Fuel Used During Exercise

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Before discussing the various exercise guidelines in the following chapters, here is an overview of the energy systems used during exercise. Your body uses the macronutrients you eat (CHO, fats, and proteins) to make a chemical called adenosine triphosphate (ATP). You need ATP to contract your muscles during exercise. ATP can be made two ways. One way makes ATP without using oxygen and is called the anaerobic energy system. The second way requires oxygen to make ATP and is called the aerobic energy system. Both of these systems (described below) are required during activity but, depending on the activity, there is a greater reliance on one system over the other.

Anaerobic Energy System

Activities that depend largely on this energy system last less than 5 minutes or have frequent rest periods. Examples include weight lifting, sprinting, and some interval training routines.

You may have heard about two of the chemicals involved in the production of ATP without oxygen: creatine phosphate and lactic acid. Creatine phosphate is present in the muscles and is used to make ATP rapidly. Creatine phosphate can make enough ATP to last for 30 seconds worth of exercise. To try and increase the amount of ATP that can be made from creatine phosphate, some people take creatine supplements. However, the research is not conclusive as to the benefits of taking creatine and the long-term risks are not known (see Chapter 14). Furthermore, your body makes creatine and it is obtained in the diet from meats. The second chemical is lactic acid. When maximal or near maximal exercise continues beyond 30 seconds, the muscle must use glucose (a simple CHO) to produce ATP. During this anaerobic energy process, the by-product lactic acid is formed. Small amounts of lactic acid can be converted back into glucose and then broken down again to form more ATP. However, as exercise continues, lactic acid begins to accumulate in the muscles and the blood, and you begin to fatigue. If maximal exercise is sustained, fatigue is inevitable within 3-5 minutes.

Aerobic Energy System

When moderate exercise continues beyond a couple of minutes, the aerobic energy system is activated to make ATP. Glucose and fats are used to make ATP in the presence of oxygen. The aerobic energy system, which produces much more ATP than the anaerobic energy system, is the primary system used during exercise lasting longer than five minutes; such as a 5K run, a 30 minute walk, or a 500 meter swim.

During most types of exercise, both the aerobic and anaerobic energy systems are involved. The amount of energy from each system depends on the duration and intensity of the exercise. Figure 4-3 illustrates the percentage of ATP each energy system contributes during exercise of various durations. As shown, when exercise duration increases there is a shift from greater use of anaerobic energy to aerobic energy.

Figure 4-3. Energy Use During Exercise

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Figure 4-3. Energy Use During Exercise

From Aerobic Energy Production

Anaerobic Energy Production

From Aerobic Energy Production

Anaerobic Energy Production

Exercise Time (minutes)

Powers S. (1993) Fundamentals of Exercise Metabolism. In Durstine JL et al (Eds). ACSM's Resource Manual for Guidelines for Exercise Testing and Prescription, 2nd ed. (p.61) Baltimore: Lea & Febiger

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