Training requires a great deal of muscular exertion. This means that throughout exercise the muscles are continually contracting and relaxing. This muscular exertion requires energy. Energy comes from a substance called adenosine triphosphate (ATP) which, during exercise, breaks down to a second substance called adenosine diphosphate (ADP) and this produces energy.

The energy that is stored in ATP comes from the reaction between glucose and oxygen.

KEY POINT - ATP is stored in muscle fibres but is used up quickly.