Short-term heat acclimation training improves cycling performance in the heat and enhances knee extensor strength
J.Osborne, I.Stewart, D.Borg, K.Beagley & G.Minett
Introduction: The redistribution of blood flow from splanchnic regions during exercise in the heat may compromise gastrointestinal permeability and facilitate endotoxin leakage. Subsequent inflammatory responses are suggested to cause neuromuscular fatigue, resulting in reduced exercise performance This study examined the protective neuromuscular and inflammatory effects of short-term heat acclimation (HA) on cycling performance in the heat.
Methods: Eight recreationally-trained male athletes (VO2peak: 49.3 ± 4.9 mLkg-1min-1; Pmax: 347 ± 56 W) undertook a 5-day cycling block (60 minday-1 at 50% Pmax) in hot (HA: 35 ± 1 °C, 53 ± 4% relative humidity (RH)) and thermoneutral (CON: 22.2 ± 2.6 °C, 65 ± 8% RH) environmental conditions in a counterbalanced cross-over order. Performance changes were assessed via 20-km cycling time trials (TT) in the heat, completed before and after the training intervention. Neuromuscular function of the right knee extensors was assessed pre- and immediately post-exercise on the first and last day of both training blocks, as well as pre and post TT's. Blood samples were also collected at these same time points and analysed for markers of gut damage, inflammation and endotoxins. Data were analysed using Bayesian hierarchical regression, and Cohens d effect sizes were also calculated. Data are presented as the posterior mean [95% credible interval].
Results: Statistically faster TT completion times were observed after HA compared to CON (MD = 55 s [11,98], d = 2.51 [0.49, 4.46]). There was evidence post-intervention TT performance was statistically faster for HA (MD = 62 s [18,104], d = 2.86 [0.82, 4.75]), but not CON (MD = 30s [-6, 67]). Knee extensor strength increased with HA but declined in CON following 5 days of training. Despite the faster post-intervention TT performance for HA, there was little evidence of differences in measures of central fatigue, circulating endotoxin levels, inflammation, or markers of gut damage between HA and CON.
Conclusion: Short-term HA training improves subsequent 20TT cycling performance in the heat, without an associated increase in intestinal damage or inflammation. These findings may provide further support for short-term HA training being a feasible and time-efficient method for improving TT performance before athletic competition in the heat.