Deficits in the coordination of multijoint arm movements in patients with hemiparesis: evidence for disturbed control of limb dynamics

This study provides a detailed analysis of disturbances in the kinematics a nd dynamics of the acceleration phase of multijoint arm movements in six pa tients with chronic hemiparesis. Movements of the dominant and nondominant limbs were also examined in three control subjects. Subjects performed rapi d movements from a central starting point to 16 targets located equidistant ly around the circumference of a circle. Support of the upper limb was prov ided by an air-bearing apparatus, which allowed very low friction movements in the horizontal plane. We found that patients retained the capacity to m odulate, in response to target direction, the initial direction of movement s performed with the paretic limb. However, in comparison to the nonparetic limb or control subjects, movements of the paretic limb were misdirected s ystematically. An inverse dynamics analysis revealed an abnormal spatial tu ning of the muscle torque at the elbow used to initiate movements of the pa retic limb. Based on electromyographic recordings, similar spatial abnormal ities were also apparent in the initial activations of elbow muscles. We ar gue that these spatial abnormalities result from a systematic disturbance i n the control signal to limb muscles that cannot be attributed to previousl y identified mechanisms such as weakness, spasticity mediated restraint, or stereotypic muscle activation patterns (muscle synergies). Instead, our an alysis of movement dynamics and simulation studies demonstrate that the spa tial abnormalities are consistent with an impaired feedforward control of t he passive interaction torques which arise during multijoint movements. Thi s impaired control is hypothesized to reflect a degradation of the internal representation of limb dynamics that occurs either as a primary consequenc e of brain injury or secondary to disuse. (C) Springer-Verlag 2000.