The effect of climatic heat stress on intermittent high intensity running

Little is known about the effects of hot and humid conditions on supramaximal performance (Sargeant, 1987). This study investigates the physiological responses of performance involving high intensity running in a hot, humid environmental compared to that in a cool environment. The study was approved by the University of Strathclyde Ethics Committee. The performance model used was the maximal anaerobic running test (MART) which has been shown to estimate anaerobic capacity (Maxwell and Nimmo, 1994). and consists of repeated 20 s sprints with 100 s passive recovery. On each occasion the MRT protocol was preceded by a standard 20 min "warm-up" in the testing environment and performed either in cool (C)[21.2±0.35°C, 43.6±1.74% rh (mean ± s.e.m.)], or a hot, humid (H) [32.9 ± 0.12°C, 80.5 ±0.84%rh] environment. Twelve recreationally active males performed each test on a motorised treadmill separated by approximately ten days. Fingertip capillary blood samples were taken pre and post-exercise for the determination of peak lactate, ammonia, glucose and the changes in plasma volume. Core and mean skin temperature and heart rate were measured continuously throughout each test. A paired t test was used to examine differences between conditions (Table 1). A significant difference (P<0.05) was found in anaerobic capacity, expressed as oxygen equivalents, in the MART protocol (C vs H; 110.8 ± 1.18 vs 107.7 ± 1.31 ml.kg-1.min-1, respectively). No differences were found in plasma volume or heart rate at exhaustion between conditions. Table 1. Mean plasma ammonia, lactate and glucose concentrations, rectal and mean skin temperatures and weight loss at exhaustion during the cool (C) and hot (H) trial, (n=12). Ammonia (µmol.l-1) C - 358.4 ± 25.43 H - 380.0 ± 43.94 Lactate (mmol.l-1) C - 20.4 ± 1.03 H - 19.3 ± 1.32 Glucose (mmol.l-1) C - 6.4 ± 0.26* H - 7.4 ± 0.32 Rectal (°C) C - 37.6 ± 0.12** H - 38.2 ± 0.12 Mean Skin Temp (°C) C - 32.6 ± 0.29** H - 36.5 ± 0.15 Weight Loss (kg) C - 0.52 ± 0.052** H - 0.98 ± 0.103 Values are means ± s.e.m.; *P<0.01; **P<0.001 significantly different from hot trial. These results would suggest that anaerobic capacity is impaired in hot and humid conditions and when using this performance model there is a decrement to intermittent high intensity running. References Maxwell, N.S. and Nimmo, M.A. (1994). Clin. Sci. 87, suppl. 15-17. Sargeant, A.J. (1987). Eur J.Appl.Physiol.56, 693-698