Improvement in exercise comes from stressing and recovering. You take a hard workout that causes your muscles to burn, which damages them. You will feel sore the next day because of the damage, and should take easy workouts until the soreness disappears. Then you take another hard workout. Muscles are damaged by hard exercise and soreness is a sign of the damage. Then when the muscles heal, they are stronger than they were before the hard workout.

If you take another hard workout before the soreness disappears, you place yourself at high risk for injury. If you want to build muscle or improve in any sport, take a hard workout that causes muscle soreness and on the next days, take easy workouts or take off until the soreness goes away. Serious weight lifters do not take off completely during their recovery days, even though resting when your muscles feel sore allows them to heal faster. If you exercises at low intensity during recovery, your muscles will become more fibrous and resistant to injury when you stress them with the next intense bout of hard exercise.