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Figure 1: Breakdown of ATP

ATPase
ATP ADP + Pi + energy

Note: Pi represents an inorganic phosphate molecule

Within a muscle, there is about 6 seconds worth of ATP stored which can be used for
immediate energy. For activity to continue past 6 seconds, ATP must be generated through
various other reactions. The first of these of these is through the creatine phosphate (CP)
system.

The creatine phosphate system

Also stored in the muscle is a substance called creatine phosphate (CP). This provides a
phosphate molecule to ADP to regenerate ATP so that muscular activity can continue. CP
donates its high energy phosphate molecule to ADP to regenerate ATP via an enzyme called
creatine kinase as shown in figure 2.

Figure 2: Breakdown of CP to regenerate ATP

Creatine kinase

ADP + CP ATP + Creatine

There is enough stored CP in a normal muscle to provide energy for approximately the first
20 seconds of muscular activity at which time intramuscular CP is depleted. The CP system
operates in the absence of oxygen (it is anaerobic) and can provide energy very quickly during
exercise. Collectively stored ATP and CP are known as the ATP-CP or phosphagen system. The
total energy yield from the ATP-CP system is low due to the small amount of ATP and CP
available in the muscle. The ATP-CP system is used to fuel maximal intensity activities of a
duration of 20 seconds or less such as low rep weight training and sprinting. At exhaustion during
these types of exercise, fatigue is most likely caused by CP depletion. It should be noted that,
even at complete exhaustion, muscle ATP stores do not decrease very much during any type of
exercise. After depletion, CP is resynthesized fully in approximately 3-4 minutes (12).

For activity to continue past 20 seconds, the body must rely on other fuel sources to
generate ATP. One of these is the breakdown of blood glucose or glycogen (the storage form of
glucose found in the muscles and liver), which is called glycolysis.

Aerobic glycolysis is discussed in the previous chapter and is not repeated here. The major
difference between aerobic glycolysis (during exercise below LT) and anaerobic glycolysis is the
ultimate fate of pyruvate. Whereas in aerobic glycolysis, pyruvate goes into the Krebs cycle to
provide more energy, in anaerobic glycolysis pyruvate is converted to lactate. The fate of lactate