We studied the mechanisms underlying impairments in voluntary movements of the spastic joint in chronic, hemiparetic stroke subjects. System identification techniques were used to characterize the mechanical abnormalities of the elbow joint and to identify the contributions of intrinsic and reflex stiffness to these abnormalities. Repeated voluntary movements of the elbow from full flexion to extension at maximum speed were also conducted and were quantified by measuring their kinematics parameters. We found that both intrinsic and reflex stiffness were abnormally modulated with elbow position in the spastic arms. These abnormalities were correlated with the changes in kinematics parameters of the stroke arms; i.e., an increase in duration of movement, and a decrease in peak velocity, peak acceleration and maximum voluntary contraction (MVC). Weakness, quantified as a decrease in MVC, was also correlated with these changes in kinematics parameters as well as with the reduction in active range of motion. These findings demonstrate that abnormal modulation of both intrinsic and reflex stiffness with position are related to extensor muscle weakness that may cause stroke patients to move slower and take longer to complete reaching tasks.