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
• heat stress
• exercise
• maturation
• sweating

Acknowledgments

The authors would like to thank the children and their parents for their enthusiastic involvement in this study.

Disclosure statement

No potential conflict of interest was reported by the authors.

Author’s contribution

G.W.M. and G.P.K. conceived and designed the experiments. G.W.M., S.S., K.E.K., S.T., B.J.R., M.R.G., and E.R.M. performed data collection and analysis. All authors interpreted the results. G.W.M. prepared figures and drafted the manuscript. All authors edited and revised the manuscript. All authors approved the final version of the manuscript.

Additional information

Funding

This study was supported by Health Canada (funds held by Dr. Glen P. Kenny). G. P. Kenny is supported by a University of Ottawa Research Chair Award. All other authors were supported by the Human and Environmental Physiology Research Unit at the University of Ottawa.