When you go to exercise, you use energy and this energy must come from somewhere. During the first few seconds of exercise, the energy comes from your body’s stored fuels. Depending on the type of exercise, the primary source of fuel for your body can vary.

Adenosine Triphosphate

Adenosine triphosphate (ATP) is the first source of energy used for any activity. It is made from three phosphate molecules attached to an adenosine molecule. ATP is stored in the muscle cells and is used to power muscle contractions, like those used when lifting weights or moving quickly.

Creatine Phosphate

Creatine phosphate is also stored in the muscles and is the primary fuel used for short bursts of exercise. It comes from a molecule of creatine attached to three phosphate molecules. When ATP is used, the phosphate molecules break off and bond with the creatine, creating ATP and ADP. This reaction releases energy to fuel your muscles for the short bursts of activity.

Glycogen

Glycogen is a complex carbohydrate stored in the muscles and liver. During exercise, glycogen is broken down into glucose molecules and used to fuel the body’s cells. When you first start to exercise, glycogen is the primary source of fuel. As the intensity of exercise increases, ATP is used instead. The body can store a limited amount of glycogen, so if you exercise for a long time it is important to maintain adequate levels of glycogen to keep going.

Fat

Fatty acids are broken down into compounds that can be used as energy by the body. During aerobic exercise, fat is the primary source of fuel. Although fat can provide a large volume of energy, the body breaks it down more slowly than glycogen and ATP. This means that when you start exercising, your body will use glycogen and ATP first, because they provide a rapid source of energy.

Conclusion

For the first few seconds of exercise, your body is able to use the energy stored in ATP, creatine phosphate, glycogen and fat to power your muscles. Knowing the source of fuel that is used for exercise can help you determine how best to fuel your body before, during and after your workout.