When people do long-term workouts like running or lifting weights, their muscle cells show tiny changes in a biological tagging system called DNA methylation—mostly fewer tags in areas that control metabolism and muscle function.

When you do short bursts of intense exercise like sprinting or HIIT, it changes how your muscle genes behave in a way that sticks around—even after you stop exercising for three months—and comes back when you start again.

After just one workout like running or cycling, your muscles quickly change how their genes are turned on—specifically, a gene called PGC-1α gets more active because a chemical tag on it gets removed, helping your body respond to the stress of exercise.

After doing intense workout intervals, your muscles keep some chemical changes for at least three months—even if you stop exercising—suggesting your body might be 'remembering' the training at a genetic level.