One has an affinity for actin, another for tropomyosin and a third for calcium. Troponin molecules are positioned along the actin-tropomyosin filaments and act to position the tropomyosin filaments over the active sites on the actin filaments. When calcium is present it binds to the troponin which changes in shape causing the movement of tropomyosin off the active sites so that myosin and actin can interact and muscle contraction can occur. When the active sites are uncovered the myosin heads bind to the sites which initiates a movement of the head toward the center of the sarcomere thus pulling the actin along and shortening the sarcomere. Each one of the myosin heads is thought to operate independently of the others, each attaching and pulling in a continuous alternating ratchet cycle until the calcium is removed and the active sites are covered up again.
Muscle contraction requires a great deal of energy.
Energy is required to break the bond between the myosin head and the actin active sites as well as for removal of calcium from the cytoplasm by the use of a special pump within the sarcoplasmic reticulum. The breakdown of ATP to ADP releases the head from the actin filament and cocks it for the next ratchet power stroke. Energy Sources Energy is required for muscle contraction. At rest and during light exercise, muscles use lipids as their energy source. The use of carbohydrate becomes more important as the intensity of exercise increases.