Heat stress and your training: what the temperature isn't telling you

Your athlete did a Thursday morning run. 45 minutes, easy pace, nothing dramatic. But they came back wrecked — heart rate drifting high, legs heavy by the end, and a rate of perceived exertion that didn't match the pace at all.

You check the temperature. 28°C. Warm, sure, but hardly extreme.

Except it was 80% humidity with barely a breeze. The actual heat load on their body was closer to what you'd expect at 33°C in dry conditions. The thermometer said one thing. Their physiology said something very different.

This is why raw temperature is a poor guide for coaching in the heat — and why we built heat stress tracking into Simma.

WBGT: the metric that actually matters

Exercise physiologists don't use air temperature to assess heat risk. They use something called WBGT — Wet Bulb Globe Temperature. It's the standard used by military organisations, sports governing bodies, and occupational health researchers worldwide.

WBGT accounts for four things at once: temperature, humidity, wind speed, and cloud cover. It measures the actual thermal load the body has to deal with — not just how hot the air is.

That's why a humid 28°C day with no wind can produce a higher WBGT than a dry 32°C day with a breeze. The first scenario makes it much harder for sweat to evaporate, which is your body's primary cooling mechanism. When evaporation stalls, core temperature rises faster, heart rate drifts, and performance drops — even at paces that would normally feel comfortable.

For coaching purposes, WBGT breaks down into categories that actually mean something:

Low (under 20°C WBGT): Normal conditions. No meaningful heat load. Train as planned.

Moderate (20–25°C WBGT): Mild heat. Most athletes handle this without issues, especially if acclimatised. Not worth flagging unless the athlete is new to warm conditions.

High (25–30°C WBGT): This is where it starts to matter. Heart rate will drift, pacing will feel harder, and hydration becomes critical. Athletes need to know this affected their session — and coaches need to account for it when reading the data.

Very high (30–35°C WBGT): Serious heat stress. Performance is meaningfully impaired. Sessions in this range are genuinely hard on the body, regardless of what the pace or power data says.

Extreme (above 35°C WBGT): Dangerous. These sessions carry real health risk. If your athlete trained in these conditions, you need to know about it.

Heat changes your training load — and Simma accounts for it

Here's something most coaches intuit but few platforms quantify: a session done in high heat is harder than the same session in mild conditions. Your athlete's body is doing double duty — executing the workout and managing thermoregulation. That costs energy, elevates cardiac output, and accelerates fatigue.

Simma adjusts training stress scores to reflect this. When an outdoor run or ride happens in heat, the stress score gets nudged upward — proportional to how far above neutral the conditions were. The same applies to cold, where the body burns extra energy maintaining core temperature.

The adjustment isn't massive — up to 20% for severe heat, up to 15% for extreme cold. But across a week of training in hot conditions, those adjustments compound. An athlete who trains through an Australian summer is accumulating more stress than their pace and power alone suggest. If you're not accounting for that, you're underestimating their fatigue.

This means the ATL, CTL, and TSB numbers you see in Simma already reflect environmental load. You don't need to mentally adjust. The data does it for you.

Every activity shows what the conditions were

When Simma processes an outdoor run or ride, it pulls the weather data for that specific location and time. Not a city-wide average — the actual conditions where and when the session happened.

On each activity, you'll see the heat stress category, the air temperature, and what it actually felt like (the apparent temperature). So when your athlete's Thursday run shows "high heat stress (33°C, felt 31°C)," you immediately understand why their heart rate was elevated and their pacing felt off.

For coaches screening across a squad, this is the difference between seeing confusing data and understanding it. An athlete whose heart rate drifted 10 beats above normal on an easy run isn't losing fitness — they ran in 30°C WBGT. Context changes the coaching response entirely.

Heat context flows into weekly coaching emails

Simma's weekly coaching emails already summarise each athlete's training, flag who needs attention, and draft personalised commentary for the coach to review and send. Heat stress data now feeds into that process.

When an athlete's week includes notable heat exposure — a session done in high or very high conditions, or multiple sessions in the heat — it shows up in the coaching email naturally. Not as a data dump, but as practical context: hydration reminders, pacing explanations, acclimatisation observations, or race-day readiness notes if they've got an event coming up.

If your athlete did three outdoor sessions in heat this week and they've got a hot-weather race in a month, the email might note that they're building meaningful heat acclimatisation. If it was a one-off scorcher and they looked flat, it explains why — and frames recovery accordingly.

The coach still reviews everything before it's sent. Simma provides the context. You provide the judgment.

Why this matters for endurance coaches

Heat is one of the biggest performance variables in outdoor endurance sport, and it's one of the hardest to track consistently across a squad. Most coaches rely on memory or manual weather checks after the fact — if they check at all.

When heat stress is tracked automatically, adjusted into training load, surfaced per activity, and woven into coaching communication, it stops being a blind spot. You see the full picture of what your athletes experienced, not just what their watch recorded.

Simma tracks heat stress across your squad's outdoor sessions automatically — adjusting training load, surfacing conditions per activity, and feeding it into weekly coaching emails. Join the early access waitlist.