Most athletes are familiar with the concept of training load and how it considers both training duration and intensity. Thermal load is a similar metric used to track heat training. Thermal load measures the amount of time that core body temperature is within an athlete's individualised heat training zones.

Accumulating thermal load

During competition, endurance athletes operate at elevated temperatures for extended periods of time. Athletes who have experienced significant and prolonged thermal load in training are the best adapted to respond to these conditions. Just like we train at intensities we expect to experience in race conditions, we also need to train at thermal loads expected during competition.

Thermal load is accumulated when one's core body temperature is elevated into the heat training zones. This heat training zones are best prescribed for each individual through a heat ramp test.

Thermal load is measured for both individual workouts and cumulatively across weeks and months, and trains the body to compete in similar conditions. Individual sessions can range from 15–60 minutes of thermal load, and different zones have unique benefits.

Health benefits

A training session of 15–20 minutes in a zone slightly above normal average temperature simulates a mild fever. This health benefits zone is centered around 38.2º C/100.8º F for many people. Training in this zone temporarily enhances immune function, allowing the body to respond more effectively to viruses and bacteria. The temperature range of this medium activity zone is generally a little lower than the heat training zone used for performance gains.

Performance benefits

A training session where the core temperature is in the primary heat training zone for 45–60 minutes will enhance athletic performance. For many, this heat training zone centers around 38.5º C/101.3º F. While most thermal load should be accumulated during activity, it can be prolonged by entering a sauna or hot bath to keep core temperature elevated. When enough thermal load is accumulated in this zone, the body will increase blood plasma and subsequently hemoglobin. This makes more oxygen available to the muscles at all temperatures, increasing performance. The extra blood volume also helps cool the body in warm conditions.

Avoid excessive thermal load

Excessive thermal load can come from excessively high core body temperatures. Core temperatures above the heat training zone will cause a performance degradation.  In training, it is best to minimize time spent at these temperatures. Similar to overtraining, it creates excessive stress which can be detrimental to training. And reaching very elevated core temperatures is dangerous and can lead to heat exhaustion or heat stroke.

Excessive thermal load can also be caused by too much time in the heat training zone – either in one session or cumulative weekly time. This can cause sluggishness and decreased performance. Cumulative thermal load should be prescribed by a coach based on specific goals (ie. a heat block training versus maintaining benefits).

Monitoring thermal load

Professional teams monitor daily, weekly and monthly thermal load. For example, most will do a focused heat block training of 2–4 weeks, consisting of 6 heat training sessions per week. This creates extra blood plasma and hemoglobin. Those plasma levels will then be maintained with 2–3 heat training sessions per week.

Monitoring thermal load in standard training software or the CORE app can give a good indication if athletes are doing proper heat training, or if they are training at too high of a core body temperature. Monitoring thermal load can also show if enough is being done for health and wellness benefits.