Using Energy Systems for Physical Activity

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  • 0:00 Energy
  • 0:53 Phosphagen System
  • 1:26 Anaerobic System
  • 2:28 Aerobic System
  • 4:24 Food As Fuel
  • 5:42 Lesson Summary
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Lesson Transcript
Instructor: John Hamilton

John has tutored algebra and SAT Prep and has a B.A. degree with a major in psychology and a minor in mathematics from Christopher Newport University.

We all need energy to go about our daily lives and accomplish the goals that we set each day. However, many of us do not know how our body energy systems work. In this lesson we review ways energy systems are used for physical activity.


If you are like most people, there are not enough hours in the day, and you do not always have the energy you need to accomplish everything. We need energy for the things we do. There are three major energy systems the body utilizes:

  1. Aerobic system
  2. Anaerobic (lactate) system
  3. Phosphagen system (immediate use)

These three systems work together, often simultaneously, and the process is like trying to solve a difficult calculus equation: quite complex, to say the least. A substance known as ATP (Adenosine Triphosphate) is sort of the building block that aids in human movement. It should be noted that the phosphagen system is anaerobic as well. The energy systems are not independent, and which one dominates depends on two main factors: the time spent exercising and intensity.

Phosphagen System

Let us begin with the system the body goes to first. Your basal metabolic rate (BMR) is how much energy you expend at rest. This is important to know because it can be increased by anaerobic exercise. The phosphagen system is used for very short bursts of energy of a few seconds, such as throwing an object far or sprinting on a run in football. Your body uses a substance called creatine phosphate (PCr) for these short periods. However, the body does not have much of this substance, so then it calls on the carbs of the anaerobic system.

Anaerobic System

So now that the phosphagen system has worked for short bursts of energy, the body moves on to the second system, which is the anaerobic system, meaning without oxygen. Have you ever watched a 400-meter race (one lap) and cheered as the runners tightened up in the straightaway and did their best to finish? There is a reason for this phenomenon. Lactic acid is a substance that builds up in the runners' bodies after about 36 seconds, but the race itself takes over 40 seconds. Lactic acid is a byproduct of anaerobic, meaning without oxygen, activity.

Okay, now let us describe what happens when you run a hard lap.

  1. Your body uses glycolysis, the breaking down of glucose (simple sugars) for fuel.
  2. Your body removes some lactic acid as more is being produced.
  3. Your body falls behind and cannot keep up with the removal.
  4. Your body starts to slow down as the lactate threshold is reached.

By training properly, athletes can delay the lactate threshold and run faster.

Aerobic System

Finally, the body can utilize the aerobic system, which is the most complex of the three systems and utilizes oxygen. An athlete may sustain an aerobic activity as a general rule much longer than an anaerobic activity. For example, a runner could go for a moderately paced run or take an aerobics class for a full hour. However, a sprinter might get winded and have to rest after a few full out sprints of 100 meters. The aerobic system uses three items for fuel:

  1. adipose tissue (used for fat storage) and intramuscular fat
  2. blood glucose (a simple sugar)
  3. liver and muscle glycogen (carbohydrate stores)

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