Time evolution of the traction stresses and corresponding progressive emergence of a tension kymograph for a chemotaxing wild-type D. discoideum cell. The tension kymograph of a representative wild-type D. discoideum cell, shown on the left, is progressively built by adding the traction tension Tx at each instant of time while the instantaneous cell contour (black) moves upward with the centroid velocity V. Red and black lines indicate the position of the front and back cell edge. Motility has been split into cycles composed of four phases, based on the quasiperiodic oscillations of the cell length and by using phase statistics. All individual time points have been assigned to the four phases and are shown as colored dots superimposed on the red line showing the front cell edge (blue, protrusion; red, contraction; green, retraction; black, relaxation). The inset in the top left shows the oscillations of the strain energy, Us(t) (blue line), and the current value of Us(t) (red circle). The graph on the right shows the instantaneous magnitude of the cellular traction stresses ∣ τ⃗ ∣ together with the instantaneous outline of the cell (black contour). Time-lapse sequences of the chemotaxing cell were acquired every 4 s on an inverted light microscope (TE300; Nikon) controlled with MetaMorph software (Molecular Devices). Calculation of the traction stresses and tension and generation of the video images were performed using MATLAB (MathWorks). The video refers to the cell shown Fig. 3 A but includes additional time points. The playback rate is 40× real time (10 frames/s).
