Fitness is most easily understood by examining its components -- cardiovascular endurance, muscular strength, muscular endurance and flexibility.
Cardiovascular endurance is the body's ability to do large muscle work, for example moving the body over a period of time. This ability is dependent on the cardiovascular system's ability to pump blood and deliver oxygen through your body. Cardiovascular endurance should be a central component of your overall fitness program. Improving cardiovascular endurance not only increases the supply of oxygen and energy to your body, it decreases your risk of important diseases that may shorten your life, such as heart disease, stroke and high blood pressure.
When a heart is well conditioned, it is like any other muscle -- it becomes stronger and more efficient. A normal heart beats at a rate of approximately 70 beats per minute at rest or about 100,000 beats a day. The well-conditioned heart can actually beat as few as 40 times a minute at rest or approximately 50,000 beats per day. A well-conditioned heart conserves energy and can supply oxygen-rich blood to the rest of the body while performing less work.
Strength is the ability of a muscle or a group of muscles to exert an amount of force, typically in a one-time burst of effort. Weight-lifting (or resistance training) is a classic example of strength training because it increases muscle strength and mass, as well as bone strength, by placing more strain on muscles and bones than they are used to. When you lift weights, muscles are forced to meet that challenge by generating more force-generating proteins to feed the fibers that grow during exercise.
Most muscles have a combination of two types of fibers that are challenged during strength-training activities: Fast-twitch fibers provide the explosive force needed for weight-lifting or activities such as sprinting. Slow-twitch fibers are for endurance, such as the ability of muscle to withstand fatigue. Most muscles have a 50-50 blend of fast- and slow-twitch fibers, but some have more of one than the other. When you make muscles work harder, you actually injure these fibers. As they rebuild, they get stronger and bigger, resulting in harder, tighter and larger muscles.
Muscular endurance is the ability to resist fatigue and continue to exercise over long periods of time. Whereas strength training is needed to maintain muscle strength, endurance training is required to achieve stamina. Muscular endurance is the ability of muscles to continue working strong without rest, such as the ability of a baseball pitcher to throw hard over and over.
Flexibility is the ability of joints and muscles to achieve a full range of motion. This results in the prevention of injuries and helps keep your body feeling comfortable after exercise. Despite popular opinion, there's no evidence that you must lose flexibility as you build muscle.
Unfortunately, there is truth to the notion that the aging process can rob you of muscular strength, endurance and flexibility -- if you don't maintain them. That's why a regular fitness regimen becomes increasingly important as you age.