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A parallel-cascade system identification technique was used to measure dynamic elbow stiffness in hemiparetic stroke subjects. Modulation of intrinsic and reflex stiffness of elbow flexor muscles was studied by applying perturbations to the elbow at different initial joint angles with subjects' muscles relaxed. Intrinsic stiffness increased smoothly from mid-flexion to mid-extension and increased sharply at the extreme range of motion in both spastic and contralateral (control) sides. However, intrinsic stiffness was significantly larger in spastic than control side. Reflex stiffness increased monotonically from full flexion to mid-extension then decreased at full extension in both sides. However, reflex stiffness was significantly larger on the spastic than control side; the difference reached its maximum at the middle of elbow extension. The intrinsic stiffness estimates were consistent in all stroke subjects, whereas enhanced reflex stiffness was significant in only a portion of the stroke cohort. These findings suggest that enhanced intrinsic stiffness of passive tissues is a major mechanical abnormality in stroke subjects.