Skin temperature modifies the impact of hypohydration on aerobic performance

Kenefick RW, Cheuvront SN, Palombo LJ, Ely BR, Sawka MN. Skin temperature modifies the impact of hypohydration on aerobic performance. J Appl Physiol 109: 79-86, 2010. First published April 8, 2010; doi:10.1152/japplphysiol.00135.2010.-This study determined the effects of hypohydration on aerobic performance in compensable [evaporative cooling requirement (E-req) < maximal evaporative cooling (E-max)] conditions of 10 degrees C [7 degrees C wet bulb globe temperature (WBGT)], 20 degrees C (16 degrees C WBGT), 30 degrees C (22 degrees C WBGT), and 40 degrees C (27 degrees C WBGT) ambient temperature (T-a). Our hypothesis was that 4% hypohydration would impair aerobic performance to a greater extent with increasing heat stress. Thirty-two men [22 +/- 4 yr old, 45 +/- 8 ml.kg(-1).min(-1) peak O-2 uptake ((V) over dotO(2peak))] were divided into four matched cohorts (n = 8) and tested at one of four T-a in euhydrated (EU) and hypohydrated (HYPO, -4% body mass) conditions. Subjects completed 30 min of preload exercise (cycle ergometer, 50% (V) over dotO(2peak)) followed by a 15 min self-paced time trial. Timetrial performance (total work, change from EU) was -3% (P = 0.1), -5% (P = 0.06), -12% (P < 0.05), and -23% (P < 0.05) in 10 degrees C, 20 degrees C, 30 degrees C, and 40 degrees C T-a, respectively. During preload exercise, skin temperature (T-sk) increased by similar to 4 degrees C per 10 degrees C T-a, while core (rectal) temperature (T-re) values were similar within EU and HYPO conditions across all T-a. A significant relationship (P < 0.05, r = 0.61) was found between T-sk and the percent decrement in time-trial performance. During preload exercise, hypohydration generally blunted the increases in cardiac output and blood pressure while reducing blood volume over time in 30 degrees C and 40 degrees C T-a. Our conclusions are as follows: 1) hypohydration degrades aerobic performance to a greater extent with increasing heat stress; 2) when T-sk is >29 degrees C, 4% hypohydration degrades aerobic performance by similar to 1.6% for each additional 1 degrees C T-sk; and 3) cardiovascular strain from high skin blood flow requirements combined with blood volume reductions induced by hypohydration is an important contributor to impaired performance.