Some of the major goals of exercise are to improve muscle tone, strength, and endurance. All of this is made possible by, and requires, the body's energy production-and-use system. Central to that system is a complex molecule called ATP, Adenosine Triphosphate.

ATP is a core element of a process known in biochemical studies as (ready for it?): the tricarboxylic acid cycle, or the Kreb's cycle. But don't worry about the complicated names. The basic ideas are very simple.

Carbohydrates are broken down into sugars which produce ATP. Simple sugars break down more easily and therefore, on average, more quickly. Complex carbohydrates take longer - and therefore supply the body with a longer lasting storehouse of compounds needed to produce ATP.

Sugar, per se, is NOT bad, only excess sugar, consumed in unhelpful forms, can lead to poor health effects.

ATP is broken down into ADP (Adenosine Diphosphate) and releases energy in the process. ADP later in the process then picks up the needed molecules to produce more ATP. That's why it's known as a cycle, since the process 'cycles around' to the beginning and starts over. That energy is used to maintain and repair cells, fuel respiration and organ systems and - more to our purpose here - produce the energy needed to fuel muscle contractions.

As byproducts of the cycle, heat and carbon dioxide are produced. The heat is eliminated by a number of means, including respiration and sweating. The carbon dioxide is carried through the system and some of it is expelled during respiration.

In order to carry out exercise, one essential element of which is muscle contraction, ATP must be produced continuously over varying stretches of time. In order to carry out this task, the body actually has three different ATP producing systems, with different production rates.

The phosphagen system replenishes ATP quickly, but only for short periods. That aids sprinters, fast-twitch fibers and other short-term uses. The glycogen-lactic system produces more slowly, but lasts up to 90 seconds or so. Aerobic respiration (normal oxygen breathing) makes ATP the most slowly, but can continue indefinitely.

As you exercise, ATP is consumed. That's one of the chief reasons you have to eat - in order to replenish the building blocks that can produce more ATP. Once you have more ATP, you have the basic molecule needed to engage in exercise and we're back where we began.

The body is an amazing, self-regulating complex of interconnected systems. None is more fascinating or central than the way it produces and consumes energy, an essential component of life itself.

Being flexible - having well lubricated joints and stretchable muscles - is one of the best ways to reduce the prospect of injury and give you the basis for a wide variety of workout routines.

Your overall goal is to attain good range of motion, while gradually extending that range to a degree appropriate to your fitness level and body type. There are several different ways to accomplish that goal, and you should use at least a few of them before every workout.

Warm-ups are fundamental. Cold muscles are much more likely to tear and lead to stretched or torn cartilage and other harmful results. Warm-ups and stretches help produce the fluid that lubricates the joints, and it helps the muscles become more elastic. Those both lead to safer, higher performance workouts.

Ten to fifteen minutes is the minimum for most people. This can be done by low-impact jogging in place, simple stretches and other techniques.

Static stretching, for example, is the old-fashioned stretch and hold for 30 seconds. This should be done with the arms, trunk, legs and neck. Dynamic stretching and ballistic stretching involve more active, bouncing-style or weight-assisted stretching, sometimes with extra force applied. Both types are helpful.

Some dynamic stretches involve holding the arms out to the side, then swinging them back and forth across front of the body, repeating for 30 seconds. Another technique involves using a short bar across the neck, lying on the shoulders. Place your hands on the bar, then bend slowly left, then right, moving the head toward the outer edge of the foot.

Abdominal muscles can be prepared by lying backwards on a large rubber ball. Push back slowly and raise the arms above your head. Repeat 10 times. Loosen your hamstrings by lying on your back, and raising one outstretched leg using a large towel wrapped around the foot. Grab the ends and pull up slowly. Switch legs, then repeat for 10 reps.

A full back stretch is accomplished by lying on your back and bringing both knees to the chest, hands clasped behind the knees. Roll forward until your feet hit the floor, then roll back until the head touches. Do 10 rolls.

Groin stretches can be done safely by using a large rubber ball. Place one knee on the ball and slowly rotate the ball a few inches away from your body. Move the ball back toward your body, then switch legs. Do 10 movements.

Both the legs and back muscles can get a good warm-up stretch by doing toe taps. Stand up straight, feet apart about shoulder width. Lean forward, touching the big toe on one foot with the opposite hand - left hand to right foot, and vice-versa. Those with lower back problems should consult a trainer or physician before attempting these.

For maximum flexibility, stretching routines should be carried out at least a few times per week. This will help maximize the range of motion and decrease the potential for injury.