Fig. 1.

Illustration of the sequence of events in the ultra-fast force clamp method. Changes between each frame are highlighted in yellow. A) An actin dumbbell is held under pretension load by two optical traps which can be independently controlled via force-feedback loops and beam steering devices (either AODs or EODs). B). The setpoint of the feedback loop for one bead (right) is increased by F, which causes a force to be exerted to the right, resulting in the motion of the dumbbell at a constant velocity (v) which creates hydrodynamic drag on the dumbbell that matches the applied force, F. The feedback loops work to maintain a constant force on both beads during the dumbbell’s motion. C) After a set excursion distance, the force setpoints are changed for both beads, with the applied force, F, now being applied to the left bead in the opposite direction. This results in motion of the dumbbell in the opposite direction. D)This cycle repeats until the myosin interacts with the actin filament, at which point the force is quickly transferred from hydrodynamic drag to the myosin molecule and the dumbbell’s motion stops within ~30-50 microseconds. This allows precise detection of the attachment time and the ability to study myosin under set loads from the beginning of its interaction with actin.