Thermoregulatory and metabolic responses to a half-marathon run in hot, humid conditions

Highlights

• A half-marathon performed outdoors in hot, humid conditions induced core temperatures ranging from 38.49 to 41.05 °C.

• Runners regulated their pace with a core temperature of 39 °C which occurred between 6 and 9 km of the run.

• Runners demonstrated an “end spurt” as they were able to increase their pace toward the end of the run.

Abstract

This study describes the thermoregulatory and metabolic responses during a simulated half-marathon (21 km) run performed outdoors in a hot, humid environment. Ten male runners were recruited for the study, The run was carried out individually under solar radiation on a predetermined path in the following environmental conditions (ambient temperature: 27.96 ± 1.70 °C, globe temperature: 28.52 ± 2.51 °C, relative humidity: 76.88 ± 7.49%, wet bulb globe temperature: 25.80 ± 1.18 °C). Core temperature, skin temperature, head temperature, heat storage, heart rate, expired gases, rating of perceived exertion, and speed were measured or calculated before the start, every 3 km, and immediately following the run. Comparisons were made for each dependent variable using one-way repeated measures analysis of variance tests, and a Bonferroni test. Average run time and pace were 101:00 ± 9:52 min and 4:48 ± 00:16 min km^-1, respectively. Participants significantly reduced their running speed, oxygen consumption, and heat storage at 9 km (p < 0.05). While core temperature was significantly increased at 6 km (p < 0.05) before plateauing for the remainder of the run. The key finding was that most of the runners reduced their pace when a T_core of 39 °C was reached which occurred between 6 and 9 km of the run, yet runners were able to increase their speed demonstrating an “end-spurt” near the end of the run.

Introduction

During exercise, core temperature (T_core) is regulated through a number of thermoregulatory responses to the altered heat load. At lower exercise intensities heat production is matched primarily by evaporative and convective cooling, while at higher intensities heat production exceeds heat loss leading to heat storage and ultimately exertional hyperthermia (i.e. T_core > 39.5 °C). In runners, metabolic heat production indicated by run pace is considered the main factor for rising in T_core (Noakes et al., 1991). Indeed, T_core above 41 °C has been reported following marathon (42 km) (Maron et al., 1977) and half-marathon (21 km) (Byrne et al., 2006; Dion et al., 2013) races. In addition, environmental factors such a heat or humidity can impair heat loss through sweat evaporation and increase the risk for heat related illness. Among these heat related illnesses is exertional heat stroke (EHS), an acute and potentially fatal condition that is typically characterized by a T_core greater than 40.5 °C and central nervous system dysfunction (Leon and Bouchama, 2015). In the United States EHS is the fifth leading cause of non-cardiovascular sudden death amongst young competitive athletes (Maron et al., 2016). The incidence of EHS in long races (≥21 km) is approximately 1 in 10,000 runners in cool weather increasing to 2 in 1000 runners in hot weather (Roberts, 2007). Since the risk of EHS increases during runs in hot or humid environmental conditions, understanding the thermoregulatory responses in these types of environments is crucial.

Previous research has established the thermoregulatory and metabolic responses to exercise in hot and humid environments (Ely et al., 2010; Lee et al., 2010; Noakes et al., 1991; Tatterson et al., 2000; Tucker et al., 2006). However, much of the research has been done indoors which is problematic because it excludes the effect of solar radiation, a known risk factor for heat related illness, as well as air movement around the athlete (Casa et al., 2015). Thus, field-based research is needed to better understand the impact that exercise in these outdoor environments has on thermoregulation. Recreational and competitive runners, such as marathoners and half-marathoners, are among those at increased risk for heat illness because of their high metabolic rate and exposure to hot environments (Backer et al., 1999; Krueger-Kalinski et al., 2001; Yanturali et al., 2015). The wet bulb globe temperature (WBGT) index has been used to mitigate the risk of exertional heat illnesses during running races (Armstrong et al., 1996). The most current American College of Sports Medicine guidelines recommends that if the WBGT index is above 28 consideration should be given to canceling the race events (American College of Sports Medicine (2007)). However, race events (5K, 10K, half and full marathon) in tropical regions are growing in popularity and the physiological strain of these participants is not well understood.

In a previous study, Lee et al. (2010) investigated T_core, pacing strategy, and fluid balance responses of 31 heat-acclimatized males (professional soldiers) during a half-marathon race conducted in a tropical environment (WBGT = 25). The authors reported that 68% of the finishers completed the race with a T_core greater than 40 °C and concluded that hyperthermia is common in trained individuals running in the heat, without evidence of heat illness. However, the race started at 05:45 a.m. and might have resulted in low solar radiation (globe temperature not presented – sunrise at 07:03 a.m.), thus the impact of solar radiation on thermoregulatory and metabolic responses remains unclear. Further, Lee et al. (2010) used an equation to predict oxygen consumption (VO₂), rather than measurement via indirect calorimetry, which may limit the interpretations of those results. The measurement of metabolic responses with simultaneous measurement of T_core during outdoor running in hot and humid environments is scarce. These data are important because they can help coaches and athletes adjust training programs and develop pacing strategies. Therefore, the purpose of this study was to describe the thermoregulatory and metabolic responses during a simulated half-marathon (21 km) run performed outdoors in a hot, humid environment.