It is customarily assumed that myosin motors act as independent force generators. Here, we tested this assumption by analyzing the fluctuation of the actin sliding movement in vitro over long native thick filaments from shell fish smooth muscle. This analysis is based on the prediction that the effective diffusion coefficient of actin, a measure of the fluctuation, is proportional to the inverse of the number of myosin motors generating the sliding movement of an actin filament, hence proportional to the inverse of the actin length, if the actions of the motors are independent. Contrary to this prediction, we found the effective diffusion coefficient to be independent of the actin length. This result shows that the myosin motors are not independent force generators when generating the continuous sliding movement of actin in vitro.