What filaments that move to shorten muscle according to sliding filament theory of muscle contraction?

The filament that moves to shorten a muscle during contraction is actin.

Actin is a thin protein filament that slides inward during the contraction process. According to the sliding filament theory, the thick filament called myosin stays mostly in place and forms cross-bridges with actin. These cross-bridges pull the actin filaments toward the center of the sarcomere. As actin slides inward, the sarcomere becomes shorter which leads to overall muscle shortening. This means that actin is the filament that actually changes position during contraction. Myosin only helps in pulling actin by using ATP but it does not move itself. That is why actin is the correct answer for the filament responsible for shortening the muscle.

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