Action needs ATP. ATP is the coenzyme, which is used by the cells to transfer energy to the cell when a muscle cell demands energy to contract. ATP is short for Adenosine Triphosphate and is critical for muscles to contract. APT prepares the myosin to bind with actin - it moves the myosin to a high energy state ready to move. The myosin is the protein that converts the chemical energy, ATP, into mechanical energy. This cycle keeps on repeating itself and the muscle expands and contracts.