Physical fitness is to the human body what fine-tuning is to an engine. It enables us to perform up to our potential. Fitness can be described as a condition that helps us look, feel, and do our best. More specifically it is: The ability to perform daily tasks vigorously and alertly, with energy left over for enjoying leisure-time activities and meeting emergency demands. It is the ability to endure, to bear up, to withstand stress, to carry on in circumstances where an unfit person could not continue, and it is a major basis for good health and well-being.

Physical fitness involves the performance of the heart, lungs, and muscles. And, since what we do with our bodies also affects what we can do with our minds, fitness influences to some degree qualities such as mental alertness and emotional stability.

THE BASICS--Physical fitness is most easily understood by examining its components or parts. The five ingredients are:

1. Cardiorespiratory endurance--the ability to deliver oxygen and nutrients to tissues, and to remove wastes, over sustained periods of time.
   
2. Muscular strength--the ability of a muscle to exert force for a brief period of time.
   
3. Muscular endurance--the ability of a muscle, or a group of muscles, to sustain repeated contractions or to continue to apply force against a fixed object.
   
4. Flexibility--the ability to move joints and use muscles through their full range of motion. The sit-and-reach test is a good measure of flexibility of the lower back and backs of the upper legs.
   
5. Body composition--is considered a component of fitness. It refers to the makeup of the body in terms of lean mass (muscle, bone, vital tissue, and organs) and fat mass. An optimal ratio of fat to lean mass is an indication of fitness. The right types of exercises will help you decrease body fat and increase or maintain muscle mass.

Because a weak abdomen can interfere with the proper functioning of the organs inside it. Apart from facilitating movements of the trunk and legs, the stomach muscles provide support and protection for the liver, kidneys, pancreas, and of course the stomach. Abdominal muscles that lose their ability to serve as a kind of natural girdle allow these organs--and the lower spine--more internal movement than they were designed to handle. Digestive disorders can result, as can back problems. An estimated 80% of all lower back pain can be traced to lack of abdominal strength.

Sitting gives stomach muscles little to do, and so they weaken. And because sitting also gives the rest of our bodies little to do, we fatten. The combination is devastating and develops into a vicious circle: As a burgeoning waistline makes movement more difficult, movement becomes less likely. And less movement means fewer calories are burned. And fewer calories burned means more calories stored--around the middle. The force behind the evolution of a potbelly, as you can see, is not one to be taken lightly.

You don't have to be overweight in order to suffer from abdominal prolapse. Excess weight increases one's chances, but a potbelly can develop in anyone who neglects to maintain sufficient abdominal strength and proper posture. Also, osteoporosis can cause potbellies in the slimmest and most fit women.

FITNESS ACTIVITIES--More than 30% of Americans age 55 and older say they walk for exercise. Walking is their first choice for fitness activities followed in order by: swimming, fishing, bicycle riding, camping, golf, bowling, exercise equipment, hiking, hunting, aerobics, calisthenics, jogging, and tennis. Women prefer fitness-related activities and men prefer outdoor sports.

AEROBICS--Aerobic means using oxygen. To achieve aerobic conditioning, you must engage in vigorous, sustained exercise at least three times each week. You must raise your pulse rate to a target range and maintain it in that range for the entire time.

MEASURING YOUR HEART RATE--Heart rate is widely accepted as a good method for measuring intensity during aerobic exercises. Exercise that doesn't raise your heart rate to a certain level and keep it there for 20 minutes won't contribute significantly to cardiovascular fitness.

The heart rate you should maintain is called your target heart rate. If you are in good health, you need to set a target zone for the heartbeat, according to your age. Your target zone should be 60% to 75% of your maximum heart rate.

Your maximum heart rate is usually 220 minus your age. However, the above figures are averages and should be used as general guidelines. Take your pulse during your exercise to determine your heart rate: Put the first and second finger of your right hand on the radial artery of the inner wrist of your left hand. Count the number of beats in 30 seconds and multiply by 2 (equals heartbeats per minute). Take your pulse 5 minutes into your exercise, and again just before ending it.