1. Figure 3. Measurement of passive ankle stiffness using anklebot. (Reprinted with permission from Roy A, Krebs HI, Bever CT, Forrester LW, Macko RF, Hogan N. Measurement of passive ankle stiffness in subjects with chronic hemiparesis using a novel ankle robot. J Neurophysiol. 2011;105:2132–49.) (a) Commanded ramp-and-hold displacement perturbation (θcommand) of 15° in dorsiflexion (DF) with constant velocity (v) of 5°/s and hold time (thold) of 1 s. Raw traces of (b) ankle angle and (d) torque resulting from each commanded positional perturbation taken from single representative subject with stroke, shown with initial (θ0, τ0) and final (θ∞, τ∞) conditions for a single trace. (c) Steady-state torque (τstatic) and angular displacement (θstatic) data are obtained by anklebot “slowly” stretching subject’s ankle over passive range of motion in sagittal plane (right positive: DF, left negative: plantar flexion) and computing resultant net torque (τ∞–τ0) and angular displacement (θ∞–θ0) under static conditions. Data are then fitted with least-squares linear regression line in each direction within plane of movement, slope of which is estimate of passive ankle stiffness in that direction. Roy A, Forrester LW, Macko RF, Krebs HI. Changes in passive ankle stiffness and its effects on gait function in people with chronic stroke. J Rehabil Res Dev. 2013; 50(4):555–72.http://dx.doi.org/10.1682/JRRD.2011.10.0206 ResearcherID/ORCID: Anindo Roy, PhD: E-4312-2012