In Depth

• Aerobic Exercise • Anaerobic Exercise • Resistance Exercise • Flexibility Exercise

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, i.e. 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 increases your supply of oxygen and energy to your body. It also decreases your risk of heart disease, stroke, high blood pressure and other life-threatening diseases.

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 with half the effort.

Strength is the ability of a muscle or 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 sprint racing. Slow-twitch fibers are for endurance, such as the ability for muscle to withstand fatigue. Most muscles have a 50-50 blend of fast-and slow-twitch fibers, but others have an advantage one way or the other. When you make muscles work harder, you actually tear these fibers. As they rebuild, they get stronger and bigger, resulting in harder, tighter and larger muscles.

Muscle Endurance is the ability to resist fatigue and continue to exercise over long periods of time. While 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 quarterback to throw long pass after pass.

Flexibility is the ability of joints and muscles to achieve a full range of motion. This results in the preventing injuries and helps keep your body feel comfortable after exercise. Despite popular opinion, there's no evidence that you should lose flexibility as you build muscle.

Unfortunately, there is truth that the natural 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.

Last updated June 10, 1997

1996-1999 The Johns Hopkins University. All rights reserved. Source: Inteli-Health Inc. All information is intended for your general knowledge only and is not a substitute for medical advice or treatment for specific medical conditions. You should seek prompt medical care for any specific health issues and consult your physician before starting a fitness regimen. Use of this online service is subject to InteliHealth's disclaimer and the terms and conditions.