The effect of 10 days of heat acclimation on submaximal exercise economy and efficiency at 1600 m and 4350 m

Heat acclimation is known to increase exercise economy. Previous examinations suggest heat acclimation may preserve performance at altitude. This study examined the effect of using heat acclimation as a cross environmental stressor to improve exercise economy and efficiency during acute exercise at altitude. Eight trained males (VO2peak: 53.3 ± 6.7 ml/kg/min) performed maximal exercise tests, submaximal exercise bouts, and heat tolerance testing in a temperate environment (21°C) at 1600 m and 4350 m before and after a 10-day heat acclimation (40C and 20% RH) on a cycle ergometer (~43% peak power). To investigate heat stress mechanisms, C2C12 myocytes were heat stressed for 24 hours (40°C, 5% CO2). Heat acclimation did not alter VO2peak at 1600 m (53.3 ± 6.7 vs. 53.7 ± 3.7 ml/kg/min, p > 0.05) or 4350 m (45.3 ± 4.1 versus 45.9 ± 3.4 ml/kg/min, p > 0.05). Heat acclimation increased exercise economy by 1.6% and 2% in the low intensity and high intensity exercise, respectively at 1600 m with only a 0.48% increase at 4350 m. In the cell study, heat stress significantly reduced UCP3 expression, reduced mitochondrial uncoupling (71.1% ±1.2%) and suppressed basal and peak oxidative metabolism (75.5% ± 4.9% and 64.4% ± 5.9%, respectively) compared to control. Heat stress also significantly increased PGC-1α, NRF1 and TFAM leading to increased mitochondrial content. These data demonstrate that while heat stress reduces UCP3 expression, thereby reducing uncoupling and leading to enhanced mitochondrial efficiency, these adaptations are not observed in the whole body. At this time, I am unable definitively promote the use of heat acclimation as a cross environmental stressor for acute exercise at altitude.Exercise ScienceDoctoralUniversity of New Mexico. Division of Physical Performance and DevelopmentMermier, ChristineParker, DarylSuzanne, SchneiderKravitz, Le