Running head: PASSIVE ANKLE STIFFNESS IN CHRONIC STROKE 1 Measurement of Passive Ankle Stiffness in Subjects with Chronic Hemiparesis

36 Our objective in this study was to assess passive mechanical stiffness in the ankle of chronic hemiparetic stroke 37 survivors and to compare it with those of healthy young and older (age-matched) individuals. Given the importance 38 of the ankle during locomotion, an accurate estimate of passive ankle stiffness would be valuable for locomotor 39 rehabilitation, potentially providing a measure of recovery and a quantitative basis to design treatment protocols. 40 Using a novel ankle robot, we characterized passive ankle stiffness both in sagittal and in frontal planes by applying 41 perturbations to the ankle joint over the entire range of motion with subjects in a relaxed state. We found that 42 passive stiffness of the affected ankle joint was significantly higher in chronic stroke survivors than in healthy adults 43 of a similar cohort, both in the sagittal as well as frontal plane of movement in three out of four directions tested 44 with indistinguishable stiffness values in plantarflexion direction. Our findings are comparable to the literature, thus 45 indicating its plausibility and, to our knowledge, report for the first time passive stiffness in the frontal plane for 46 persons with chronic stroke and older healthy adults. 47 48 49 50 51 52 53 54