A: Spontaneous oscillations of a 1-µm bead attached to a single actin filament interacting with myosin motor molecules on a glass substrate. The bead is subjected to an elastic restoring force F exerted by a laser trap, that opposes the active force F_a produced by the motors (Plaçais et al, 2009). B: Active transport of 200-nm beads coated with heavy meromyosin-II motors in antiparallel networks of actin filaments. The beads move toward the center of the network, from left to right (red trajectories) or from right to left (blue trajectories), where they accumulate: the bead distribution is nearly flat when the beads are first detected (dashed line) but shows a sharp peak at steady state (solid line) (Richard et al, 2019).