Fig. 2

Lean-and-release postural perturbation. The participant leans forward so that approximately 10 % of body weight is supported by a cable attached to her back. At an unexpected time, the cable is released, causing the participant to start falling forward; a reactive step is required to regain stability. The research assistant stands close to provide assistance if the participant is unable to regain stability by stepping. A safety harness attached to an overhead support frame is worn, which prevents a fall to the floor. Panel a shows the ‘usual response’ condition where the participant is free to respond naturally. Panel b shows the ‘encouraged use’ condition; the preferred stepping limb (typically the non-paretic limb) is blocked, preventing step initiation with that limb and forcing use of the opposite limb to regain stability. (The individual shown is not a research participant. Consent was obtained for publication of the picture in this manuscript)