The effect of progressive heat acclimation on fatigue following intermittent-sprint exercise in the heat: Training and Competing in the Heat Conference, Aspetar, Doha, Qatar

Aim: The aims of this study were to examine central and peripheral contributions to fatigue during intermittent-sprint exercise in 33°C, 50% RH using transcranial magnetic stimulation (TMS) and electrical femoral nerve stimulation (FNS) and to investigate whether progressive heat acclimation could ameliorate fatigue following exercise of this type. Method: Seventeen male games players matched for peak oxygen uptake (V̇O2peak), peak power and body surface area were divided into two groups; progressive heat acclimation (PA, n = 9; 4 d, 50% V̇O2peak, 30.8 ± 0.7°C, 49 ± 5% RH, 4 d 33.2 ± 0.6°C, 50 ± 6% RH and 4 d 35.4 ± 0.6°C, 62 ± 6% RH) and training (TG, n = 8; 12 d, 50% V̇O2peak, 21.2 ± 0.9°C, 31 ± 6% RH). Pre and post acclimation or training, participants completed a 40 min cycling intermittent-sprint protocol (CISP) in 33.7 ± 0.6°C, 50 ± 3% RH, with neuromuscular fatigue assessment immediately before and after exercise. Results: Maximal voluntary contraction (MVC) and potentiated twitch force in both PA and TG were reduced after CISP 1 (Table 1; all P values < 0.05), whereas cortical voluntary activation was not significantly different but tended to decline. PA reduced resting Tre and heart rate prior to CISP 2 (37.26 ± 0.21 vs. 37.02 ± 0.24°C; 65 ± 9 vs. 55 ± 4 b.min-1 respectively, P < 0.05). Further, exercise heart rate was reduced by PA (168 ± 19 vs. 155 ± 19 b.min-1, P < 0.05) and Tre tended to decline (37.9 ± 0.4 vs. 37.6 ± 0.4, P = 0.10). Despite the improved overall physiological strain, peak power output and work done were not different between CISP 1 and 2. Further, the reduction in MVC and potentiated twitch force during intermittent sprinting remained after twelve days of progressive heat acclimation (Table 1). Conclusions: These data indicate that during forty minutes of intermittent-sprint exercise in the heat, neuromuscular fatigue may be primarily peripheral in origin and PA or TG does not reduce the extent of fatigue despite a reduced physiological strain. Table 1. Neuromuscular function pre-post CISP 1 and 2 in Progressive Acclimation (PA) and Training Group (TG). PA TG CISP 1 CISP 2 CISP 1 CISP 2 Pre-Post Pre-Post Pre-Post Pre-Post MVC (N) - 101* - 84* - 99* - 121* Potentiated twitch (N) -45* - 31* - 36* -33* Cortical voluntary activation (%) - 3 - 5 - 9 -7 Data presented are change (Δ) from pre to post CISP 1 and pre to post CISP 2 in both PA and TG. * Indicates significant difference pre to post CISP 1 or CISP 2 (within groups).