Author(s): ZAGATTO, A., MENDONÇA, P.T., MURIAS, J., DUTRA, Y.M., Institution: SAO PAULO STATE UNIVERSITY, Country: BRAZIL, Abstract-ID: 1445

INTRODUCTION:
Exercise performed in hot environments is associated with exacerbated physiological strain and reduced endurance performance1,2. However, most studies assess neuromuscular function following exercise to task failure, where longer exercise duration in the thermoneutral environment may confound the isolated effects of heat stress. This study aimed to evaluate the effects of cycling exercise performed in the heat, within the heavy-intensity domain, on endurance performance and knee-extensor neuromuscular function using both task-failure and isotime experimental designs.
METHODS:
Thirteen physically active participants (11 males, 2 females; V̇O₂max: 43 ± 5 mL·kg⁻¹·min⁻¹) completed a step–ramp–step test under thermoneutral (20 ± 2 °C; 55 ± 7% relative humidity) and hot (38 ± 2 °C; 43 ± 9%) conditions. Participants subsequently performed constant work-rate cycling at Δ80% between the gas-exchange threshold and the maximal metabolic steady state, determined separately for each condition, until task failure. An additional isotime trial was conducted under thermoneutral conditions, with exercise duration matched to the time to task failure (TTF) observed in the heat condition. Maximal voluntary contraction (MVC), voluntary activation (VA), and evoked knee-extensor forces (single twitch, Qtw; low- and high-frequency doublets, Db100 and Db10; Db10/Db100 ratio) were assessed pre-exercise and up to 3 h post-exercise to quantify performance fatigability.
RESULTS:
TTF was significantly reduced in the heat (31±14 min) compared with thermoneutral conditions (60±24 min; p<0.001). Skin temperature was higher in the heat(36.8±0.6 °C;), compared to thermoneutral (34.5±1.0 °C; p<0.001) environment, whereas post-exercise core temperature was slightly but significantly greater following thermoneutral exercise (38.1±0.5 °C) compared with heat (37.7±0.5 °C; p=0.017). Following task failure, no condition × time interactions were observed for MVC (p=0.422), VA (p=0.277), Qtw (p=0.079), Db10 (p=0.182), Db100 (p=0.021, not confirmed by Bonferroni post hoc), or Db10/Db100 ratio (p=0.082). Significant main effects of time relative to baseline were observed for MVC (up to 3 h post-exercise), Db10, and Db100 (all p<0.001), with no evidence of central fatigue. For the isotime comparisons, only main effects of time were observed for MVC, Qtw, Db10, and Db10/Db100 ratio (all p<0.001).
CONCLUSION:
Cycling exercise performed in a hot environment markedly reduces endurance performance but does not exacerbate neuromuscular impairments compared with thermoneutral conditions when exercise duration is controlled. The observed global and peripheral fatigue responses appear primarily attributable to exercise time rather than heat stress per se.

1. HARGREAVES (2008); 2. TATTERSON et al. (2003)