If you make a rat’s leg do repeated tough stretching exercises, that leg gets stronger faster the next day and doesn’t get as sore—because the muscle itself adapts and protects its internal machinery.

If you make one leg of a rat work hard by stretching its muscles on purpose, the other leg doesn’t get stronger or less sore—even if you repeat the workout many times. It’s like training your right arm but expecting your left arm to benefit too, and it doesn’t.

When male rats do 100 forced muscle-lengthening moves (like lowering a heavy weight slowly), their muscles get weaker for at least four days — and this weakness comes with tiny tears in the muscle fibers, a cleanup enzyme turning on, and key muscle signals getting messed up.