Joint position dependence of weakness during maximum isometric voluntary contractions in subjects with hemiparesis

Objective: To determine the distribution of weakness across elbow range of motion (ROM) in subjects with hemiparesis. Design: A detailed analysis of elbow torque and associated electromyographic signals of 5 prime elbow muscles generated during maximum isometric voluntary flexion (MIVF) and extension (MIVE) at 8 different elbow positions. Setting: Rehabilitation center research laboratory. Participants: Convenience samples of 5 controls and 10 subjects with hemiparesis with sufficient passive (>90°) and active (>60°) ROM on their paretic side. Interventions: Not applicable. Main Outcome Measures: Measured and normalized MIVF and MIVE torques and normalized moving average electromyographic signals of each muscle at each testing position. Results: Measured MIVF and MIVE torques generated by the hemiparetic group were marginally and significantly smaller than those of the control group (2-factor repeated-measures analysis of variance [ANOVA]: P=.053 for MIVF, P=.011 for MIVE). Distribution of weakness was nonuniform across elbow positions, as shown by normalized torque-position curves. Normalized MIVE torque of the hemiparetic group was significantly and marginally smaller than that of the control group at 15° and 30° (Student t test: P<.0001, P=.054), respectively. Although statistically not significant, the normalized MIVF torque of the hemiparetic group was slightly larger than that of the control group but became smaller than the control group's as the elbow flexed beyond 90°. Our electromyographic recordings supported the normalized MIVF torque findings, showing a significant increase in brachioradialis activation in the control group at flexed positions during MIVF (1-factor repeated-measure ANOVA, P=.003), but not in the hemiparetic group (P=.392). Conclusion: Our findings suggest that measuring the strength in multiple joint positions is useful for characterizing the basic changes in muscle activation strategies and properties and provides a relevant measure of elbow weakness from a clinical and functional perspective. Various mechanisms of action are discussed to better understand the relation between joint position and weakness.