Running in the Cold: What Your Body Is Actually Doing

Every athlete knows the feeling. The first kilometre of a cold run is miserable, the fifth feels great, and the moment you stop, the shivering starts. There is real physiology behind all three, and understanding it makes winter running safer and far more comfortable. This one is for everyone heading into winter, and for anyone racing in cooler conditions, wherever you live.

Your blood vessels get smaller. In the cold, your body triggers peripheral vasoconstriction, which simply means the blood vessels in your skin and extremities get smaller. This keeps blood closer to your core to protect your vital organs, and it is why your hands and feet go numb first. Protect the extremities first, with mitts over gloves and a warm hat.

Hydrate like it is hot. Hydration matters just as much in the cold as it does in the heat. Cold air increases urine output (cold-induced diuresis), blunts your thirst response, and you still lose water with every breath of dry air and every bit of sweat. Drink on a schedule, because thirst will not tell you.

Cold air is dry air. Your lungs do not freeze. Air is warmed to body temperature before it reaches them. The challenge is that cold air is dry, so your airways have to warm and humidify every breath, and when you breathe hard that extra work can cause irritation, coughing and chest tightness. A fleece-lined buff or neck gaiter over your mouth helps by trapping warmth and moisture from your own breath, and both are easy to find for winter running. On easy runs, breathe through your nose when you can.

My cutoff is -20°C (-4°F). Research from the University of Alberta suggests paying attention from about -15°C (5°F), especially for hard efforts. Between -15°C and -20°C, keep it easy and cover your mouth. Below -20°C, I move it inside.

Finish outside, too. Take an easy 5 to 10 minute warm-down before you head indoors. Walking straight into warm air makes already-stressed airways work harder to humidify it, and that is when the coughing and burning start.

Muscles need more time. Cold muscle contracts less efficiently and stiffens, so the first 10 to 15 minutes carry more injury risk. Warm up indoors, start easy, and treat your early pace as unreliable.

Dress for kilometre two. Dress for about 5 to 10°C (9 to 18°F) warmer than the thermometer says, because you will heat up. I use Dress My Run to help me decide what to wear. Sweat that cools is the real problem, so change quickly when you stop, because that is when body temperature drops fastest.

Keep the numbers in perspective. Power leads, then pace, since pace drifts on snow, ice and in wind. Heart rate stays a warning signal, not the target. Easy runs should still finish at about 2 to 3 on Foster’s CR-10 scale, even when they feel harder in the cold. If you rate it higher, slow down.

Respect the cold, prepare for it, and winter becomes one of the best training seasons of the year.