Heat strain in children during unstructured outdoor physical activity in a continental summer climate

Abstract

The purpose of this study was to assess the heat strain experienced by children during unstructured physical activity outdoors in a temperate continental summer climate. Eighteen children (7 girls, 12.1 ± 1.7 years) performed up to 4 h of outdoor free-play (duration: 218 ± 33 min; air temperature of 24.5 ± 3.9°C and relative humidity of 66.2 ± 9.2%). Urine specific gravity (USG) was measured pre- and post-free-play, while body core temperature (T_co, ingestible pill) and heart rate (HR) were measured continuously. Physiological strain index (PSI) was calculated from T_co and HR (scale: 0 (none) to 10 (very high)). Activity levels were categorized as rest, light, moderate, and vigorous based on the metabolic equivalent of task, estimated from video analysis. Most children were euhydrated pre (78%, USG ≤ 1.020), but not post-free-play (28%, USG ≤ 1.020). Mean and peak T_co, HR, and PSI responses were 37.8 ± 0.3°C and 38.4 ± 0.3°C, 133 ± 14 bpm and 180 ± 12 bpm, and 4.7 ± 1.1 (low) and 7.4 ± 1.0 (high), respectively. All children reached peak T_co≥38.0°C, with seven ≥38.5°C, and the highest at 38.9°C. The children spent 58 ± 15% of free-play engaged in moderate-to-vigorous intensity physical activity. During free-play, all of the children performed moderate-to-vigorous intensity physical activity, which was associated with pronounced elevations in heat strain.

Keywords: Hydration; exercise; heat stress; maturation; sweating.

Figure 1.

Urine specific gravity (USG) prior to (Pre) and following (Post) free play. The dashed line at USG = 1.020 represents the threshold between euhydration and hypohydration based on the American College of Sports Medicine exercise and fluid replacement position stand [16]. Values are presented for all children (n = 18; 7 girls)

Figure 2.

The proportion of free play (% time) spent at varying levels of (a) physical activity, (b) core temperature (°C), (c) heart rate reserve (%HRR), and (d) physiological strain index (PSI). Physical activity levels were determined by video analysis with energy expenditure estimates based on the metabolic equivalent of task, defined as the ratio of metabolic work rate to the resting metabolic rate, obtained from the Youth compendium for physical activity [17]. Intensity categories for %HRR are based on the American college of sports medicine guidelines for exercise testing and prescription [20]. PSI categories are based on Moran et al. [21]. Values are presented for all children (n = 18; 7 girls)

Figure 3.

Correlations between (a) mean wet bulb globe temperature (WBGT) and mean physical activity levels (MET; metabolic equivalent of task) during free play, and between (b) mean WBGT and change in body core temperature from baseline to peak (∆°C) during free play. Values are presented for all children (n = 18; 7 girls), *p < 0.05