Position-dependent torque coupling and associated muscle activation in the hemiparetic upper extremity

Abstract

Previous studies have demonstrated abnormal joint torque coupling and associated muscle coactivations of the upper extremity in individuals with unilateral stroke. We investigated the effect of upper limb configuration on the expression of the well-documented patterns of shoulder abduction/elbow flexion and shoulder adduction/elbow extension. Maximal isometric shoulder and elbow torques were measured in stroke subjects in four different arm configurations. Additionally, an isometric combined torque task was completed where subjects were required to maintain various levels of shoulder abduction/adduction torque while attempting to maximize elbow flexion or extension torque. The dominant abduction/elbow flexion pattern was insensitive to changes in limb configuration while the elbow extension component of the adduction/extension pattern changed to elbow flexion at smaller shoulder abduction angles. This effect was not present in control subjects without stroke. The reversal of the torque-coupling pattern could not be explained by mechanical factors such as muscle length changes or muscle strength imbalances across the elbow joint. Potential neural mechanisms underlying the sensitivity of the adduction/elbow extension pattern to different somatosensory input resultant from changes in limb configuration are discussed along with the implications for future research.

Fig. 1

Image of the experimental setup in position 2. a Forearm interface plate mounted directly to load cell. b Fiberglass cast. c 6-DOF load cell. d Delrin cast interface ring. e Video feedback monitor

Fig. 2

Mean with standard error bars of normalized elbow flexion torque (a) and extension torque (b) during various percentages of maximum shoulder adduction in positions 2 and 4. Subjects were able to generate more elbow flexion and less elbow extension during adduction in position 4

Fig. 3

Mean with standard error bars of normalized elbow flexor and extensor EMG occurring during the adduction/elbow extension task and adduction/elbow flexion task. The graphs are organized by muscle group (flexors, BRD and BIC; extensors, TRILT and TRILH) and task such that the effects of position (P2 position 2, P4 position 4) and adduction level (25, 50, and 75% of maximum adduction) can be visually appreciated. During the adduction/elbow flexion task, there was an effect of position and adduction level for the BIC (a) and an effect of level and position on the TRILH (b). During the adduction/elbow extension task, there was an effect of position for both elbow extensors (d) but not for the flexors (c). These EMG data reflect the torque data

Fig. 4

Mean with standard error bars of normalized elbow extension (−) and flexion (+) torque during various percentages of required adduction in positions 2 and 4 for control subjects. Individuals without stroke were able to generate near maximum elbow torques regardless of required adduction level