What happens is a classic example of the use it or lose it principle of human physiology. Unlike automobile engines that wear out with use, our musculoskeletal system seems to rust out with lack of use. This is why it is so important to do strength training to maintain muscle tissue and bone density. However, it is equally essential to do stretching exercise to maintain joint flexibility and functional movement ability.

For example, many people have become aware of neck inflexibility when they have to turn their whole upper body to check traffic when backing up their vehicles. Others experience stiffness in the morning or after sitting for extended periods of time. Some people become abruptly aware of their rigid bodies when they take a ski trip, go sailing, or hit a few tennis balls. The first golf or softball game of the season can also be a rude awakening, resulting in injuries to tight muscles, such as the lower back and hamstrings.

Unfortunately, as movement becomes more difficult, people tend to move even less, leading to further debilitation and lifestyle limitations. There is good news, however, for those who are willing to work towards better musculoskeletal function. By systematically stretching the muscles, they can become more extensible and the joint structures can become more flexible. Regardless of age, muscles have the ability to respond positively to a progressive program of stretching exercises. Let's take a look at how to improve joint flexibility in a safe, effective and efficient manner.

Principles of Stretching
1. The first principle of stretching safely is to always stretch within your comfort zone. In other words, never stretch to the point of pain. Although a mild muscle taughtness may be desirable, discomfort has no part in a sensible stretching program.

2. The second principle for stretching safely is to relax. It is almost impossible to stretch effectively when you are tense, and an up-tight stretching session can certainly increase the risk of tissue injury.

3. The third principle of sensible stretching is to exercise first. It may actually be counterproductive to stretch a cold muscle. After exercising your body temperature is elevated and your muscles are more extensible. Although the example of salt-water taffy may be a bit extreme, the analogy has some application from an injury-prevention perspective.

4. The fourth stretching guideline is to stretch slowly. Fast muscle movements and bouncing actions trigger the stretch reflex that causes the muscle to contract rather than relax. Be sure to move slowly and gently into each stretched position, avoiding abrupt actions.

5. The fifth stretching guideline is to pause for 10 to 30 seconds in the fully-stretched position. While it is neither necessary nor advisable to stretch to the point of discomfort, it is important to maintain each stretched position long enough for the muscles to make the desired adaptations. Although stretches may be held for longer time periods, research indicates that most of the flexibility benefits can be attained in 10 to 30 seconds.

6. The sixth stretching guideline is training consistency. Unlike strength and endurance exercise that requires relatively high-effort training for best results, stretching must be essentially effortless (relaxed) to be fully effective. Therefore, you must commit to stretching regularly. Plan to perform 10 to 15 minutes of stretching at the end of every exercise session. Try not to view stretching as an add-on that you may include if time permits, as the catch-as-catch-can approach typically results in infrequent stretching sessions.

While there is no rule on what stretches you should do, I suggest at least one stretch for the rear thigh (hamstring), low back, and shoulder joint muscles. If I had to recommend just one exercise that involves all of these muscles to some degree it would be the Figure- Four Stretch. This basic stretching exercise is performed as follows: Begin by sitting on the floor with your left leg straight and your right leg bent at the knee so that your right foot touches your left thigh. Reach your left hand toward your left foot slowly, until your hamstrings feel comfortably stretched. At this point, grasp your foot, ankle, or lower leg and hold the stretched position for 10 to 30 seconds. Change leg positions and repeat the same procedure for your right hamstrings. You should also feel some stretching effects in your calf, hip, low back, and shoulder muscles as you do the figure "4" stretch.

Wayne L. Westcott, Ph.D., is fitness research director at the South Shore YMCA in Quincy, MA., and author of several fitness books including the new releases, Building Strength and Stamina and Strength Training Past 50.