Have you ever thought about what happens to your body during exercise?  No?  Well I have.  In fact, just the other day while I was on my elliptical, I thought to myself, “How is it that the body can exercise and exert this amount of energy?” What does the body go through every time it hits the weights, pounds the pavement or runs the distance?”

Well, to say it is complicated would be the biggest understatement of the year.  So for the sake of brevity, remember these three letters:  ATP.  Without ATP, your body doesn’t do a darn thing.

ATP is short for adenosine triphosphate and it’s what the body uses for fuel while you’re exercising.  The harder you work—the more weight you lift, the faster you sprint, or the heavier you breathe—the more ATP the body demands.

Because the body needs so much ATP, it has three different ways of producing it:  through phosphate, through glycogen and through aerobic respiration.

These three sources of ATP are produced in installments, sort of like how employees produce goods in shifts.

The first installment, or shift, is the phosphagen system.  This system uses ATP that the muscles already have stored away.  But because there’s only a small amount of ATP stored away at any given time, the phosphagen system works the smallest amount of time (the phosphagen system works for about eight to 10 seconds).  To really get a longer lasting supply of ATP, the body turns to the glycogen-lactic acid system.

This system is where the food we eat—and what energy we already have stored away as fat—factors into how (and how well) we exercise.  The food we eat is stored as energy in our muscles in the form of glycogen.  Glycogen is broken down into glucose and ATP is produced.  There are about 12 chemical reactions that have to take place before ATP can be produced, but it is eventually produced.

While this process supplies a longer batch of energy then the phosphagen system does, it’s not by much.  About one and a half minutes of the energy you expend is controlled by the glycogen-lactic acid system.

Where the hay is really made is in the aerobic respiration phase.  Here, the oxygen you breathe in is broken down into carbon dioxide and water to produce ATP with glucose’s help.  In this phase, the glucose the body uses for ATP production comes from one of three sources:  the glycogen that’s left in the muscles, the glycogen that’s broken down in the liver, or from the food we eat.  These last two—from the liver or the food we eat— supply our working muscles with the energy they need through the bloodstream.

With aerobic respiration’s ability to produce ATP, the body can go for a long, long time.  Ultra-marathoners and triathletes would be nothing were it not for aerobic respiration.

You’d be amazed how long the body can exercise with relatively small amounts of stored energy.  But like a car, it needs its fuel.  Once the body empties of glycogen stores, it’s only a matter of time before the body shuts down.  This is why you’ll often see triathletes and marathoners eating some type of carbohydrate-rich supplement while they’re jogging or peddling.  The body prefers to use carbohydrates for energy, but if it’s low on carbs, it will use the next best thing (i.e. first fat, and once that runs out, protein).

This is ultimately how weight is lost—when the body relies on energy stores as opposed to the energy we’re supplying our bodies with through food.  This is why people trying to lose weight will exercise on an empty stomach, as the body will rely on the body’s glycogen stores for fuel.

And folks, that’s just one aspect of exercise.  We haven’t even gotten into how the heart and lungs factor into the exercise equation, supplying the body with the extra oxygen it needs so it can continue to produce ATP.  Or how the body cools itself through sweat (another aspect of weight loss).  Or the differentiation in how the body uses oxygen depending on the exercise  (e.g. Are you strength training or endurance training?).

For that and a whole lot more, I encourage you to enroll in an exercise science class.  There you’ll get a real taste of just how amazing exercise is and how the body is the most complex machine ever created.

Source
health.howstuffworks.com