Regardless of whether you're a novice athlete or a professional athlete, it's always important to make time for a little R & R. Periods of rest are instrumental in helping to make sure you get the most out of your workout. In fact, the days off from working out are equally as important as the physical routine itself. Exercise breaks down the muscles on a microscopic level. On the days when you aren't working out, the muscles have time to recover and strengthen. Recovery enables the body to refill energy stores and fix damaged tissues. During a workout, the muscle tissue breaks down and energy stores are depleted -- there is also fluid loss. Not allowing time for recovery can cause the muscles to grow stiff or sore. This in turn can result in injury. Experts recommend that people only work each muscle group once or twice a week to prevent overtraining, injuries and other complications.