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3. Aerobic glycolysis: During exercise of about 20’ maximal duration, the muscle primarily breaks
   down stored muscle glycogen as well as blood glucose to provide energy. This produces pyruvate
   as an end product which is used to make more energy. This energy system is used during
   activities such as a 2 mile run.


4. Aerobic lipolysis: During exercise of longer than 20’, the body will break down free fatty acids to
   produce energy. This energy system is used during low intensity aerobic activities.

The body may also use other fuel sources (ketones, protein, and intramuscular triglyceride)
during exercise to varying degrees. The involvement of each fuel is discussed in specific
situations. A brief overview of the four energy systems and their duration appears in table 2.

Summary

Muscle fibers are typically delineated into three different types, depending on their
characteristics. During exercise, depending on the intensity, the various muscle fibers are
recruited as necessary to produce force. As muscles produce force, they utilize the fuel
adenosine-triphosphate (ATP). Since there is limited ATP stored in the muscle itself, a variety of
energy systems exist to produce more ATP. The energy system which is used will depend on the
duration and intensity of exercise.

Table 2: Overview of energy systems for different activities

Energy System Time Exercise Intensity Example Activity

ATP-CP 1-20” Maximal Shot putting, low rep

weight training (1-5 reps)

Anaerobic glycolysis 20-60” Near maximal 400 meter sprint, medium

rep weight training (6

reps+)

Aerobic glycolysis 1-10’ High 2 mile run

FFA Oxidation 10’ and up Low slow walking, jogging

Source: Hawley JA and Hopkins WG. Aerobic glycolytic and aerobic lipolytic power systems: A
new paradigm with implications for endurance and ultraendurance events. Sports Med (1995) 19:
240-250.

References Cited

1. “Designing Resistance Training Programs, 2nd edition” W. Kraemer and S. Fleck, Human
   Kinetics 1996.


2. “Physiology of Sport and Exercise” Jack H. Wilmore and David L. Costill. Human Kinetics
   Publishers 1994.


3. “Exercise Physiology: Human Bioenergetics and it’s applications” George A Brooks, Thomas D.
   Fahey, and Timothy P. White. Mayfield Publishing Company 1996.