Heat Zones

 

Preventing Performance Loss and Guiding Heat Training 

CORE's Heat Strain Index indicates how hard your body is working to stay cool. It delivers a real-time value from 0 (no heat strain) to around 10 (extremely high heat strain). Monitoring Heat Strain Index can help you to prevent performance loss and to effectively adapt to the heat. (See the article CORE’s Heat Strain Index for more information on this metric.) To guide you in training and racing, CORE experts have developed four Heat Zones based on the Heat Strain Index. 

  

CORE’s Heat Zones 

Disclaimer: The CORE sensor is not a medical device and is not meant to diagnose heat stroke or heat illness. Additionally, training or racing in Heat Zones 1-3 does not guarantee that you will not experience heat stroke or heat illness.

CORE’s Heat Zones help you to understand the effects of heat stress on your body. They are defined as follows:  

 

 

 

Please see the article Heat Safety for more information about elevated heat strain. 

The Paradox of Heat Zone 3 

Heat Zone 3 is of critical importance. It’s best avoided during competition, as heart rate will rise and perceptions of heat may become very uncomfortable. While finishing a race in this heat zone may be tolerable, many athletes will prefer to avoid long durations in this zone. 

In contrast, Heat Zone 3 is ideal for heat training. Regular and repeated training in this zone will result in heat adaptation. It is the zone best for adapting to the heat. While some adaptations may occur from training in Heat Zone 2, they will take longer to develop and will not reach the body’s full potential. For more information, see the article Heat Training Basics. 

 

Individuality of Heat Zones 

Thermoregulation and associated performance losses are highly individual. Therefore, the exact thresholds for the Heat Zones may vary across individuals. For example, Heat Zone 3 has a fairly wide range of values, and not everyone will react the same way to each of those values. For some, a Heat Strain Index of 3.0–4.0 will cause great performance loss and may be an effective range for heat training . Others may see relatively little performance loss between 3.0–4.0 and may need a Heat Strain Index of 5.0 or 6.0 for effective heat training. Similarly, exercise in Heat Zone 4 may be dangerous for some athletes, but not for all. 

These individual differences can be caused by various factors, such as heat adaptation status, fitness level, height, weight, body composition, sex, and age. Also, some people simply have a ‘low thermostat’ or ‘high thermostat’ and may see lower or higher values than their peers. 

Please see the article Heat Training Basics for tips on how to identify your ideal Heat Strain Index for heat training. 

 

References 

Alkemade P. Mitigation of heat strain during exercise in hot-humid conditions: One size does not fit all. 2023. 236 p. https://doi.org/10.5463/thesis.263  

Casa DJ, DeMartini JK, Bergeron MF, et al. National Athletic Trainers’ Association position statement: exertional heat illnesses. J Athl Train. 2015;50(9):986–1000. https://doi.org/10.4085/1062-6050-50.9.07. 

Foster J, Hodder SG, Lloyd AB, Havenith G. Individual responses to heat stress: Implications for hyperthermia and physical work capacity. Front Physiol. 2020; 11:541483. https://doi.org/10.3389/fphys.2020.541483. 

Nybo L, Rasmussen P, Sawka MN. Performance in the heat - physiological factors of importance for hyperthermia-induced fatigue. Compr Physiol. 2014;4(2):657–89. https://doi.org/10.1002/cphy.c130012. 

Périard JD, Racinais S, Sawka MN. Adaptations and mechanisms of human heat acclimation: Applications for competitive athletes and sports. Scand J Med Sci Sports. 2015;25(1):20–38. https://doi.org/10.1111/sms.12408. 

Périard JD, Eijsvogels TMH, Daanen HAM. Exercise under heat stress: thermoregulation, hydration, performance implications, and mitigation strategies. Physiol Rev. 2021;101(4):1873–979. https://doi.org/10.1152/physrev.00038.2020.