Bionic Power produces one of the few energy harvesting exoskeletons in the world. Part of the idea is that the natural swing of the leg at the knee requires the leg to come to a stop. Why not harvest this energy?
The PowerWalk® Kinetic Energy Harvester appears to be surprisingly small and flexible.
In theory, the device and technology could be reversed to apply force(s) on the user, either to propel or brace them.