The effect of elbow posture on biceps function at the shoulder in brachial plexus birth injuries: A computational sensitivity analysis.

BACKGROUND Individuals with a brachial plexus birth injury have changes in optimal muscle length of affected muscles and diminished motion at the elbow and shoulder, impacting active shoulder elevation. The long head of the biceps, a bi-articular muscle, is of particular interest in this population due to its reduced optimal muscle length and simultaneous shortening at the shoulder and elbow during active shoulder elevation. Prior studies have shown immobilization of a joint may improve the moment-generating capacity of a biarticular muscle crossing that joint. The aim of this study was to investigate the impact of joint stabilization on shoulder elevation isometric moment-generating capacity and active and passive fiber force of the long head of the biceps in simulated elbow postures and altered optimal fiber length of the biceps.

METHODS Simulations, via a musculoskeletal model, were used to predict maximum isometric shoulder elevation moment, active-fiber force, and passive-fiber force versus shoulder elevation in stabilized elbow postures from 10° to 120° with a range of optimal muscle lengths from typical to reduced longitudinal length by 30%, 20%, and 10% to the long head of the biceps.

FINDINGS The highest shoulder elevation moment was produced with optimal fiber length reduced 30% for biceps long head and the elbow stabilized in 30°, causing lower active fiber force, but higher passive fiber force.

INTERPRETATION Understanding the biomechanical advantage of immobilizing the elbow in a specified posture that improves shoulder elevation in this population, establishes future directions for therapeutic interventions for birth brachial plexus injuries.