As we saw last week, fitness is more than just exercising. It may help to understand the 'big picture' of fitness by looking at its individual components:

1. Cardiorespiratory endurance - the ability to deliver oxygen and nutrients to the body's tissues and to remove wastes, over sustained periods of time. Long runs and swims are among the methods used to measure this component.

2. Muscular strength - the ability of a muscle to exert force for a brief period of time. Upper-body strength, for example, can be measured by various weight-lifting exercises.

3. Muscular endurance - the ability of a muscle, or a group of muscles, to sustain repeated contractions or to continue applying force against a fixed object. Pushups are often used to test endurance of arm and shoulder muscles.

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.

Next week, we'll look at the types of exercise suited to each of these components.