In a resting muscle, the active binding sites on actin are masked by

1. Contraction requires myosin heads to bind actin. 2. In the resting state those binding sites are not available. 3. A subunit of troponin masks the active binding sites for myosin on the actin filaments. 4. Only when that masking is lifted can cross bridges form. 5. Calcium is what removes the masking rather than what causes it. _Source: syllabus Biology Ch17 'Locomotion and Movement'_