The Source of Energy of a Muscle Contraction

Woman Bodybuilder Doing Exercise For Legs

Your muscles depend on metabolic energy systems such as the phosphagen system, glycolysis and the aerobic system to convert fuel sources into units of usable energy needed to produce locomotion of your skeletal muscles and to power varying durations of activity. Each of these systems is simultaneously active; however, the predominant energy system is determined by the type of physical activity you are performing.

Phosphagen System

Like all energy systems, the phosphagen system requires the breakdown of a chemical energy component known as adenosine triphosphate. Within the phosphagen system a series of reactions occurs that breaks down creatine phosphate into ATP. Due to limited amounts of CP stores in your skeletal muscles, the phosphagen system is dominant in very intense, short-duration activities, about 10 to 15 seconds, such as power lifting or shot put.


Glycolysis generates energy by breaking down glucose and glycogen. Glucose is essentially your blood sugar, while glycogen is a stored form of glucose within your muscles and liver. This energy system is made up of several more reactions than that of the phosphagen system and involves the breaking down of carbohydrates in order to rebuild ATP molecules that fuel muscle contractions. Glycolysis requires the use of energy in order to create greater amounts of energy. Because this system is a cycle, it can act as the dominant fuel source for longer bouts of activity lasting from approximately 30 to 45 seconds to upwards of two to three minutes, such as the 200- or 400-meter sprint.

Aerobic System

The aerobic system can be further divided into two categories: aerobic lipolysis or aerobic glycolysis. During aerobic lipolysis, fat is the main source of fuel, whereas during aerobic glycolysis, carbohydrates are the main source of fuel. This system contains a greater number of reactions than either the phosphagen system or glycolysis; it generates lots of ATP but takes longer to do so. Therefore, the aerobic system is the dominant energy system during prolonged cardiovascular activities. Aerobic glycolysis is favored during long-duration, intense events upwards of two to three hours, such as an 8K or 10K race. Aerobic lipolysis is favored during long-duration, lower intensity events such as marathons or ultramarathons.

Fuel Sources

To provide your body with healthy types of fuel, the U.S. Departments of Agriculture and Health and Human Services recommend focusing your diet around healthy carbohydrates, lean proteins and good fats. Healthy carbohydrate choices include whole grains, fresh fruits and vegetables. Choose lean meats and low-fat dairy options as your sources of protein, and nuts and fish for your healthy fat choices.