Effect of pre-cooling, with and without thigh cooling, on strain and endurance exercise performance in the heat

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Abstract

Body cooling before exercise (i.e. pre-cooling) reduces physiological strain in humans during endurance exercise in temperate and warm environments, usually improving performance. This study examined the effectiveness of pre-cooling humans by ice-vest and cold (3°C) air, with (LC) and without (LW) leg cooling, in reducing heat strain and improving endurance performance in the heat (35°C, 60% RH). Nine habitually-active males completed three trials, involving pre-cooling (LC and LW) or no pre-cooling (CON: 34°C air) before 35-min cycle exercise: 20 min at ∼65% V̇o2peak then a 15-min work-performance trial. At exercise onset, mean core (T̄c, from oesophagus and rectum) and skin temperatures, forearm blood flow (FBF), heart rate (HR), and ratings of exertion, body temperature and thermal discomfort were lower in LW and LC than CON (P<0.05). They remained lower at 20 min [e.g. T̄c: CON 38.4±0.2 (±S.E.), LW 37.9±0.1, and LC 37.8±0.1°C; HR: 177±3, 163±3 and 167±3 b.p.m.), except that FBF was equivalent (P=0.10) between CON (15.5±1.6) and LW (13.6±1.0 ml·100 ml tissue−1·min−1). Subsequent power output was higher in LW (2.95±0.24) and LC (2.91±0.25) than in CON (2.52±0.28 W kg−1, P=0.00, N=8), yet final T̄c remained lower. Pre-cooling by ice-vest and cold air effectively reduced physiological and psychophysical strain and improved endurance performance in the heat, irrespective of whether thighs were warmed or cooled.

Introduction

Metabolic heat production during exercise causes body temperature to rise. Sustained exercise thus requires a sympathetically-mediated increase and redistribution of cardiac output to meet the increased demand for perfusion of skin and metabolically-active muscle. Moderate heat strain or hypohydration will compromise cutaneous perfusion (Gonzalez-Alonso et al., 1998, Gonzalez-Alonso et al., 1999a), whereas severe heat strain or hypohydration will also compromise cardiac output and muscle perfusion (Gonzalez-Alonso et al., 1998), facilitating even higher Tc, muscle temperature (Tm) and rate of muscle glycogen depletion (Gonzalez-Alonso et al., 1998). The elevation in Tc and/or Tm per se, appears to limit exercising-heat tolerance in rats (Fuller et al., 1998), dogs (Kruk et al., 1985) and humans (Gonzalez-Alonso et al., 1999b).

Heat-related physiological strain is reduced and endurance exercise performance is improved by lower ambient heat stress (Tatterson et al., 2000), auxiliary cooling of the skin during exercise, e.g. in dogs (Kruk et al., 1985, Kozlowski et al., 1985) and humans (MacDougall et al., 1974, Livingstone et al., 1995) or during rest periods (Constable et al., 1994, House, 1998), and by initiating exercise from a lower resting body temperature. The latter is an adaptive benefit of aerobic training and heat acclimation (Nielsen et al., 1993, Cheung and McLellan, 1998), but can also be induced artificially prior to exercise (i.e. pre-cooling). Pre-cooling reduces thermal, cardiovascular, and metabolic strain during endurance exercise in temperate and hot conditions (Kozlowski et al., 1985, Lee and Haymes, 1995, Gonzalez-Alonso et al., 1999b). Thus, whereas reduced muscle and core temperatures have been found to impair power output in short-term exercise of up to ∼8 min (Bergh and Ekblom, 1979, Crowley et al., 1991), reduced skin and core temperatures have usually been found to improve endurance exercise performance in temperate (Hessemer et al., 1984, Olschewski and Bruck, 1988, Lee and Haymes, 1995, Gonzalez-Alonso et al., 1999b) conditions. One issue to be considered is whether the application of surface cooling garments should include the limbs to be used in exercise. The additional skin coverage would augment the body's heat deficit but would also reduce Tm. Reducing Tm impairs force development (Davies and Young, 1983) and high intensity exercise performance (Bergh and Ekblom, 1979, Crowley et al., 1991), but may not adversely affect (and might even enhance) endurance exercise because of the sub-maximal power requirement and the reduced glycogenolytic rate, which is partly mediated by Tm per se (Starkie et al., 1999). Indeed, pre-cooling studies in which the limbs to be worked were exposed to the water during cooling, found intermittent and continuous endurance exercise to be unimpaired (Bolster et al., 1999, Drust et al., 2000) or improved (Lee and Haymes, 1995, Gonzalez-Alonso et al., 1999b).

Water immersion, by virtue of its surface coverage and heat transfer capacity, provides effective pre-cooling (Lee and Haymes, 1995). A practical method of pre-cooling is by cooling jacket (e.g. ice vest), potentially in combination with cold air (e.g. air-conditioned compartment). Ice vests are gaining popularity in sporting events in Australia, but there appear to be few data available on the impact of this method of pre-cooling on perceived and actual exercise-related strain and work performance effects. Therefore, since we were examining the effects of pre-cooling by ice vest and cold air — with and without thigh cooling — on high intensity power output (see companion paper, by Sleivert et al.), we further sought to determine the effectiveness of such pre-cooling in reducing exercise-related strain and improving endurance performance in the heat. It was hypothesised that pre-cooling would decrease heat-related strain and improve performance, and there would be no net adverse effect on strain or performance following surface cooling over the muscles to be worked.

Section snippets

Subjects

Nine males participated voluntarily as subjects. Their physical characteristics are shown in Table 1. They were habitually active, but were of lower average aerobic fitness than subjects used in previous studies on the effects of pre-cooling. Trials were conducted in accordance with the approval obtained from the Australian Defence Medical Ethics Committee.

Protocol

Subjects completed one to two familiarisation sessions, during which peak oxygen uptake (V̇o2peak) for cycling was determined from a maximal

Pre-cooling

All physiological and psychophysical variables were equivalent (P>0.05) between LC, LW and CON prior to the exposure onsets, and all variables were different (P<0.05) between CON and pre-cooling (LW and LC) at completion of the 2nd exposure (see left side of Fig. 1, Fig. 2, Fig. 3 below). Pre-cooling reduced mean body temperature by 1.9°C from baseline in LW and 2.8°C in LC, or body heat content by ∼273 kJ m−2 and ∼401 kJ m−2, respectively. Subjects’ average perception of body temperature at

Discussion

This study has shown that pre-cooling of male humans by ice vest and cold air was effective in reducing thermal, cardiovascular and psychophysical strain during subsequent exercise in the heat. In particular, thermal (T̄c and T̄sk), cardiovascular (HR and FBF) and psychophysical (perceived temperature and exertion) indices of strain were generally still attenuated after 20 min of moderately-stressful upright cycling, allowing subjects to perform more work throughout a subsequent 15-min period

Acknowledgements

The effort and time of the subjects is most appreciated. This study was partially supported by Sport Science New Zealand.

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