why do people who exercise vigorously outdoors on a cold day risk a reduced body temperature when they stop?

Answer 1

A basic lesson in thermodynamics here: Exercise makes you sweat, the sweat evaporates, an evaporating surface loses heat energy through the change of state from a liquid to a gas.

Sweating is one of Nature's ways to keep the body at a steady temperature. Now if you are sweating at a cold temperature, the loss of heat will be much higher than if you are sweating at a warm temperature, thereby risking hypothermia. A sudden stoppage of vigorous exercise in cold weather leaves a lot of sweat on the skin but reduces the heat output, and presto, you can reduce the body's temperature excessively. In other words, the body's cooling system is set on high, but the heat engine has been shut off.

Answer 2

As your internal body temperature rises, the blood vessles dilate and allow the warmer blood to come closer to the surface so that it can cool, hence reducing the internal temperature. That's a normal part of homeostasis. And, if they vigorously exercise outdoors on a cold day, they could risk a much lower body temperature because of this. The body sweats during vigorous exercise, causing evaporation (which is used to cool the core temperature of the body -- again homeostasis), and the dilation of the blood vessels again bring the warmer blood to the surface so that it can become cooled and return to the core of the body.

These two mechanisms used by the body to protect itself from reaching too high a temperature on the inside could, in this situation, cause too cool a core body temperature.

Answer 3

When it's warmer outside, it makes you lethargic and not want to do anything....but with it being cold, it wakes you up more and gets you going