Training Guide – Core Body Temperature Zones

The science of core body temperature for performance sports is comparatively young, but even a lot of seasoned athletes are familiar with the adage, “Train Hot, Race Cold”. While this remains a good general guide, now it is possible to easily capture core body temperature data and view this data in real-time. This represents a giant leap forward in training effectively.  
 
From the perspective of endurance sports, splitting core body temperature into five zones aligns with the common approaches for simplifying and quantify this data. 

Keep in mind that this is general information for the purposes of understanding the body behaviour and impact on performance as it gets hot. Each person has different thermoregulatory behaviour and it is strictly advised to seek medical and professional coaching advice first to understand how this applies to you and so that you can train and race safely. 

 
One approach to help define an individual’s core body temperature zones is with the heat ramp test. At this stage there is no automated approach for setting these zones.  

As a generalisation, for best athletic performance a sports person training in their Heat Training Zone which is between their ideal performance zone and where their power declines. In training, athletes increase their individual temperatures and remain in this zone - this conditions their body to performan better when hotter.  
 
Working from hot to cold, let’s look at each of these core body temperature zones with respect to sports performance.  

Zone 5 – Red Zone 

If an athlete reaches their Red Zone , this is a safety risk that could even become life-threatening. In this zone, power output is usually severely degraded and athletes risk heat stress which can encompass a series of risks including heat stroke, dizziness, exhaustion and collapse.  

Reaching the Red Zone is not only a result of physical intensity, it is also influenced by prior training and race preparation, hydration and environmental conditions. Monitoring the core body temperature in real-time can help athletes avoid this risk by taking steps to cool down. 

If this zone is reached, medical attention is required. Athletes who experience these conditions may also face a long physical recovery.  
 
If the heat stress is life-threatening, medical staff rely on rectal thermometers to monitor the condition of the patients and utilise rapid cooling approaches such as placing the patient inside an ice-baths for rapid cooling. 

Zone 4 – Power Declines 

With an increased core body temperature, the threshold power output begins to decline as the body uses more energy to pump blood to the skin. This means there is less blood reaching the muscles for power generation.  

During training and racing, an athlete may enter this zone with high intensity efforts and it can be compounded by environmental conditions such as high air temperatures and low wind.  
 
In competition, athletes approaching the finish line or undertaking major efforts may knowingly move into Heat Zone 4. Otherwise, for effective and sustained performance, athletes should seek to reduce their core body temperature with active cooling and/or reducing power/pacing and aim to return to Heat Zone 3.  

If an athlete remains in Heat Zone 4 for too long, they will usually experience a continual decline in power output and suffer more fatigue and slower recovery. Power and pace need to be reduced to avoid going into exhaustion. Zone 3 – Ideal Performance Zone  

This fairly broad temperature range covers increased core body temperature from physical intensity and is the ‘ideal’ zone for optimal power output during training and racing.  

Athletes should aim to spend most of their effective training and racing time within this temperature range.  
 
Heat training conditions the body and extends the upper limits of Heat Zone 3, where performance is best. In addition, active cooling strategies are used to keep the core body temperature within this zone. 

Zone 2 – Everyday Living 

For everyday activity including resting, sleeping, walking and office work, the core body temperature is not static and fluctuates within a zone throughout the day and night. This is known as the circadian rhythm and is relevant with regard to observing the well-being and general health of sports people.  

For sports training, this zone is well suited to strength training, which generally should be undertaken with a cool core body temperature. 

Zone 1 – Cold / Hypothermic 

In most sports, there is typically minimal danger of entering this zone as it is usually triggered by extreme environmental conditions and coupled with low (or no) physical activity.  

When the body cools too much, this becomes life-threatening. Sporting performance is significantly degraded and due to the safety risk, immediate medical attention is required. 

Core Body Temperature Zones for Athletes 

Trained sports people, with their improved efficiency and conditioning, typically have higher heat thresholds than untrained people, who may be more susceptible to heat stress. 

However, core body temperature is individual and even some trained athletes may have lower heat thresholds. Heat Training is used to improve the Heat Training Zones by increasing the cooling efficiency and power output of athletes. This provides an advantage over competitors who are not conditioned. 

These temperature zones are a simple guide for orientation and each person can then identify, with the guidance of professionals, specific temperatures individual to them and define the zones to suit. An advantage is that a histogram can plot the duration in specific temperature zones - for example an athlete can ensure that their heat training was sufficient.

Heat Training Zone and thermal load

The ideal Heat Training Zone, to condition the body, is between Zone 3 and Zone 4 and ensures that the athlete is hot enough to trigger the physiological benefits that will help condition their body, but is not too hot that it creates fatigue and impacts recovery. 

The amount of time spent in the Heat Zone 3 and 4 can be considered an athlete’s thermal load. A structured training plan can prescribe the thermal load to be accumulated during each week of the season. Importantly, training in Heat Zone 5 does not add to the thermal load and can in fact be harmful. For training, it is not more beneficial to train hotter than needed. 

Heat Adaption (also known as Heat Acclimation) is a form of Heat Training which is often conducted ahead of races in hot climates such as the Kona Ironman in Hawaii and ensures that an athlete is physically prepared. Heat Training can also be used with the goal of improving competitive performance for mild and cool weather competition. Further information on Heat Training can be found here.