You train for competitive sports by taking a hard workout, which makes your muscles sore for the next day or two and then when your muscles feel fresh again, you take another hard workout. Every intense workout causes muscle damage and soreness. Biopsies taken on the day after a hard workout from the muscles of athletes show bleeding into the muscles and disruption of the muscle fibers. If you try to exercise intensely when your muscles are still sore from a previous workout, you are at great risk for injuring yourself. Regular exercisers and competitive athletes improve most with a weekly schedule that includes one or two intense workouts and one longer session for endurance. To prevent injury, they follow each of these three harder workouts with easy workouts or days off.
Intense workouts cause far more muscle damage than longer endurance workouts. That means that an athlete can exercise harder on the day after an endurance workout than the day after an intense one. So weightlifters should not lift weights with the same muscle groups on the day after the one day a week that they lift very heavy weights. Runners should run very slowly on the days after the two days a week that they run very fast. Most training programs include two intense workouts, say Tuesday and Friday, followed by days of very easy workouts on Wednesday and Saturday and a longer workout on Sunday followed by a moderate workout on Monday.