Running in the heat reduces time to task failure, attenuates biomarkers of muscle microtrauma, and exacerbates low-frequency force depression and cardiac vagal withdrawal over 72-h post-exercise

Fig. 1. Schematic representation of the investigation design. CWR: Constant work-rate; GXT: Graded exercise test; vRCP: Velocity associated to respiratory compensation point; CK: Creatine kinase concentration; LDH: Lactate dehydrogenase concentration; V̇O2:Rate of oxygen consumption; V̇O2max: Maximal rate of oxygen consumption; V̇E: Ventilation; PetCO2: Partial expiratory pressure of carbon dioxide; HR: Heart rate; RER: Respiratory exchange ratio

Fig. 2. Time to task failure during treadmill running until task-failure at velocity associated with the respiratory compensation point. Temperate, exercise performed in a room at 22.4 °C; Heat, exercise performed in a room at 38.8 °C. *, different from temperate (paired t test); ES = effect size calculated as Cohen´s d. Dashed lines highlight females’ participants

Fig. 3. Exercise responses during running until task-failure at velocity associated with the respiratory compensation point. Temperate, exercise performed in a room at 22.4 °C; Heat, exercise performed in a room at 38.8 °C. V̇O2, rate of oxygen consumption (A); V̇E, ventilation (B); HR, heart rate (C); fB, breathing frequency (D); RER, respiratory exchange ratio (E); RPE, ratio of perceived exertion (F). Analyses were performed using the first 30 s of exercise, each 2 min until the shortest time to task failure evidenced among conditions (i.e., 12.05 min evidenced in the heat), and the 30 s at task failure. The statistical reports come from a two-way repeated measures ANOVA (condition × time). *, different from temperate in this time point (condition × interaction effect, post-hoc p < 0.05). Main effects of time and condition were not reported in the figure to improve clarity. n = 12 (crossover design)

Fig. 4. Percentage change from baseline in knee-extensors function outcomes. Temperate, exercise performed in a room at 22.4 °C; Heat, exercise performed in a room at 38.8 °C. B, baseline assessments; iMVCForce, highest voluntary force recorded during maximal voluntary isometric contractions of knee-extensors (A); VAPNS, voluntary activation measured through twitch interpolation technique (B); RMS/M-waveamp, ratio between the root mean square of electromyography signal from vastus lateralis during iMVCForce and peak-to-peak M-wave amplitude ‒ M-waveamp (C); Qtwpot, force evoked by a potentiated single stimulus at relaxed knee-extensors (D); Db10Force/Db100Force, ratio between the force evoked by a supramaximal high-frequency (100 Hz) and low-frequency (10 Hz) doublet stimulus at relaxed knee-extensors (E); M-waveamp (F). The statistical reports come from a two-way repeated measures ANOVA. #, different from baseline (main effect of time, post hoc p < 0.05); *, different from temperate (main effect of condition, post-hoc p < 0.05). n = 12 (crossover design)

Fig. 5. Percentage change from baseline in heart rate-variability outcomes. Temperate, exercise performed in a room at 22.4 °C; Heat, exercise performed in a room at 38.8 °C. B, baseline assessments; Log10RMSSD, base-ten logarithm of root-mean-square of successive differences of R-R intervals (A); Log10SDNN, base-ten logarithm of the standard deviation of N-N intervals (B); Log10SD1, base-ten of the standard deviation perpendicular to the line of identity (C); Log10SD2, base-ten of the standard deviation parallel to the line of identity (D); Log10LF, base-ten low-frequency power (E); Log10HF, base-ten high-frequency power (F). The statistical reports come from a two-way repeated measures ANOVA. #, different from baseline (main effect of time, post-hoc p < 0.05); *, different from temperate (main effect of condition, post-hoc p < 0.05). n = 12 (crossover design)