Calcium initiates a skeletal muscle contraction by

During excitation-contraction coupling in a skeletal muscle fiber, calcium is released from the sarcoplasmic reticulum (not the sarcolemma) and binds to troponin (not tropomyosin or myosin). The binding of calcium to the troponin causes the troponin to shift the position of the tropomyosin to which it is attached, thus uncovering the actin binding sites for binding with myosin.