Summer training gets treated as normal training with more water.
It isn’t. Heat changes the physiology of endurance exercise substantially, it degrades performance in measurable and predictable ways, and at the extreme end it’s genuinely dangerous — exertional heat stroke sits among the leading causes of death in sport.
The good news is that the body adapts to heat remarkably well, faster than to almost any other training stimulus, and the adaptation is worth having. Here’s what’s happening and what to do about it.
What Heat Does to a Working Body
When you exercise, your muscles produce heat. Your body dumps that heat mainly by moving blood to the skin and by sweating.
In hot conditions, both routes get harder. The temperature gradient between your skin and the air narrows, so less heat escapes by radiation. Humidity impairs sweat evaporation, which is the mechanism that actually cools you — sweat dripping off you has done no work.
Meanwhile you’re competing for blood. Your muscles want it for oxygen delivery, and your skin wants it for cooling. Both can’t win, so heart rate rises to compensate at the same workload.
The performance consequence is well documented. Marathon performance progressively declines as environmental heat stress increases. That easy pace feeling harder in July isn’t a fitness problem.
Then dehydration compounds it. Fluid deficits of as little as 3 to 5 percent of body weight result in diminished sweat production and declining skin blood flow — meaning your cooling system degrades exactly when you need it most.
Heat Acclimation: The Adaptation Worth Having
Here’s the encouraging part. Repeated exposure to exercise in heat produces a substantial set of adaptations, and they arrive quickly.
What changes: lower heart rate at the same workload, lower internal body temperature, lower skin temperature, reduced perceived exertion, expanded plasma volume, higher sweat rate, increased skin blood flow, and more dilute sweat — your body conserves sodium rather than dumping it.
How long it takes: benefits appear from protocols of seven days or fewer, with greater benefits from protocols running roughly seven to fourteen days.
How much it’s worth: one study of twelve consecutive days of heat acclimation training found roughly a 5.5% improvement in cycling time trial distance in hot conditions compared with a control group training in cool conditions.
How to do it: train in the heat deliberately, at reduced intensity, for around an hour a day, on consecutive days. Reduced intensity is important — you’re chasing the thermal stimulus, not a hard session. Then return to normal training with the adaptations banked.
The catch: it fades. A couple of weeks away from heat and you’ll have lost a meaningful share of it.
The Three Sports Are Not Equal in Heat
Cycling is the most forgiving. You generate significant airflow, which drives both convective cooling and sweat evaporation. The danger is that this masks the problem — you feel fine while riding and the heat load only becomes obvious when you stop at a light, or on a slow climb where the airflow disappears. Climbs in still air are where cyclists get into trouble.
Running offers far less airflow relative to the heat you’re producing, and you’re often exposed with no shade. Runners feel heat stress earlier and more honestly than cyclists.
Rowing deserves particular attention. Water reflects sunlight, there’s often no shade at all, and the boat may be moving at a pace that doesn’t generate meaningful cooling airflow relative to effort. On top of that, rowing is a full-body effort producing substantial metabolic heat. And there’s a specific hydration concern worth knowing about — one study documented exercise-associated hyponatremia in 70% of a group of thirty elite junior rowers during an extended training period.
Hydration: Both Directions Matter
Under-drinking is the risk everyone knows about. Over-drinking is the one that kills people, and it gets far less airtime.
Exercise-associated hyponatremia — dangerously low blood sodium — occurs when fluid intake exceeds losses, diluting sodium. It’s been documented across endurance sports: in Ironman triathletes, endurance cyclists, ultramarathon runners, and marathon and half-marathon runners tested after racing.
Confusingly, the early symptoms overlap with heat illness — headache, nausea, confusion, feeling unwell. Which is why the instinct to drink more when someone feels bad can be exactly wrong.
Sensible principles: drink to thirst rather than to a schedule. Include sodium in longer or hotter sessions, particularly if you’re a heavy or salty sweater — white streaks on your kit after a session is a reasonable clue. Don’t force fluids beyond thirst. And if you finish a session heavier than you started, you drank too much.
Warning Signs, and What They Mean
Stop, get out of the heat, and cool down for: unusual fatigue relative to the effort, headache, nausea, dizziness, muscle cramping, goosebumps or chills in hot conditions, or feeling that your effort suddenly disproportionate to your pace.
Call 911 and begin cooling immediately for confusion, disorientation, aggression or altered behavior, loss of coordination or stumbling, collapse, seizure, or loss of consciousness.
Altered mental status is the critical sign. Exertional heat stroke is defined by a core temperature above 40°C alongside mental status changes, and it requires immediate rapid cooling. Cold water immersion, where available, is the priority — cooling should begin before or during transport rather than waiting.
Do not send someone home to rest because they seem confused but are still talking. That’s the presentation that becomes an emergency.
Practical Adjustments Worth Making
Move the session. Early morning is meaningfully cooler than evening, since ground heat persists after sunset.
Adjust by effort, not pace or power. Your usual numbers are not available in heat, and chasing them is how people get into trouble. Run and ride by feel on hot days.
Shorten hard sessions rather than pushing through them, and shift intensity work to cooler times or indoors.
Use the terrain. Shaded routes, out-and-backs that keep you near water or your car, and loops rather than point-to-point routes when it’s very hot.
Pre-cool where practical. Starting a session cooler buys you time.
Wear light, loose, light-colored kit, and don’t neglect sun protection.
Watch cumulative fatigue. Heat adds physiological load on top of training load. A normal training week in a heat wave is a harder week than the plan says.
Who Needs Extra Caution
Anyone returning from illness, particularly with fever or gastrointestinal illness. Anyone poorly slept or under-recovered. Anyone on medications affecting fluid balance, sweating, or heart rate — worth checking with your physician or pharmacist. Anyone with cardiovascular conditions. Older athletes, whose thermoregulation is generally less efficient. And anyone in their first days of hot weather, before acclimatization has developed — heat syncope characteristically occurs in that early window.
If you’ve had a heat illness before, you’re at higher risk of another, and return to full training should be managed with medical guidance rather than by feel.
Train Through Summer, Sensibly
Heat is a training variable like any other — one that rewards deliberate adaptation and punishes ignoring it.
Marin Peak Physio offers a free discovery visit at no cost and no obligation. You’ll get an assessment of your movement, strength, and training load, plus practical guidance on structuring a summer block around the conditions and the events you’re actually training for.
If you’re carrying an injury or your performance has dropped for reasons that don’t add up, we’ll help you work out why.
