CONTROL OF LIMB DYNAMICS IN NORMAL SUBJECTS AND PATIENTS WITHOUT PROPRIOCEPTION

1. We recently showed that patients lacking proprioceptive input from their limbs have particular difficulty performing multijoint movements . In a pantomimed slicing gesture requiring sharp reversals in hand pa th direction, patients showed large hand path distortions at movement reversals because of failure to coordinate the timing of the separate reversals at the shoulder and elbow joints. We hypothesized that these reversal errors resulted from uncompensated effects of inertial inter actions produced by changes in shoulder joint acceleration that were t ransferred to the elbow. We now rest this hypothesis and examine the r ole of proprioceptive input by comparing the motor performance of five normal subjects with that of two patients with large-fiber sensory ne uropathy. 2. Subjects were to trace each of six template lines present ed randomly on a computer screen by straight overlapping out-and-back movements of the hand an a digitizing tablet. The lines originated fro m a common starting position but were in different directions and had different lengths. Directions and lengths were adjusted so that tracin g movements would all require the same elbow excursion, whereas should er excursion would vary. The effects of varying interaction torques on elbow kinematics were then studied. The subject's dominant arm was su pported in the horizontal plane by a low-inertia brace equipped with b all bearing joints and potentiometers under the elbow and shoulder. Ha nd position was monitored by a magnetic pen attached to the brace 1 cm above a digitizing tablet and could be displayed as a screen cursor. Vision of the subject's arm was blocked and the screen cursor was blan ked at movement onset to prevent visual feedback during movement. Elbo w joint torques were calculated from joint angle recordings and compar ed with electromyographic recordings of elbow joint musculature. 3. In control subjects, outward and inward paths were straight and overlapp ed the template lines regardless of their direction. As prescribed by the task, elbow kinematics remained the same across movement direction s, whereas interaction torques varied substantially. The timing of the onsets of biceps activity and the offsets of triceps activity during elbow flexion varied systematically with direction-dependent changes i n interaction torques. Controls exploited or dampened these interactio n torques as needed to meet the kinematic demands of the task. 4. In c ontrast, the patients made characteristic errors at movement reversals that increased systematically across movement directions. These rever sal errors resulted from improper timing of elbow and shoulder joint r eversals. Instead of adapting biceps and triceps activity to direction -dependent changes in interaction torques, the patients cocontracted a ntagonists throughout the reversal phase. Although this may have incre ased joint stiffness the strategy was nor effective in controlling elb ow dynamics: elbow joint acceleration varied directly with the amplitu de of the interaction torques. Interaction torques, transferred to the elbow by upper arm deceleration, drove the elbow into flexion prematu rely. This decoupled the normally synchronous reversals at the shoulde r and elbow and resulted in large hand path distortions at movement re versals. 5. Our data indicate that interaction torques are normally co ntrolled through feedforward mechanisms and that this control is sever ely impaired in patients deprived of proprioception because of sensory neuropathy. We therefore conclude that proprioceptive information pla ys an important role in interjoint coordination during multijoint move ments. We hypothesize that information during movement serves to updat e an internal model of limb dynamics that is then used to program moto r commands.