Muscle crossbridges: absence of direct effect of calcium on movement away from the thick filaments.

Fluorescence depolarization experiments show that the rotary mobility of myosin heads is hindered by the assembly of the thick filament. Calcium, with or without magnesium adenosine triphosphate, dose not alter this hindrance in synthetic filaments. This implies that calcium does not directly move the crossbridges toward thin filaments on activation of muscle.