The effects of single versus twice daily short term heat acclimation on heat strain and 3000 m running performance in hot, humid conditions

Endurance performances are impaired under conditions of elevated heat stress. Short term heat acclimation (STHA) over 4-6 days can evoke rapid adaptation, which mitigate decrements in performance and alleviate heat strain. This study investigated the efficacy of twice daily heat acclimation (TDHA) compared to single session per day heat acclimation (SDHA) and normothermic training, at inducing heat acclimation phenotype and its impact upon running performance in hot, humid conditions.Twenty one, moderately trained males were matched and assigned to three groups; SDHA (mean±SD) (peak oxygen consumption [V˙O2peak] 45.8±6.1 mL kg-1 min-1, body mass 81.3±16.0 kg, stature 182±3 cm), TDHA (46.1±7.0 mL kg-1 min-1, 80.1±11.9 kg, 178±4 cm) or control (CON) (47.1±3.5 mL kg-1 min-1, 78.6±16.7 kg, 178±4 cm). Interventions consisted of 45 min cycling at 50% V˙O2peak, once daily for 4d (SDHA) and twice daily for 2d (TDHA), in 35 °C, 60% relative humidity (RH), and once daily for 4 days (CON) in 21 °C, 40% RH. Participants completed a pre- and post-intervention 5 km treadmill run trial in 30 °C, 60% RH, where the first 2 km were fixed at 40% V˙O2peak and the final 3 km was self-paced.No statistically significant interaction effects occurred within- or between-groups over the 2-4 days intervention. While within-group differences were found in physiological and perceptual measures during the fixed intensity trial post-intervention, they did not statistically differ between-groups. Similarly, TDHA (-36±34 s [+3.5%]) and SDHA (-26±28 s [+2.8%]) groups improved 3 km performances (p=0.35), but did not differ from CON (-6±44 s [+0.6%]).This is the first study to investigate the effects of HA twice daily and compare it with traditional single session per day STHA. These STHA protocols may have the ability to induce partial adaptive responses to heat stress and possibly enhance performance in environmentally challenging conditions, however, future development is warranted to optimise the administration to provide a potent stimuli for heat adaptation in athletic and military personnel within a rapid regime

Defining the determinants of endurance running performance in the heat

In cool conditions, physiologic markers accurately predict endurance performance, but it is unclear whether thermal strain and perceived thermal strain modify the strength of these relationships. This study examined the relationships between traditional determinants of endurance performance and time to complete a 5-km time trial in the heat. Seventeen club runners completed graded exercise tests (GXT) in hot (GXTHOT; 32°C, 60% RH, 27.2°C WBGT) and cool conditions (GXTCOOL; 13°C, 50% RH, 9.3°C WBGT) to determine maximal oxygen uptake (V̇O2max), running economy (RE), velocity at V̇O2max (vV̇O2max), and running speeds corresponding to the lactate threshold (LT, 2 mmol.l¡1) and lactate turnpoint (LTP, 4 mmol.l¡1). Simultaneous multiple linear regression was used to predict 5 km time, using these determinants, indicating neither GXTHOT (R2 = 0.72) nor GXTCOOL (R2 = 0.86) predicted performance in the heat as strongly has previously been reported in cool conditions. vV̇O2max was the strongest individual predictor of performance, both when assessed in GXTHOT (r = ¡0.83) and GXTCOOL (r = ¡0.90). The GXTs revealed the following correlations for individual predictors in GXTHOT;V̇O2max r =¡0.7, RE r = 0.36, LT r =¡0.77, LTP r =¡0.78 and in GXTCOOL; V̇O2max r =¡0.67, RE r = 0.62, LT r =¡0.79, LTP r =¡0.8. These data indicate (i) GXTHOT does not predict 5 km running performance in the heat as strongly as a GXTCOOL, (ii) as in cool conditions, vV̇O2max may best predict running performance in the heat. © 2018 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group