Exercise and 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,
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
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
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
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.