THE 3 ENERGY SYSTEM

The human body uses energy from
food to fuel movement and essential
body functions , but the body cells
don ’ t get energy directly from food .
After food is digested , the
carbohydrates , protein and fat break
down into simple compounds - - glucose ,
amino acids and fatty acids - - which
are absorbed into the blood and
transported to various cells
throughout the body . Within these
cells , and from these energy sources ,
adenosine triphosphate (ATP ) is
formed to provide fuel . The body
uses 3 different systems to supply
cells with the necessary ATP to fuel
energy needs. Most of the body ’ s
activities use a continuum of all three
energy systems , working together to
ensure a constant supply of energy .
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The body needs a continuous supply of ATP for
energy -- whether the energy is needed for lifting
weights, walking, thinking or even texting. It’s
also the unit of energy that fuels metabolism, or
the biochemical reactions that support and
maintain life. For short and intense movement
lasting less than 10 seconds, the body mainly
uses the ATP-PC, or creatine phosphate system.
This system is anaerobic, which means it does
not use oxygen. The ATP-PC system utilizes the
relatively small amount of ATP already stored in
the muscle for this immediate energy source.
When the body’s supply of ATP is depleted,
which occurs in a matter of seconds, additional
ATP is formed from the breakdown of
phosphocreatine (PC) -- an energy compound
found in muscle.
The lactic acid system, also called the anaerobic
glycolysis system, produces energy from muscle
glycogen -- the storage form of glucose.
Glycolysis, or the breakdown of glycogen into
glucose, can occur in the presence or absence of
oxygen. When inadequate oxygen is available, the
series of reactions that transforms glucose into
ATP causes lactic acid to be produced -- in
efforts to make more ATP. The lactic acid
system fuels relatively short periods -- a few
minutes -- of high-intensity muscle activity, but
the accumulation of lactic acid can cause
fatigue and a burning sensation in the muscles.
The most complex energy system is the aerobic
or oxygen energy system, which provides most
of the body's ATP. This system produces ATP as
energy is released from the breakdown of
nutrients such as glucose and fatty acids. In the
presence of oxygen, ATP can be formed through
glycolysis. This system also involves the Krebs
or tricarboxylic acid cycle -- a series of chemical
reactions that generate energy in the
mitochondria -- the power plant inside the body
cells. The complexity of this system, along with
the fact that it relies heavily on the circulatory
system to supply oxygen, makes it slower to act
compared to the ATP-PC or lactic acid systems.
The aerobic system supplies energy for body
movement lasting more than just a few minutes,
such as long periods of work or endurance
activities. This system is also the pathway that
provides ATP to fuel most of the body’s energy
needs not related to physical activity, such as
building and repairing body tissues, digesting
food, controlling body temperature and growing
hair.
Three energy systems work in the body to
provide energy. While these systems are well
known for their role in fueling athletic
performance, ATP is essential for every energy
need in the body -- including all the automatic
body processes of growth, development and
maintaining vital body functions. These energy
systems do not work independently and do not
function in isolation. Rather, all systems operate
at all times, but some may predominate based
on the body’s activities, including the type,
intensity and duration of physical activity as well
as a person's fitness level.
System