Training in summer heat: adjusting load, fluids and pace

Training in the heat is not just less comfortable, it is physiologically different. At 30 °C your body pushes a large share of blood towards the skin to dump heat, which leaves less for the muscles: the same weights feel heavier, your heart rate runs 10-20 beats higher at the same pace, and fatigue arrives earlier. Here is what to change, how much to drink, and when to stop.

When to train

The worst window is 12:00 to 17:00. If you train outdoors, aim for before 9:00 or after 19:30. Humidity matters more than most people assume: 28 °C at 70% humidity is harder on you than 34 °C at 20%, because sweat does not evaporate and your cooling system stops working.

In a gym without air conditioning, look at the thermometer. Up to 24 °C, train normally. Above 28 °C, cut the session to 45-60 minutes and expect your last sets to be clearly weaker than they were in April.

How much to drink

• 2 hours before: 400-600 ml of water.
• During: 150-250 ml every 15-20 minutes, which works out to 0.5-1 litre per hour in real heat.
• After: weigh yourself before and after. Replace each kilogram lost with 1.2-1.5 litres of fluid spread over the next 2-4 hours, not all at once.

A 2% loss of body mass (1.8 kg for a 90 kg person) measurably drops both strength and endurance. Pale straw urine is fine; dark urine means you are behind. Do not overshoot either: more than 1.5 litres per hour of plain water, with no salt and no sweat rate to justify it, can dilute your blood sodium.

Salt and electrolytes

Sweat carries roughly 400-1500 mg of sodium per litre. If you train longer than 60-75 minutes in the heat, or if white streaks dry on your dark shirt, add 300-700 mg of sodium per litre of fluid, which is about a quarter teaspoon of ordinary table salt. For anything under an hour, water and normal meals cover it.

Adjusting strength work

• Keep the weight, cut the volume: 3-4 working sets instead of 5, same load. Losing a set beats losing form.
• Extend rest. On squats and deadlifts take 3-4 minutes instead of 2, because your heart rate comes down more slowly when you are hot.
• Leave 2-3 reps in reserve instead of grinding to failure. Failure work at 32 °C is paid for the next day.
• Target 45-60 minutes total. If your session runs 90 minutes, cut accessories, not the main lifts.
• Save max testing for autumn.

Adjusting cardio

Your pace drops, and that is normal. Expect 15-30 seconds per kilometre slower at 30 °C than at 15 °C for the same effort. Run by heart rate and feel, not by the splits you hit in spring. Move intervals to early morning or shorten them, for example 10 rounds of 30 seconds hard and 60 seconds walking instead of 6 times 3 minutes. Plan long runs as a 2-3 km loop around one spot where your water sits, rather than an out-and-back you are committed to finishing.

Acclimatisation

Adaptation takes 7-14 days: you start sweating earlier, the sweat gets more dilute, and plasma volume rises 5-10%. In practice, spend the first 4-5 days at 60-70% of your usual intensity, 30-60 minutes per day in the heat, then rebuild the load. After two weeks the same session feels noticeably easier. Miss a week in air conditioning and much of that adaptation fades within 2-3 weeks.

Cooling that actually works

• 400-500 ml of a cold or iced drink 20-30 minutes before you start.
• A cold wet towel on the neck and forearms between sets.
• Light, loose, breathable clothing, plus a peaked cap if you are in direct sun.
• 10-15 minutes under a cool shower afterwards, before you sit down or get into a hot car.

Signs to stop immediately

Heat exhaustion looks like this: dizziness, nausea, headache, cold clammy skin, chills in the middle of a hot day, cramps, and a heart rate that will not settle after 3-4 minutes of rest. What to do: stop, get into shade, lie down with your legs raised, take salted fluid, and put cold water on the neck, armpits and groin.

Heat stroke is a medical emergency: core temperature above 40 °C, confusion, slurred speech, sweating that shuts off, or loss of consciousness. Call your emergency number (112 across most of Europe) and keep cooling the person while you wait.

When to see a doctor

• Same day, if there was any loss of consciousness or disorientation, or if your urine after training is the colour of dark tea (possible rhabdomyolysis).
• Chest pain, breathlessness out of proportion to the effort, or a fluttering or skipping heartbeat.
• If you take blood pressure medication, diuretics, antihistamines, antidepressants or thyroid medication, ask your doctor or pharmacist how they affect heat tolerance and fluid loss.
• Diabetes, heart disease, pregnancy, or recovery from a febrile infection: agree a plan before you start training at 30 °C and above.
• Cramps lasting for days, or weakness that persists after 2 days of rest and normal hydration.

The short version

• Train before 9:00 or after 19:30, and indoors above 28 °C cut the session to 45-60 minutes.
• Drink 400-600 ml two hours before, 150-250 ml every 15-20 minutes during, and 1.2-1.5 litres per kilogram lost afterwards.
• Anything over 60-75 minutes in the heat needs 300-700 mg of sodium per litre of fluid.
• Keep the weights, drop from 5 sets to 3-4, rest 3-4 minutes, and stay 2-3 reps short of failure.
• Acclimatisation takes 7-14 days; dizziness, nausea and chills mean stop now, and confusion or collapse means call emergency services.