Figure 1. Active motor-filament bundle dynamics involves a competition between contraction and extension.

(a) Motors bind filaments and move towards their plus ends. (b) This motion results in local contraction or extension depending on the local arrangement of the filaments. (c) In a full bundle, a given filament arrangement can generate contraction or extension depending on the localization of the motors and crosslinks. The present work shows that motor and crosslink self-organization can bring about either outcome.