energy

In biology, the basis for conducting living processes. Much of life involves energy transfer. Energy is transferred from the surroundings of an organism into its body, and is also transferred within an organism's body. Energy is used by organisms to do things, such as growing or moving. When they do these things energy is transferred from one substance to another or from one place to another.

An important part of being fit is being able to use energy effectively during exercise to allow muscles to contract.