Individual k-fibers lengthen under applied force, related to Fig. 2 . (A and B) Additional representative time-lapse images of spindle and k-fiber (GFP-tubulin, white) movement and remodeling in response to sustained force from a microneedle (Alexa 555, yellow) as in Fig. 2. (C) Plot of k-fiber length oscillations over time in neighboring middle k-fibers (blue) during the manipulation of the neighboring outer k-fiber (black). (D) The average duration of the lengthening phase of the k-fiber is longer for the manipulated outer k-fibers (black; n = 21 growth events in 18 cells) than for the unmanipulated neighboring middle k-fibers (blue; n = 17 growth events in 13 cells; plot shows mean ± SEM, P = 0.02, two-sided Mann–Whitney U test). Lengthening phases are scored as three or more consecutive time points with increasing k-fiber length.