FB model description. (A) Forces exerted on a (right) sister KT and a single kMT (note, each KT binds a bundle of kMTs, or k-fiber). (B) Close-up of the KT–MT interface and the viscoelastic Ndc80 complexes. (C) Mechanical properties of cohesin and Ndc80 complexes and positions of the kMT plus end, the KTs, and the Ndc80 bonds along the pole–pole axis. (D) Model equations.