The stiffness of myosin subfragment-1 changes with the physiological state of muscle

► The anisotropy decay of spin-labeled S1 is due to rotation around the S1-S2 joint. ► The rotation is due to thermal energy and is opposed by viscous and elastic reactions. ► The rotation's amplitude is inversely proportional to the stiffness assigned to S1. ► Thus a different wobbling amplitude signals a different stiffness of S1.