They were designed to create electrical impulses in damaged tissue, or route weak nervous signals around nonfunctional tissue. Although they were very sophisticated devices, it could take several days and even weeks to master their use and even then, patients rarely recovered one hundred percent of their mobility.
The Zalkonian named John Doe used motor assist bands during his rehabilitation in late 2366. He had much success with the bands due to his extraordinary physiology. (TNG: "Transfigurations") Lieutenant Worf also used the bands following his spinal injury in 2368, but he had little patience for learning how to use them. He contemplated suicide before turning to the risky genitronic replicator as treatment. (TNG: "Ethics")