The site where myosin heads bind to actin in skeletal muscles are covered by

Tropomyosin Molecules: The actin filament also contains tropomyosin. Each molecule of tropomyosin has a molecular weight of 70,000 and a length of 40 nanometers. These molecules are wrapped spirally around the sides of the F-actin helix. In the resting state, the tropomyosin molecules lie on top of the active sites of the actin strands so that attraction cannot occur between the actin and myosin filaments to cause contraction.Ref: Guyton; 13th edition; Chapter 6; Contraction of Skeletal Muscle