How do Muscles Contract?
When a nerve signals a skeletal muscle to contract, calcium is released inside the muscle fibers. The presence of calcium causes attachments to form between the myosin and actin filaments. The actin filaments are pulled inward toward the center of each sarcomere, shortening the sarcomere. As the attachment is formed between myosin and actin, ATP is broken down to provide the energy for the muscle contraction. When the muscle relaxes, the filaments slide back into their original positions.