Triathletes are injured only about one third as often as marathon runners even though they do far more work in their program of swimming, cycling and running. Training intelligently for three sports is less likely to injure you than training very hard for one. Training is limited by damage to skeletal muscles. Every time you exercise, your muscles develop small tears with bleeding. It takes at least 48 hours for muscles to heal from exercise. Each sport stresses a particular group of muscles most. Marathon runners who train every day stress the same muscles and do not allow adequate time to recover from the previous day's workout, so they are at increased risk for injury.
Top triathletes train in different sports on consecutive days. Running stresses the lower leg muscles most, cycling stresses the upper leg muscles most and swimming stresses the arms and shoulders most. Triathletes usually set up a workout schedule that includes two sports on one day and one on the next. Of the three sports, running causes the most muscle damage. Muscles are protected by the water in swimming and by the rotary pedal motion in cycling. However, the force of the footstrike in running tears up muscles. So a knowledgeable triathlete runs on one day and cycles and swims on the next. This way each stressed muscle group has a 48-hour recovery before the next workout. You can apply these same rules for alternating any two or three different activities.