The new muscle fibers that replace the dead ones are stronger than the old ones were. The new fibers are more dense with sacomeres, which means they can be strained more without snapping. The next time you try the same activity, you won't be as sore. You're more likely to experience this kind of muscle soreness with movements that are known as eccentric exercise. Most movements cause one group of muscles to contract while the other group stretches.
Eccentric movements cause muscles to stretch and contract at the same time.
One example is walking downhill. Because eccentric motions are less common, they're more likely to cause muscle soreness. Exercise experts at UT Southwestern Medical Center say it's important to strive for consistency in your workout. If you give up at the first sign of soreness, you won't benefit. You have to keep up a regular schedule to continue growing stronger.