Cardiac and skeletal muscle myosins have the central role in contraction transducing ATP free energy into the mechanical work of moving actin in transduction/mechanical coupling. Inheritable cardiomyopathies are more frequently linked to myosin mutations than other sarcomeric proteins. Hereditary skeletal myopathies linked to myosin are less common. They lead to muscle weakness or affect myosin isoforms expressed during development leading to arthrogryposis syndromes. Myosin has a motor domain containing ATP and actin binding sites and light chains stabilized lever-arm that undergoes rotation impelling bound actin. The lever-arm converts torque generated in the motor into linear displacement (step-size). Relative myosin and actin filament sliding is modeled in vitro with a motility assay quantitating actin filament translation over a myosin coated surface. A novel quantum dot super-resolution in vitro motility assay confirmed a 5 nm step-size for fast skeletal myosin while β cardiac myosin (βMys) had multiple unitary steps, most frequently 5 and 8 nm, and a rare 3 nm displacement. The myosin lever-arm is stabilized by bound essential and regulatory light chains (ELC and RLC). RLC phosphorylation at S15 is linked to modified lever-arm mechanical characteristics contributing to disease and to myosin filament based contraction regulation. We have studied the effect of RLC phosphorylation on the step-size of porcine βMys. Phosphorylated βMys has ∼85% of the myosin phosphorylated. We find RLC phosphorylation causing the distribution of longest step increasing from 37% to 71%. This dramatic re-distribution of step-sizes provides significant gain in average step-size. The results indicate a mechanism for contraction regulation by step-size adaptation using post-translational modification of the myosin filament via RLC phosphorylation. Research supported by R01AR049277, R01HL095572 and the Mayo Foundation.