Predicted experimental outcomes of single needle experiments for different types of matrices assuming flux forces can be transmitted to the needle by lateral microtubule cross-links. (A–C) Thin red lines represent the matrix, whereas green lines represent spindle microtubules. Thicker sections along the microtubules depict the movement of a photoactivated mark on the microtubule lattice to illustrate poleward flux over time. The blue shapes within the spindle midzone represent aligned metaphase chromosomes. The dashed line runs through the midpoint of the stationary needle tip, as seen in cross-section (depicted as a white annulus with a filled, black center). See “Does a spindle matrix play a mechanical role in spindle length regulation?” for explanations of each cartoon.