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 pull 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 a confromational change to the Tropomyosin which exposes the myosin binding sites on the actin thus enabling the myosin heads with ADP already bound, enabling the power stroke to occur.