What is preventing the myosin globular head from bonding to the active site of actin?

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Actin is a family of globular proteins that form microfilaments, one of the three fundamental components of the cytoskeleton of the cells of eukaryotic organisms (also called eukaryotes). It can be found as a monomer in free form, called actin G, or as part of linear polymers called microfilaments or actin F, which are essential for cellular functions as important as cell mobility and contraction during cell division.