Fitness refers to your heart, and the harder you exercise, the more fit you become. But every time you exercise vigorously your muscles are injured, and the harder you exercise, the longer it takes for your muscles to heal. Muscle biopsies done the day after a person exercises vigorously show bleeding into the muscles and disruption of the Z-bands that hold muscle filaments together. You are not supposed to exercise vigorously again until the muscle soreness disappears.
Most competitive athletes set up training programs so they exercise vigorously enough on one day to make their muscles feel sore for the next day or two and then after the soreness disappears, they exercise vigorously again. You can use the same principle in your exercise program to achieve a higher level of fitness. You can exercise vigorously on one day and easy on the next few days or until the soreness disappears, OR you can train in two or three sports. This is called cross-training, and it can make you very fit and help to prevent injuries.
Each sport stresses specific muscle groups. Cycling stresses the upper legs, while rowing stresses your back and upper body. If you cycle and row on the same day, you stress your upper legs and upper body on the same day. To reduce your chances of injuring yourself, you should take the next day off, or at least exercise at a very low intensity. If you cycle on Monday and row on Tuesday, you allow your muscles 48 hours to recover from each sport. Pick two sports that use different muscle groups and do them on alternate days. You can then exercise more intensely in each sport and achieve a higher level of fitness.
