Fitness experts have identified 4 keys to success in helping you choose the right exercises to develop and maintain each of the basic components of fitness that we've been discussing: muscular strength, muscular endurance, cardiorespiratory endurance, and flexibility. These keys to success are:

• Specificity - Pick the right kind of activities to affect each component - that is, do strength training exercises to increase specific strengths. And train for the specific activity you are interested in. For example, you'll get your best swimming performance if you have trained the muscles involved in swimming. A good runner is not necessarily a good swimmer, since different muscles are involved.

• Overload - Work hard enough, at levels that are vigorous, and long enough to overload your body above its resting level, in order to bring about improvement.

• Regularity - You can't hoard physical fitness. At least three balanced workouts a week are needed to maintain a desirable level of fitness.

• Progression - Increase the intensity, frequency, and/or duration of activity over periods of time in order to improve.

You can use some activities to fulfill more than one of your basic exercise requirements. For example, running builds muscular endurance in the legs while it also increases cardiorespiratory endurance.