, Doha, Qatar

Aim We investigated whether equal cardiovascular (Heart rate; HR), thermoregulatory (Rectal temperature; Trec), sudomotor (Sweat-rate; SR), and cellular (Leukocyte Heat shock protein-72 mRNA; Hsp72) adaptation followed fixed-intensity (FIXED; workload=50%V.O2peak) or isothermic (ISO; Trec=38.5°C) heat acclimation (HA). Method Following heat stress tests (HST; 30 min, 9 km.h-1/2%, 40°C/25%RH), twenty-seven males performed five 90 min sessions (STHA;40°C/25%) utilising FIXED (age 19.8 ± 0.9 years, mass 77.7 ± 16.5 kg, V.O2peak 3.61 ± 0.80 L.min-1), or ISO (24.4 ± 5.4 years, 74.6 ± 7.7 kg, 3.71 ± 0.61 L.min-1) followed by repeat HST. LTHA comprised five additional sessions, ISO dividing into continuous (ISOCONT; Trec=38.5°C) and progressive (ISOPROG; Trec=39.0°C) groups, and a final HST. Hsp72 was measured via quantitative PCR (RT-QPCR). Results STHA improved HRrest (-6 ± 1 b.min-1), Trecrest (-0.22 ± 0.01 °C), HRpeak (-8 ± 1 b.min-1), Trecpeak (-0.31 ± 0.02 °C) and SR (+0.38 ± 0.20 L.hr-1) overall (p<0.05). Post-hoc analysis observed adaption for HRpeak, Trecrest Trecpeak, and SR in ISO, and HRrest in FIXED. LTHA improved HRrest (-7 ± 0 b.min-1), Trecrest (-0.21 ± 0.05 °C), HRpeak (-8 ± 1 b.min-1), Trecpeak (-0.33 ± 0.05 °C), and SR (+0.57 ± 0.16 L.hr-1) overall (p<0.05). Post-hoc analysis observed adaption in HRrest in ISOCONT, HRpeak, in ISOCONT and ISOPROG, Trecrest in FIXED and Trecpeak in FIXED and ISOCONT. ISO achieved STHA for less (p<0.05) exercising duration (ISO 303.5 ± 59.8 min; FIXED = 446.5 ± 7.5 min), and work (ISO 2,730 ± 481 kJ; FIXED = 3,353 ± 749 kJ). ISOCONT (706.9 ± 102.5 min) and ISOPROG (597.5 ± 87.2 min) achieved LTHA for less exercising duration (p<0.05) than FIXED (891.9 ± 17.7 min). Hsp72 increased in ISO and FIXED (p<0.05) pre-post session one and five, FIXED Hsp72 reduced (p<0.05) between post one and five. ISOCONT, ISOPROG and FIXED elicited increases (p<0.05) pre to post session one and ten, FIXED Hsp72 reduced (p<0.05) between post one and ten. Conclusion Short and long term Isothermic HA elicits superior adaptation more efficiently than fixed-intensity. Hsp72 increased post-exercise following HA, however increases were attenuated post-exercise following sessions five and ten, compared to one in FIXED.