The cytoskeleton, despite comprising relatively few building blocks, drives an impressive variety of cellular phenomena ranging from cell division to motility. These building blocks include filaments such as microtubules and actin, motor proteins such as kinesins and myosins, and static crosslinkers. Outside of cells, these same components can form novel materials exhibiting active flows and nonequilibrium contraction or extension.

Reconstituted actin-myosin mixtures typically contract, and microtubule-kinesin mixtures typically extend along the filament axis. A longstanding puzzle is the microscopic origin of active stresses in motor-filament mixtures and the mechanisms underlying the balance between contraction and extension.  Using a minimal physical model of filaments, crosslinking motors, and static crosslinkers we dissect the microscopic mechanisms of stress generation. 

We demonstrate the essential role of filament steric interactions and develop a unified picture of active forces in motor-filament systems. With this insight, we are able to tunecontractile or extensile behavior through control of motor-driven filament sliding and crosslinking. Our results help explain why flowing reconstituted motor-filament mixtures are extensile while gelled systems arecontractile. This work provides a roadmap for engineering stresses in cytoskeletal active matter and a framework for understanding the cellular cytoskeleton.