In the sliding filament model, two myofilaments - actin and myosin must slide past each other to shorten the sarcomere, thus producing the contraction. 

Myosin has myosin heads, proteins that always want to bind to the actin to form cross-bridges. In order to prevent constant contraction, tropomyosin is a protein that exists between the actin and myosin. 

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For a muscle contraction to occur, several steps must be accomplished:

1. the tropomyosin must be removed by another protein, troponin, which is activated by calcium cations released from the sarcoplasmic reticulum. 

2. The third phosphate must be removed from adenosine triphosphate to release energy

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