Marathon runners are injured more frequently than triathletes, who compete in three sports, even though triathletes exercise more. So to protect themselves from injuries, athletes plan a hard workout one day, followed by an easy one the next day. In this way, the hard workouts cause less muscle damage. Hard and easy refer to intensity, not quantity. For example, on an easy day, a marathon runner might run 20 miles but at a much slower pace than on a hard day.
Weight lifters lift very heavy weights only once a week and lighter weights the rest of the week.
Basketball players play long, exhausting practice games one day, then practice plays and shooting the next day. Usually, the muscles ache for about 48 hours. Then, athletes exercise less intensely the next few days, until their muscles stop aching. Exercising intensely when muscles ache causes injuries and decreases performance, whereas waiting for the muscles to stop aching before exercising intensely again strengthens muscles. Two types of muscle discomfort may be felt after exercise.