Training in the Heat: Why Ventilation is the True Measure of Effort

When the temperature rises, the rules of training change. If you are cycling in hot conditions but relying on standard metrics like heart rate and power, these can be misleading and even detrimental to your training progress.

As summer temperatures climb, many athletes find themselves confused by their data. Why is my heart rate higher at a lower power output? Why am I working harder but moving slower?

The answer lies in how your body manages heat. However, one metric you can rely on while training in the heat is ventilation.

The Problem with Power and Heart Rate in the Heat

In normal conditions, power and heart rate track more closely to your internal workload. But heat is a stress and your body has to adjust to cope with this load. 

When your core body temperature rises, your body starts to divert oxygen rich blood away from your muscles (where you generate power) and towards your skin where the sweat evaporation cools blood and helps to cool you down.

 Increasing heat creates a stress response that impacts physical effort.

• Power Drift: Your power output will naturally drop in the heat for the same internal effort. If you try to hit your usual power zones, you risk overtraining and overheating.
• Heart Rate Drift: Your heart rate will spike relative to the work you are doing. If you train strictly by heart rate zones, you might end up undertraining because your heart rate is elevated purely due to thermal stress, not cardiovascular effort.

Arnar Larusson, Founder and CEO of Tymewear, explains, “Heat is a stress on the body and if you follow HR or power alone these drift and shift, you can then be under or overtraining. The VitalPro breathing sensor can quantify actual physical effort without the physiological and environmental distortion, in this case, heat. This means it is a much better training guide, especially for heat training and altitude training.”

Breathing as the Anchor

If power and heart rate are distorted by the heat, how do you avoid under or overtraining while continuing to drive performance gains? Your ventilation metrics are the key.

Ventilation is the body’s direct response to metabolic demand. It reflects the true physical intensity of your effort, stripping away the noise caused by factors such as heat, altitude, nutrition, hydration or fatigue.

The Science

• Metabolic Demand: For every 1°C increase in core body temperature, your metabolic demand increases by approximately 10%. Your body is working harder just to cool itself.
• The Stress Response: Rising core temperature triggers a stress response that increases breathing rate independently of muscle work.
• The Accuracy Gap: When using heart rate or power to determine training zones, there is a 29% error margin applied to each zone. Stresses such as heat, hydration, and fatigue widen this error margin even further.

By using the VitalPro breathing sensor to guide your training intensity, you can train more precisely as you know the actual physical effort. This allows you to adjust your power or pace and ensure you are inside the exact training zones that will drive your fitness and performance. 

Practical Takeaways for Training in the Heat

1. Accept that your power will be lower in the heat. This is not a loss of fitness; it's a necessary physiological response. Instead use heat adaptation to gradually lift your power numbers in hot... and cool conditions.
2. Ignore Heart Rate Spikes: Don't panic if your heart rate in the heat is 10-15 beats higher than usual at an easy pace. Your body needs to work harder in the heat so use your ventilation to guide the intensity.
3. Trust Your Personal Ventilation Thresholds: Use breathing rate and ventilation thresholds to guide your effort. Low intensity Zone 2 training are typically better suited towards heat training activities as they provide the heat exposure while avoiding the additional stress of high intensity effort.
4. Managing heat is not one-dimensional. Heat training drives physiological adaptation though this is just one aspect. You need to consider environmental acclimation (conditioning your body for the actual environmental conditions for a race) as well as cooling strategies before, during and after racing help to reduce core body temperature and stress.

The VitalPro breathing sensor doesn't estimate core body temperature, rather it identifies the impact of external stresses such as temperature and altitude because they directly impact physical intensity. 

Shifting the training intensity targets from power/pace and heart rate to ventilation unlocks a more accurate training approach, no matter the weather.