Heat Stress and Your Training: What the Temperature Isn't Telling You

An athlete runs 45 minutes on a Thursday morning at an easy pace and comes back wrecked: heart rate drifting high, legs heavy, effort nowhere near the pace. The forecast said 28°C. Warm, not extreme.

It was also 80% humidity with no wind. The thermometer measured the air. The body was dealing with something else.

Simma records the conditions at the place and time of every outdoor run and ride a squad records, and adjusts the session's training load for them. This piece explains the measure we use, the categories it produces, and exactly what the adjustment is, so you can judge whether to trust it.

Air temperature is the wrong number

The body cools mainly by evaporating sweat. Humidity slows evaporation; wind and shade speed it; radiant heat from direct sun adds load that shade does not. Air temperature captures none of that, which is why a humid, still 28°C can be harder on the body than a dry, breezy 32°C.

Exercise physiologists and sports governing bodies use wet bulb globe temperature, WBGT, for this reason. It combines temperature, humidity, wind and solar radiation into one number that tracks the thermal load the body actually has to manage.

The categories

Simma computes WBGT for each outdoor run and ride from the weather at that location and time, not a city average, and classifies it:

• Low: WBGT below 20°C
• Moderate: 20 to 25°C
• High: 25 to 30°C
• Very high: 30 to 35°C
• Extreme: 35°C and above

Cold is classified separately from the apparent temperature, with its own bands down to extreme cold.

The category, the air temperature and the apparent temperature are recorded on the session. A run that shows "high heat stress (33°C, felt 31°C)" is a run whose elevated heart rate has an explanation before anyone has to look for one.

Exactly how it changes training load

A session in heat costs more than the same session in mild conditions, because the body is doing the work and managing its temperature at once. Simma adjusts the session's stress score for that. The adjustment is:

• Roughly 1.5 to 2 percent per degree of WBGT above 20°C, capped at 20 percent.
• For cold, a smaller adjustment below 5°C apparent, capped at 15 percent.

So a run at 30°C WBGT carries a score somewhat under 20 percent higher than the same run in mild air, and the fitness, fatigue and form numbers built from it (Training Load Explained) already include that. Across a week of training through an Australian summer the adjustments add up, which is the point: an athlete training in heat is carrying more than pace and power alone suggest.

The constants are ours. They are in the range the literature on heat and perceived exertion supports, and they have not yet been validated against the outcomes of athletes on Simma.

What it does not do

It does not know how acclimatised the athlete is. A runner in their third week of a hot summer and one who flew in yesterday get the same adjustment for the same conditions, and the first will have handled it far better. It does not know hydration, clothing, whether the route was shaded, or whether they slowed down sensibly. It reads the sky, not the body.

Pool swims get no heat adjustment, because Simma has no way to know the water or air temperature indoors.

Where the context goes

When a draft to an athlete is written, whether in the web app or through your assistant, the heat category on each session is part of what the draft reads. A week with a session in high or very high heat, or several in the heat, is something the draft can name, and it is asked to connect heat training to race-day readiness when a hot-weather event is on the athlete's goals. Whether that is the right thing to say to this athlete this week is the coach's read, and nothing is sent without it.

What should your assistant be able to do here?

Heat is recorded on every outdoor session, so there is more an assistant could do with it: how many hot sessions an athlete has had in the last three weeks, how their heart rate at a given pace responds as conditions rise, what the forecast means for Saturday's long run.

If there is a heat question you want to put to your assistant, email coaching@getsimma.com and tell us. We build tools from those.

Simma records the conditions on every outdoor run and ride your squad records, adjusts the load for them, and carries the context through to the status you see and the drafts written in your name. Create your coaching account.