This basic science study demonstrates that calcium does not directly move muscle crossbridges toward thin filaments upon muscle activation.
Calcium does not reposition myosin heads in synthetic filaments; leaves open mechanisms of crossbridge activation in intact muscle.
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.
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Mendelson et al. (1976) studied this question.
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