Quick Answer: Heat adds a second job to every session - your cardiovascular system has to move blood to your skin for cooling at the same time as it supplies working muscle, which raises heart rate at the same workload and increases the total physiological cost. Training load models built on pace or power miss this entirely, and models built on heart rate can overstate it. The practical result is that a normal summer training week produces more accumulated fatigue than the same week in spring. Adjusting expected paces and adding recovery is the correct response, not pushing harder.

Which matters most right now, because late-August humidity persists after the enthusiasm of a new season has started.

What is heat actually costing you?

Three overlapping costs, and only one of them shows on a watch.

Cardiovascular competition is the main one. Cooling requires blood at the skin; working muscle requires blood too. Your heart compensates by beating faster at the same workload - typically a substantial rise at moderate intensity in hot conditions, and more as the session goes on.

Plasma volume drops as you sweat, which means less blood volume returning to the heart per beat. Stroke volume falls, so heart rate rises further to maintain output. This is cardiac drift, and it is why hour two of a hot session has a much higher heart rate than hour one at identical pace.

Core temperature itself limits performance directly. As core temperature rises toward its ceiling, central drive to the muscles is reduced - a protective mechanism that makes the same effort feel harder and caps how hard you can push.

And the recovery cost extends past the session. Elevated core temperature, fluid deficit, and electrolyte loss all persist for hours. A hot session interferes with sleep quality that night in the same way an evening run does, which compounds into the next day.

Meanwhile pace and power are unchanged as metrics. Running 8km at the same pace in 30C and 15C logs identically and costs very differently.

Why do the training load numbers mislead?

Because every common model measures a proxy, and heat breaks the proxies differently.

Pace and power-based models miss heat entirely. They calculate load from external work done. The external work in heat is identical; the internal cost is not. These models systematically under-report summer load.

Heart rate-based models over-report it in a particular way. They see a higher heart rate and score a higher load, which is directionally right. But some of that elevated heart rate is thermoregulation, not training stimulus - so the model credits you with an adaptation you did not get. You accumulated the fatigue without the corresponding training benefit.

Perceived exertion is actually the most honest metric here and is the one most people ignore. RPE naturally incorporates thermal strain, which is precisely what the objective metrics cannot see.

Recovery scores lag by design. Most consumer recovery metrics respond to overnight HRV and resting heart rate, which react a day late. By the time the score drops, you have already trained through the accumulation.

The practical rule: in hot conditions, trust RPE for how hard a session was and treat pace as a downgraded target. If you must use a number, session RPE multiplied by duration is more useful than anything your pace-based app is calculating.

5 Mistakes People Make Training Through a Hot Spell

1. Holding the same pace targets. A pace that was moderate in May is a threshold effort in humid August. Chasing the number turns every easy session into a moderate one, which is the classic route to accumulated fatigue without adaptation.

2. Interpreting a slow session as lost fitness. Nothing has been lost. Heat is a tax on expression of fitness, not on fitness itself - the same runner is measurably faster within days of conditions cooling.

3. Treating heat acclimation as automatic. It is a real, trainable adaptation - plasma volume expands, sweating starts earlier and more dilutely - but it takes roughly one to two weeks of consistent exposure and it fades within a couple of weeks of stopping. Sporadic hot sessions produce the cost without the adaptation.

4. Hydrating only during the session. Fluid and sodium replacement in the hours after is what determines whether you start tomorrow in deficit. Two consecutive under-replaced days is the usual mechanism behind a mysteriously terrible third session.

5. Adding volume because the sessions feel unproductive. The instinct to compensate for slower paces with more mileage is exactly backwards. The physiological cost is already higher; more volume on top is how a hot August becomes an injured September.

The third one is the mistake most likely to surprise experienced athletes, because acclimation is widely known and its timeline rarely is.

This is general information, not medical advice.

How should you actually adjust?

Six adjustments that keep the training productive.

1. Train to effort, not pace. Set sessions by RPE or by heart rate zone with the acceptance that pace will be slower. This is the single change that resolves most of the problem.

2. Move the session, not the intensity. Early morning is genuinely cooler and less humid. A 6am session in August is a different physiological event from a 6pm one, and moving it is easier than restructuring your training.

3. Cut duration before cutting quality. For hard sessions, a shorter session at the right intensity beats a full-length session at a compromised one. Do four intervals properly rather than six badly.

4. Replace sodium as well as fluid. Heavy sweating in humid conditions loses meaningful sodium, and water alone can leave you worse off. Salted food after a session does the job for most people without any product.

5. Add a recovery day, not a rest week. The accumulated cost of a hot block usually needs one extra easy day per week, not a full deload - though a genuine deload is right if warning signs stack up.

6. Watch the honest signals. Morning resting heart rate creeping up over several days, appetite dropping, waking at 3am, warm-ups feeling harder than they should, and motivation falling before performance does. Any two of those together in a hot block is a reason to back off, regardless of what your app says.

And take the seasonal timing seriously. The last humid weeks of summer overlap with the autumn restart enthusiasm, which is a genuinely risky combination: new ambition, old conditions. Cooler weather is coming and will do a lot of the work for you.

Heat illness is a real risk. Dizziness, confusion, stopping sweating, or nausea in the heat means stopping and cooling immediately - and seeking medical help if it does not resolve quickly.

FAQ: Heat and Training Load, Answered

How much slower should I expect to be in the heat?

Enough to notice and enough that chasing your usual paces turns easy sessions into hard ones. The exact amount depends on humidity, acclimation, and the individual - which is precisely why effort is a better target than pace.

Does heat training make me fitter?

Heat acclimation is a genuine adaptation - expanded plasma volume, earlier and more dilute sweating - and it takes one to two weeks of consistent exposure. It improves performance in heat, and the transfer to cool conditions is smaller than sometimes claimed.

Should I use heart rate or pace in summer?

Effort first, heart rate second, pace last. Heart rate captures some of the thermal cost; pace captures none of it.

Why is my heart rate higher in the second half of every hot session?

Cardiac drift - plasma volume falls as you sweat, so stroke volume drops and heart rate rises to maintain output. It is normal in heat and is not by itself a sign of overtraining.

Is humidity worse than temperature?

Often yes. High humidity blocks evaporative cooling, which is your main heat-loss route during exercise. A humid 26C day can be more demanding than a dry 32C one.

When should I actually stop a session?

Dizziness, confusion, nausea, chills, or stopping sweating are stop-immediately signals. Get cool, get fluid, and seek medical help if symptoms do not resolve quickly.

TL;DR:

• Heat makes your circulation do two jobs at once, raising heart rate at the same workload and adding real physiological cost.
• Pace and power models miss it entirely; heart rate models credit thermoregulation as training stimulus - so both mislead.
• RPE is the most honest metric available in hot conditions.
• Adjust by training to effort, moving sessions earlier, cutting duration before quality, and replacing sodium as well as fluid.
• Heat acclimation is real but takes one to two weeks of consistent exposure and fades quickly.
• Warning stack: rising morning heart rate, falling appetite, 3am waking, harder warm-ups, motivation dropping before performance.

You have not lost fitness. You have been paying a tax the app cannot see - and it is about to get cheaper.

This is general information, not medical advice.