There are a few culprits, here. First up: When you exercise, your heart pumps more blood to your hard working muscles. This momentum then causes your capillaries to expand, and as they do so, they stimulate the surrounding nerve cells in your body—and your brain interprets that signal as an itch. “This is a more likely scenario for the body versus the face,” says board-certified dermatologist Deanne Mraz Robinson, M.D., FAAD, co-founder of Modern Dermatology and assistant clinical professor of dermatology at Yale New Haven Hospital, (since your larger muscle groups are powering the hardest, and thus require more blood flow), “But the same science applies.”
Or, if you’re prone to flushing while working out, itch can certainly follow. “Exercise leads to dilation of your blood vessels to enhance circulation to your skin and muscles,” says board-certified dermatologist Joshua Zeichner, M.D. This is generally a good thing—after all, it’s increased circulation that delivers nutrients to the cells and lends the beloved “post-workout glow.” But for some people (especially those with fair to medium skin tones, sensitive skin, or conditions like rosacea), it can lead to inflammation, which can result in flushing, bumps, and, yes, itching.
In some cases, that prickly sensation can also represent a type of heat rash, called miliaria rubra. “This is a condition where there are blockages within the sweat glands, leading to tiny, red bumps and a prickly or itchy feeling,” says Zeichner. “This develops when sweat is trapped in the glands because of issues like non-breathable, wet workout clothing sticking to the skin.” It’s also much more common in hot, humid weather—say, if you’re jogging outside midday in the throes of summer.
And, finally, people can just react to their own sweat. While sweat can effectively clear pores, some individuals may be hypersensitive to its contents. “Moisture, heat, and sweat can all cause irritation of the skin,” says Mraz Robinson. Simple as that.