These fibers are made up of myofibrils, which are composed of myofilaments. Skeletal muscle contraction requires huge amounts of ATP to power the return of calcium and to break the connection between the actin and myosin. A nerve cell and all its connecting muscle fibers constitute a motor unit. A single stimulation causes a twitch. Interacting muscles, joints, and bones form biological lever systems.
Regular exercise strengthens the muscular system and enables it to use energy efficiently.
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