In the skeletal muscle fibers, there are two main parts.
1. thick fragments, made of the protein molecule called myosin
2. thin fragments, made of the protein molecule called actin
When an action occurs, a biochemical reaction is set off by a nerve impulse that causes myosin and actin to stick together. Together, they pull the thick and thin filaments towards each other. When thousands of these filaments are pulled at once, muscle contraction will happen. Once the two molecules are stuck together, they cannot separate unless adenosine triphosphate(ATP) attaches to myosin and forces it to release. The body uses oxygen to make ATP. After death, there is no more oxygen, hence ATP cannot be made. The thick and thin filaments cannot slide away from each other, thus, the muscle remains contracted, causing rigor mortis.
process of muscle contraction
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