Why Summer Heat Makes Your Workout Feel Harder

Everyone knows a workout feels worse in the heat. Fewer people know why — and that the "why" is a real, measurable change in your cardiovascular system, not just a motivation problem. Understanding it lets you train around it instead of just suffering.

What heat does to your blood

When you exercise in the heat, your body has to do two jobs at once: deliver blood to your working muscles, and send blood to your skin to dump heat through sweating. To make that possible, acute heat exposure triggers an initial increase in plasma volume — a fluid reservoir that supports sweating and cushions the drop in central blood volume as blood redirects to the skin. Without that increase, body temperature would climb faster and heat tolerance would fall.^[1]

Hydration is the whole game

Hydration status flips the math. During exercise, plasma volume increased about 4% when subjects were well hydrated and decreased about 4% when they were under-hydrated.^[2] That swing matters: dehydration from sweat loss raises blood concentration and lowers blood volume, both of which reduce sweat output and skin blood flow — the exact tools your body uses to cool itself. Those losses in cardiovascular function are largely prevented when you maintain hydration by drinking during exercise.^[3]

The single highest-leverage thing you can do to make hot training feel easier is to start hydrated and keep drinking — not to tough it out.

Why your heart rate spikes

In the heat, more blood pools at the skin for cooling, so less returns to fill the heart each beat. To keep output up, heart rate rises — which is why the same pace or load feels harder and your watch reads higher than usual on a hot day. It's not weakness; it's thermoregulation competing with performance.

Your body can adapt — to a point

Heat acclimation is real: with consistent exposure, the body increases the rate and sensitivity of sweating and skin blood flow, and lowers both whole-body temperature and heart rate during submaximal work in the heat. With adequate fluid and electrolytes, the plasma-volume expansion persists and reduces cardiovascular strain.^[4] It typically takes one to two weeks — and it only goes so far. Extreme heat still degrades hard efforts.

How to train through a Prince George heat wave

• Pre-hydrate. Arrive topped up, not planning to "catch up" mid-session.
• Drink during. Small, regular sips beat one big bottle at the end.
• Replace electrolytes on long or very sweaty sessions, not just water.
• Move hard sessions indoors. For PRs and brutal conditioning, climate-controlled air removes the heat tax entirely so you can hold intensity.

Bottom line

Heat training feels harder because it genuinely is harder on your cardiovascular system — and hydration is the dial you control. Start hydrated, keep drinking, give yourself a week or two to acclimate, and on the hottest days train inside so the workout you planned is the workout you get.

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