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.

Doing intense interval workouts for 8 weeks can change how your genes are switched on or off in your muscles — and this study says it affects over 14,000 spots in your DNA.

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.