Contraction

Contraction occurs by the mechanism of the sliding filament theory. This refers to the overlapping of actin and myosin protein filaments. Myosin heads bind to the myosin binding sites on the actin filament and push the actin filaments towards the centre of the sarcomere, this is the power stroke and results in muscle contraction. the myosine head will detatch from the actin binding site when ATP is present. The hydrolysis of ATP rotates the myosine head so that it is able to bind back to the actin filament. Rigamortis occurs due to no ATP being synthesised. Thus the myosin head doesn't detach from the actin filament and the muscles remain contracted. 

Calcium ions released by the Sarcoplasmic recticulum bind to troponin, causing tropomyosin to move away from the actin binding sites thus enabling the myosin heads with ADP already bound, to bind with the Actin binding site, enabling the power stroke to occur.