Helping Prevent Injury
Flexibility is the range of movement of a joint, or series of joints, and their associated muscles. Good flexibility can help you efficiently accomplish physical tasks like lifting, loading, climbing, parachuting, running, and rappelling with less risk of injury. Stretching during your warm-up and cool-down helps you maintain overall flexibility—it should not be painful but should cause some discomfort because the muscles are being stretched beyond their normal length. Because people differ in their physical make-up, you shouldn't compare one person's flexibility with another's. If you have poor flexibility, trying to stretch as far as someone else could cause an injury.
For further strengthening techniques please see the calisthenics section of the Army PT Guide.