Your body operates off of three main energy systems. Your ATP-PCr system is involved in short-term anaerobic energy. Your glycolytic system produces energy through the breakdown of carbohydrates stored in your muscles and liver. Your aerobic system uses oxygen and fat to produce slow, yet long-lasting energy. During most types of exercise, your body uses some combination of all three energy systems at the same time, but may emphasize the use of one over another based on the needs of your body and the type of activity you are performing.
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The simplest of the three energy systems is your ATP-PCr system. ATP stands for adenosine triphosphate, which is the chemical form of raw energy in your body. PCr stands for phosphocreatine, which is a compound that is attached to every ATP molecule. Inside your skeletal muscles, when a muscle fiber receives a signal from a nerve to contract, the ATP-PCr molecules separate from one another as a result of a complex chemical reaction. This separation releases the energy that causes a muscle to contract. This energy system is primarily used during very short duration exercise lasting less than 10 seconds, such as a quick jump or a sprint.
The glycolytic energy system produces ATP through the breakdown of carbohydrates and sugars in your body. Glucose, or sugar, is stored in your liver and skeletal muscles. When needed, your muscles will break glucose down with the use of special enzymes and eventually convert sugar into ATP. The ATP is then used for muscle contraction. This system is best used for activities that take less than two minutes to perform at a high intensity. This includes sprints and other bouts of short yet intense exercise.
The aerobic system primarily uses fat for energy production but can also use stored carbohydrates and proteins. Aerobic means "with oxygen," meaning that oxygen is required in the process of breaking down fat stores for energy. Fat molecules are pulled from various stores around your body and converted to ATP through a complex chemical reaction that takes place inside your muscles. Your aerobic system yields large amounts of continuous energy and is ideal for long-duration activities such as long runs or bike rides.
Using All Three Energy Systems Together
Although each of the three energy systems works independently from one another, they may all be utilized by your body at the same time. The energy system that is utilized the most at any given time is dependent on the intensity and duration of the exercise you are performing. All three energy systems will be turned on at the same time at the initiation of exercise; however, the recruitment of each energy system occurs when the current energy system that is primarily being used is depleted.