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Energy usage


While most people know that aerobic exercise is good for the heart and that resistance training helps build lean body mass, most people do not fully understand how these two types of exercise affect how the body utilizes fat, carbohydrate, and protein to produce energy.

Basic energy systems

In general, there are three basic energy systems:
  • Phosphagen energy system (also known as the immediate energy system).
  • Glycolytic energy system (also known as nonoxidative or anaerobic system).
  • Mitochondrial respiration (also known as the oxidative or aerobic system).
Regardless of which energy system is used, the result is the production of adenosine triphosphate (ATP). ATP is extracted from the food we eat (fat, carbohydrate, and protein) and is required for biochemical reactions used in any muscle contraction. The intensity and duration of activity dictates which food types are broken down as well as which energy system predominates. However, no energy system acts alone. The relative contribution from each system depends on the intensity and duration of the activity.

The phosphagen system is active during all-out exercise that lasts about 5 to 10 seconds, such as a 100-meter dash, lifting a heavyweight, dashing up a flight of stairs, attacking with a flurry of techniques during sparring, or any other activity that involves a maximum, short burst of power. This system relies on stored ATP and to a larger extent, creatine phosphate, to provide immediate energy. For any maximal intensity exercise lasting longer than 10 seconds, assistance from other sources of energy is required.

The glycolytic energy system (also called glycolysis) involves the partial break down of glucose to a molecule called pyruvate. During this process, a relatively small amount of energy is produced. When oxygen demands exceed oxygen supply, pyruvate is converted to lactate. Under these circumstances, glycolysis is often referred to as “fast” or “anaerobic” glycolysis. Anaerobic glycolysis is a key contributor to total energy requirements for moderate to high-intensity exercise lasting about one to two minutes, such as a round of sparring.

Although this system can provide a rapid source of energy, it is only about half as fast as the phosphagen system. When there is enough oxygen to meet oxygen demands of an activity, such as during prolonged light to moderate intensity hand target drills, glycolysis proceeds much slower and the pyruvate that is formed participates in the formation of additional energy via aerobic processes (see Aerobic System discussion below). In this case, glycolysis is sometimes referred to as “aerobic” or “slow” glycolysis.

Mitochondrial respiration refers to aerobic processes that occur when there is sufficient oxygen to meet the oxygen demands of any activity. The aerobic energy system comes into play during any exercise lasting longer than one minute, and as the duration of exercise goes beyond several minutes, aerobic energy production becomes increasingly more important. Aerobic energy production is much slower than both the phosphagen and anaerobic glycolytic energy systems, but it has the benefit of being able to provide almost unlimited energy, as long as nutrients are available. Examples of activities that rely primarily on aerobic energy production include a 10,000-meter run, cross-country skiing, and a 1,000-meter swim.

Predominate energy system

We often think of low to moderate-intensity aerobic exercise as a good way to burn a significant amount of fat. While this is true, aerobic energy can be derived from carbohydrates and to a much smaller extent, protein. Most people do not realize that even during light to moderate exercise, carbohydrates can provide up to 40 to 60 percent of total energy requirements. In contrast, protein is not a preferred source of energy during any form of exercise (assuming an adequate diet) and generally contributes less than 10 percent of total energy requirements.

One of the most effective methods of determining the predominant energy system during a specific form of exercise is by monitoring your heart rate. Heart rate monitoring can help you determine the intensity of your workout as well as estimate the heart rate at which you transition from aerobic to anaerobic exercise (i.e., from carbohydrate and fat usage to predominantly carbohydrate). While the transition point differs from person to person, you can get a general idea of where you transition from aerobic to anaerobic exercise by watching for substantial increases in heart rate, muscle fatigue, or in breathing depth and frequency.

If you are truly engaging in anaerobic exercise, you will not be able to sustain the intensity of the exercise for longer than about one to two minutes. If you notice your intensity dropping off, you were probably performing an anaerobic exercise. In contrast, if you can sustain your exercise intensity longer than about two minutes, you are probably exercising aerobically. As your fitness improves, you will be able to perform higher intensity exercise for longer periods.

Forms of training

There are numerous forms of resistance training, each of which will result in specific fitness benefits. Powerlifting, for example, involves short bouts, on the order of 3 to 10 seconds, of maximum effort repetitions. This type of lifting primarily engages the phosphagen energy system and is useful for improving muscle strength and power. In contrast, if your goal is to increase muscular endurance, then higher repetition resistance training with lighter weights is appropriate. This may include 8 to 20 repetitions per set at about 70 to 80 percent of your maximum capacity. Fewer reps and higher intensity will rely more on anaerobic energy, whereas higher repetition, lower intensity training may invoke aerobic energy production.

Keep in mind that although resistance training does not necessarily burn a significant number of calories, it can provide significant health and fitness benefits. Not only does resistance training increase lean body mass (i.e., muscle), which burns more calories than fat even while at rest, engaging in regular resistance training can have positive effects on elements such as cholesterol, glucose metabolism, and bone density, to name a few.

Circuit training is sometimes considered a type of resistance training, but it is a compromise between resistance training and cardiovascular training. Essentially, circuit training can improve muscle endurance as well as provide modest gains in aerobic capacity. Because it is generally a low to moderate intensity workout that is sustained for an hour or more, circuit training is primarily an aerobic activity.

“Aerobic” exercise is typically touted as a great way to burn a lot of fat. While this is not necessarily incorrect, it can be misleading. For example, at about 25 percent of aerobic capacity (i.e., low-intensity exercise), fat is the primary source of fuel, but you are not burning a significant number of calories. If your goal is to lose weight, the key consideration is the net deficit in calories, not where the calories come from. As exercise intensity increases, the number of calories burned also increases. Therefore, while it is true that fat contributes a greater percentage of the total energy during lower-intensity exercise, at higher intensity exercise, the total quantity of fat utilized may be greater for exercise performed for an equivalent period.

If you do not have a specific goal in mind, but simply want to improve your overall health, the American College of Sports Medicine recommends moderate-intensity physical activity performed for at least 20 to 30 minutes, excluding time spent warming up and cooling down, 3 to 5 times a week. If, on the other hand, you are training for some type of competitive event, make sure that your training program emphasizes the type of activity involved in that event. For example, if you are training for high-rank testing where you will have to spar many rounds against fresh opponents, engaging in a powerlifting training program three days a week will not make the best use of your time. You need to engage actively in running, biking, and swimming.

Finally, if your goal is to lose weight, a caloric deficit is key. You should aim for a caloric deficit of about 500 calories a day through decreased energy intake, increased energy expenditure, or a combination of the two. Although numerous types of exercise are effective for weight loss, a combination of regular aerobic exercise and resistance training is a good place to start.


  • Smith, J. (2003). Energy Usage During Exercise: How It Affects Your Workout.
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