Exercise and the Three Energy Systems
The muscles in your body use three energy systems to create ATP and fuel cellular activity. These systems use different fuels and chemical processes and perform different, specific functions during exercise (Table 3.1).
The Immediate Energy System The immediate (“explosive”) energy system provides energy rapidly but for only a short period of time. It is used to fuel activities that last for about 10 or fewer seconds such as weight lifting and shot-putting or in daily life just rising from a chair or picking up a bag of groceries. The components of this energy system include existing cellular ATP stores and creatine phosphate (CP), a chemical that cells can use to make ATP. CP levels deplete rapidly during exercise, so the maximum capacity of this energy system is reached within a few seconds. Cells must then switch to the other energy systems to restore levels of ATP and CP. (Without adequate ATP, muscles will stiffen and become unusable. )
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The Nonoxidative Energy System The nonoxidative (anaerobic) energy system is used at the start of an exercise session and for high-intensity activities lasting for about 10 seconds to 2 minutes, such as the 400-meter run. During daily activities, this system may be called on to help you run to catch a bus or dash up several flights of stairs. The nonoxidative energy system creates ATP by breaking down glucose and glycogen. This system doesn’t require oxygen, which is why it is sometimes referred to as the anaerobic system. This system’s capacity to produce energy is limited, but it can generate a great deal of ATP in a short period of time. For this reason, it is the most important energy system for very intense exercise. There are two key limitations to the nonoxidative energy system. First, the body’s supply of glucose and glycogen Weight lifting, picking up a bag of groceries 400-meter run, running up several flights of stairs 1500-meter run, 30-minute walk, standing in line for a long time For most activities, all three systems contribute to energy production; the duration and intensity of the activity determine which system predominates. Source: Adapted from Brooks, G. A., et al. 2005. Exercise Physiology: Human Bioenergetics and Its Applications, 4th ed. New York: McGraw-Hill. Copyright © 2005 The McGraw-Hill Companies. Reproduced with permission of The McGraw-Hill Companies. Is limited.