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) Ͻ maximal evaporative cooling (E max)] conditions of 10°C [7°C wet bulb globe temperature (WBGT)], 20°C (16°C WBGT), 30°C (22°C WBGT), and 40°C (27°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 Ϯ 4 yr old, 45 Ϯ 8 ml·kg Ϫ1 ·min Ϫ1 peak O2 uptake (V O2peak)] were divided into four matched cohorts (n ϭ 8) and tested at one of four T a in euhydrated (EU) and hypohydrated (HYPO, Ϫ4% body mass) conditions. Subjects completed 30 min of preload exercise (cycle ergometer, 50% V O2peak) followed by a 15 min self-paced time trial. Timetrial performance (total work, change from EU) was Ϫ3% (P ϭ 0.1), Ϫ5% (P ϭ 0.06), Ϫ12% (P Ͻ 0.05), and Ϫ23% (P Ͻ 0.05) in 10°C, 20°C, 30°C, and 40°C T a, respectively. During preload exercise, skin temperature (T sk) increased by ϳ4°C per 10°C Ta, while core (rectal) temperature (T re) values were similar within EU and HYPO conditions across all T a. A significant relationship (P Ͻ 0.05, r ϭ 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°C and 40°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 Ͼ29°C, 4% hypohydration degrades aerobic performance by ϳ1.6% for each additional 1°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. dehydration; total work; graded ambient temperature; cutaneous blood flow; mean arterial pressure HYPOHYDRATION (Ͼ2% body mass) degrades aerobic performance in temperate and warm-hot conditions (34, 37); however, there is little knowledge of the relative impact of different environmental conditions on aerobic performance at a given level of hypohydration. Cheuvront and colleagues (3) demonstrated that 3% hypohydration did not alter aerobic performance (time trial) in cold (2°C) conditions but reduced aerobic performance in temperate (20°C) conditions. Several reviews speculate (6, 35) that hypohydration might degrade aerobic performance more in warm-hot than temperate conditions. If this is true, then it is also possible that hypohydration might degrade aerobic performance more in hot than warm conditions. While this is plausible, the impact of hypohydration on aerobic exercise performance along a continuum of air temperatures has not been experimentally evalua...

Low back pain has a 70% higher prevalence in members of the armed forces than in the general population, possibly due to the loads and positions soldiers experience during training and combat. Although the influence of heavy load carriage on standing lumbar spine posture in this population is known, postures in other operationally relevant positions are unknown. Therefore, the purpose of this study was to characterize the effect of simulated military operational positions under relevant loading conditions on global and local lumbar spine postures in active duty male US Marines.Secondary objectives were to evaluate if intervertebral disc degeneration and low back pain affect lumbar spine postures. Magnetic resonance images were acquired on an upright scanner in the following operational positions: natural standing with no external load, standing with body armor (11.3 kg), sitting with body armor, and prone on elbows with body armor. Custom software was used to measure global lumbar spine posture: lumbosacral flexion, sacral slope, lordosis, local measures of intervertebral angles, and intervertebral distances. Sitting resulted in decreased lumbar lordosis at all levels of the spine except L1−L2. When subjects were prone on elbows, a significant increase in local lordosis was observed only at L5−S1 compared with all other positions. Marines with disc degeneration (77%) or history of low back pain (72%) had decreased lumbar range of motion and less lumbar extension than healthy Marines. These results indicate that a male Marine's pathology undergoes a stereotypic set of postural changes during functional tasks, which may impair performance. This article is protected by copyright. All rights reserved

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