Joint-Angle-Dependent Neuromuscular Dysfunctions at the Wrist in Persons After Stroke

Hu X, Tong K, Tsang VS, Song R. Joint-angle-dependent neuromuscular dysfunctions at the wrist in persons after stroke. Objective: To evaluate the joint-angle-dependent neuromuscular functions at the affected wrist in hemiplegic subjects after stroke while doing isometric maximal voluntary wrist flexion and extension across different wrist angles. Design: We investigated torques during isometric maximal voluntary wrist flexions and extensions at 8 different wrist angles, ranging from -45° to 60°. We used the associated electromyographic activities of 2 agonist and antagonist muscle pairs related to wrist and elbow joints for the analysis of muscular coactivations. We compared the data obtained from poststroke subjects' affected and unaffected sides. Setting: A research laboratory in a rehabilitation center. Participants: Eleven subjects with hemiplegia after stroke with passive range of motion (ROM) in the wrist from -45° to 60°. Interventions: Not applicable. Main Outcome Measures: Directly measured torques, torques after normalization during maximal isometric wrist contractions, and normalized moving average electromyographic signals of each muscle at the tested positions. Results: The measured torques of the affected wrists were significantly lower than those of the unaffected wrists at all tested angles during wrist flexion and extension (P<.05). The angle-dependent patterns of the normalized torque across the tested wrist angles varied from those of the unaffected wrists (2-way analysis of variance, P<.05). There were decreases in normalized torques during both flexion and extension at the extended positions in the affected group (P<.05). Abnormal cocontractions were found in agonist and antagonist muscle pairs related to wrist and elbow joints, and between the elbow flexor and wrist extensor when subjects did the wrist contractions on the paretic side, especially at the wrist extended positions. Conclusions: Wrist muscle weakness was distributed unevenly across the selected wrist ROM on the affected side, as represented by the varied patterns of the normalized torque-angle relationship, compared with the unaffected wrists. There were reductions in the selective control of muscle coactivating synergies both single-jointly and cross-jointly in the impaired nervous system during wrist contractions; the extent of these reductions was also related to the wrist angle configuration.