Abnormal intrinsic and reflex stiffness related to impaired voluntary movement

We studied the relationship between mechanical abnormalities associated with spasticity and 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 contribution 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. These movements were quantified by measuring their kinematics parameters. The correlation coefficient was measured to determine the relationship between abnormal modulation of intrinsic and reflex stiffness as function of joint position with the kinematics parameters. We found that both intrinsic and reflex stiffness were significantly larger in stroke than control sides and were strongly position dependent, increasing with elbow extension. Abnormal modulation of intrinsic and reflex stiffness with position (slope) was correlated with an increase in duration of movement (DM), and a decrease in peak-velocity (Pv), peak-acceleration (Pa) and maximum voluntary contraction (MVC). Weakness, quantified as a decrease in MVC, was also correlated with the reduction in Pv, Pa and active range of motion (AROM). These findings demonstrate that abnormal modulation of both intrinsic and reflex stiffness with position are related to antagonist muscle weakness that may cause stroke patients to move slower and take longer to complete reaching tasks.