Short term heat acclimation improves the determinants of endurance performance and 5,000m running performance in the heat

This study investigated the effect of 5 days controlled hyperthermia heat acclimation (STHA) on the determinants of endurance performance and 5 km performance in runners, relative to the impairment afforded by moderate heat stress. A control group (CON), matched for total work and power output (2.7 W.kg -1), differentiated thermal and exercise contributions of STHA on exercise performance. Seventeen participants (10 STHA, 7 CON) completed graded exercise tests (GXT) in cool (13°C, 50% RH, pre training) and hot conditions (32°C, 60% RH, pre and post training), as well as 5 km time trials (TT) in the heat, pre and post training. STHA reduced resting (p=0.01) and exercising (p=0.04) TCORE alongside a smaller change in thermal sensation (p=0.04). Both groups improved the lactate threshold (LT, p=0.021), lactate turnpoint (LTP, p=0.005) and vV̇O2max (p=0.031) similarly. Statistical differences between training methods were observed in TT performance (STHA -6.2[5.5]%, CON; -0.6[1.7]%, p=0.029) and total running time during the GXT (STHA; +20.8[12.7]%, CON; +9.8[1.2]%, p=0.006). There were large mean differences in change in V̇O2max between STHA +4.0 (2.2) mL.kg-1.min-1 (7.3[4.0]%) and CON +1.9(3.7)mL.kg-1.min-1 (3.8[7.2]%). Running economy deteriorated following both training programmes (p=0.008). Similarly, RE was impaired in the cool GXT, relative to the hot GXT (p=0.004). STHA improved endurance running performance in comparison to work matched normothermic training, despite equality of adaptation for typical determinants of performance (LT, LTP, vV̇O2max). Accordingly, these data highlight the ergogenic effect of STHA, potentially via greater improvements in V̇O2max and specific thermoregulatory and associated thermal perception adaptations absent in normothermic training.