Energy Systems Used by Exercising Muscle
Before describing the methods used for physical training, it is important to
understand the three systems that provide energy to the exercising muscle. All three
systems are important. Depending on the activity, there may be a greater reliance on one
system over the others.
ATP-CP System for Speed Work
Adenosine triphosphate (ATP) is the immediate source of energy within all cells of
our body for activities such as sprinting. There are small stores of ATP within skeletal
muscle, and these energy stores provide immediate energy to sustain physical activities for
a short time. Once the ATP is used, it breaks down into adenosine diphosphate (ADP). For
regeneration of ADP into ATP for more energy, creatine phosphate (CP) is needed. It is the
CP that regenerates the ATP. Without CP, ATP could provide energy for only a few
seconds. With CP, the ATP-PC system can provide energy for about 30 seconds before
other energy systems must take over. Thus, this ATP-CP system, sometimes referred to as
the phosphate pool or reservoir, provides immediate anaerobic energy for muscle
contraction.
Lactic Acid and ATP-CP System for
Anaerobic Work
This is a transitional system. When all-out exercise continues beyond 30 seconds,
the only way to continue providing ATP to the exercising muscle is by using sugar
(glucose) in the muscle. Sugar in the muscle is obtained from glycogen, and the process of
breaking down sugar for energy is called glycolysis. However, in the process of generating
ATP from glucose, lactic acid (also known as lactate) is formed. Normally there is a only
a small amount of lactate in blood and muscle. When lactate begins to accumulate in muscle
and then blood, you will begin to experience muscular fatigue, unless it is cleared by the
body. Lactate is cleared from the muscle if the intensity of the exercise is moderate. This
happens because after a few minutes the aerobic or oxygen system, which supplies energy
for sustained work, kicks in. If an all out effort is sustained, fatigue is inevitable within
three to five minutes.
Oxygen System for Aerobic Energy
The oxygen, or aerobic, system provides energy to support long-term steady state
exercise, such as long distance running or swimming. Muscles can use both glucose and
fatty acids for energy. These fuel sources can be taken from the circulating blood and from
stores within the muscle. Glucose is stored as glycogen and fatty acids are stored as
“triglycerides” in the muscle. When long duration activities are performed at a slow pace
more “fat” in the form of fatty acids is used for energy than muscle glycogen.
During many types of exercise, all three energy transfer systems are used at various
times. The amount that each system contributes to energy metabolism is related to the
duration, intensity and type of activity. In
general, high intensity, short duration exercises rely mainly on anaerobic energy. Other
examples specific to various activities are provided
in Table 1-2.
