Analysis of the relationship between stress and actin velocity. (A) Plot of forward projection of the stress versus forward projection of F-actin speed for all positions in the central region of the cell. Positions within the front half of this region are shown in black, and positions within the back half are shown in red. The inset shows boundaries of the central region, its subdivision into the front and back halves, and the direction to which velocity and stress were projected (arrow). (B) Plot of lateral projection of stress versus lateral projection of F-actin velocity for all positions in the frontal region of the cell. Positions on the left from the cell symmetry axis are shown in red, and positions on the right of it are shown in black. A positive sign was assigned to centripetal velocity and stress. The inset graphic is as in A. Each cloud in A and B was fitted using the least squares method with a linear function σ = k v + σ₀, where k is the slope and σ₀ is a constant. (C) Box plots of k for the data corresponding to the forward stress at the front (k_f) and the back (k_b) of the cell and the lateral centripetal stress for the left and right halves of the cell mixed together (k_lr), the left (k_l), and the right (k_r) halves of the cell. (D) Box plots of σ₀ for the data corresponding to forward stress at the front (σ_0f), the back (σ_0b), the left and right mixed together (σ_0lr), the left (σ_0l), and the right (σ_0r). Box and whisker plots in all figures indicate the 25% (lower bound), median (middle line), and 75% (upper bound) nearest observations within 1.5 times the interquartile range (whiskers), 95% confidence interval of the median (notches), and outliers (+). The analysis of front–back and lateral components of actin velocity and stress was performed for 12 matched pairs of velocity and stress maps that were derived from video sequences of five different migrating cells.