Objective: Individuals with a birth brachial plexus injury present with impairments in overhead reaching, as well as elbow flexion contractures with altered muscle lengths of the biceps. This study aimed to determined the relationship between elbow flexion postures and overhead reaching in individuals with birth brachial plexus injuries by investigating how changes in elbow posture and muscle fiber length impact force-generating capacity of the biceps on the elbow and shoulder simultaneously. Methods: An experimental and sensitivitiy analysis. Ten subjects participated in the experimental study, in a repeated measures design. The sensitivity analysis utilized an upper extremity musculoskeletal model via OpenSim software. Results: Overhead reaching improved in the birth brachial plexus population when the elbow was immobilized in less flexed postures. Sensitivity analysis revealed peak shoulder elevation moments in less flexed postures in the altered long head of the biceps muscle length of 30% compared to the typical length. As such, both studies indicate and increased passive force in the altered muscle length of the biceps contributes to higher shoulder elevation in the BBPI population. Muscle activity of the biceps and triceps demonstrated a direct relationship with increased overhead reaching. However, triceps muscle activity demonstrated an inverse relationship with peak shoulder flexion. Conclusion: Elbow postures in elongated postures, via immobilization, may result in higher shoulder elevation due to increased passive forces when there is an altered muscle state of the biceps in the birth brachial plexus population, providing clinicians guidelines for interventions to assist this population with improved function.
