What is the role of Tropomyosin in myofibril contraction?

Tropomyosin helps to regulate myofibril contraction. Before contraction, tropomyosin covers the myosin binding sites on the actin molecule, preventing the myosin heads from binding. During contraction, calcium binds to troponin, causes conformational change in troponin. This pulls tropomyosin away from the myosin binding sites as troponin is attached to tropomyosin. This exposes myosin binding sites, allowing myosin to bind.