Inverse and direct correlations between traction stress and F-actin speed occur over similar ranges of F-actin speed despite perturbations to myosin II and Rho GTPase signaling. (A–D) Data ranges indicating the strongest inverse (blue) or direct (red) correlation between traction stress and F-actin speed (Fig. S3, available). Large blue and red arrows mark v_s and v_w, the F-actin speed delineating the upper and lower bounds, respectively, of the speed ranges. Gray symbols represent data outside the ranges; lines with slopes m_s and m_w show linear fits to data within the ranges. Small blue and red arrows mark σ_s and σ_w, the traction stress at v_s and v_w. Characteristic data are shown for control (A), blebbistatin-treated cells (B), and overexpression of CA-Rac (C) and CA-Rho (D). Mean slopes m_s (E, blue) and m_w (E, red), traction stresses σ_s (F, blue) and σ_w (F, red), and velocities v_s (G, blue) and v_w (G, red) for different conditions. In E–G, data reflects a mean of >1,000 data points for n = 3 cells. (H) Model for how F-actin dynamics are variably coupled to traction stress by initiation and assembly of FAs across the cell front; differently shaded regions reflect changes in the magnitude of traction stress measured at similar F-actin speeds for different conditions. Position within the cell front shown below; black bars represent FAs and crosshatching represents F-actin.