Figure 1.

Computation of Fv relationships. (a) Power spectrum density of forces measured far from the lamellipodium (red trace) and when the lamellipodium pushed the bead (blue trace), computed from the red and blue traces, respectively, shown in the inset. Green, pink, and black arrows indicate 0.2, 1, and 10 Hz, respectively. (b) Fv relationships computed with Gaussian filtering at 0.2 Hz (green) and linear regression with W = 10,000 (gray), as described in Materials and Methods. The green dotted line represents −3σ_v at the 0.2-Hz bandwidth (where σ_v was calculated from bead fluctuations measured away from the lamellipodia). (c) Relationship between the standard deviations of velocity distribution as a function of smoothing for two values of the trap stiffness, 0.005 pN/nm (squares) and 0.045 pN/nm (circles).