Thermoregulatory, Cardiovascular and Biomechanical Responses of Amateur Runners at A Large City Marathon in Temperate Conditions

Introduction: Exertional heat illnesses (EHI) remain frequent in endurance sports, posing significant risks to amateur and elite athletes. With climate change, the challenge of safely managing sporting events is set to continue. The current approach for many sports is event modification and/or rescheduling based on environmental conditions. However, exercise intensity is the major determinant of EHI risk, rather than environmental heat stress. Therefore, monitoring individual heat strain is more suitable for understanding risk associated with different sporting activities, varied individual characteristics, and when appraising sporting heat stress policies. Addressing this need, we report thermoregulatory, cardiovascular and biomechanical responses of amateur runners completing in 10 km (10KM), half-marathon (HM) and marathon (M) events in Hong Kong in February 2025. The study aims to differentiate heat strain across different event lengths and identify individual characteristics that exacerbate this strain. We hypothesised the greatest heat strain will be experienced in the 10KM.

Methods: Eighty-five runners (age 38±13 [15–65] years) ran the 10KM (n=22; 8 female, 14 male. Body fat percentage [BF%] 23±12%), HM (n=32; 14 female, 18 male. BF% 19±8%) and M (n=31; 9 female, 22 male. BF% 15±6%). Self-predicted race times were; 10KM 62±10 min, HM 02:00±00:25 hours and M 03:49±00:40 hours. Runners ingested two telemetry pills to monitor core body temperature (TCORE; pill 1 before bed, pill 2 upon waking on race day) and wore a heart rate monitor (HR), with attached skin temperature sensor (TSKIN). Running biomechanics and kinematics were monitored using shoeworn sensors. Environmental conditions were measured every 5 km using portable meteorological stations. Races began between 05:30-08:30, with course closing at 14:00. Data were integrated by a bespoke app on a smartwatch and streamed in real-time. Upon finishing, participants indicated EHI symptoms experienced during the race and provided a fingertip blood sample to assess markers of gut injury. Runners completed further surveys 12 hours, 24 hours, 48 hours and 72 hours post-race to track recovery status

Results: Environmental conditions from 10 weather stations were; ambient temperature 17.9±3.2°C, relative humidity 42.6±9.6% and Wet Bulb Globe Temperature 12.5±2.9°C. Preliminary results indicate that the highest TCORE recorded was 40.37°C in the 10KM race, while no runners in the half marathon (HM) or marathon (M) exceeded 40°C. Notably, no cases of exertional heat illness (EHI) were reported.

Conclusion: We monitored a large cohort of amateur athletes during a large city marathon to understand the thermal strain and EHI risk factors, including individual characteristics and event distance. Ongoing development of multi-sensor approaches, encompassing thermal, cardiovascular, sudomotor, and biomechanical monitoring, combined with multilocation weather data, is needed to understand EHI risk across various sports for amateur and elite athletes.