The document describes the process of muscle contraction. It involves calcium ions being released which bind to troponin, changing its shape and exposing myosin binding sites on actin filaments. The myosin heads then attach to these sites and use ATP to change position and slide the actin filaments, causing muscle contraction. The process then reverses as calcium is reabsorbed, troponin reverts and tropomyosin blocks sites, and ATP is regenerated by phosphocreatine to detach myosin heads and relax the muscle.