Fitness - The Body's Energy System

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.

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