Fig. 2. (A) Illustration of force-clamp records. An optically trapped bead, decorated with myosin V molecules, is moved into close proximity with a surface-mounted actin filament. The bead is subjected to a force-clamp, in which a feedback circuit maintains a constant trap-bead separation and hence constant system tension (left). In some cases, the bead proceeded to step along the actin filament in 36-nm increments (right) as the trap followed. (B) The fraction of beads observed to step as shown in A, as a function of surface density. The solid line reflects a fit of a single molecule model; the dotted line reflects a model posing two molecules as a minimal agent of such stepping behavior. Reduced ² values for the one- (0.04) and two-molecule (0.98) models rendered relative confidence in the former 25 times that in the latter. Figure reproduced, with permission, from Rief et al., 2000.