A parallel-cascade system identification technique was used to measure dynamic elbow stiffness in spinal cord injured (SCI), hemiparetic stroke and normal 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. We found that reflex stiffness was similar in both spastic SCI and stroke subjects and was significantly larger than in normal subjects. The difference reached its maximum at the middle of elbow extension. Intrinsic stiffness was larger in SCI than normal subjects, whereas it was within normal range in stroke arms. The mechanical properties of the contralateral arms in stroke subjects were not normal; reflex stiffness was larger and intrinsic stiffness was lower. Both reflex and intrinsic stiffness were strongly position dependent; they both increased with increasing elbow extension. These findings demonstrate that increased reflex stiffness is the major mechanical abnormalities in SCI and stroke whereas enhanced intrinsic stiffness is important only in SCI subjects. Moreover, the contralateral side of stroke subjects is also affected.