Determining whether altered muscle activation influences shoulder stability in multidirectional instability

Multidirectional instability (MDI) is a shoulder pathology affecting younger adults participating in high-demand upper-extremity activities. Clinicians hypothesize that MDI develops when muscle activation is not sufficient to compensate for deficits in passive stability of the shoulder. Although altered shoulder muscle activation has been observed in MDI, it is not yet known how these changes influence shoulder stability. The objective of this proposal is to quantify glenohumeral stiffness (resistance to translational joint motion) during muscle activation in subjects with MDI and asymptomatic controls. I hypothesize that individuals with MDI will have a reduced ability to increase glenohumeral stiffness through muscle activation. I will use a robotic system to perturb the shoulder as subjects generate volitional contractions. System identification will be used to estimate stiffness from the resulting displacements and forces. These results will provide the first characterization of active shoulder stability in MDI, informing future studies designed to optimize rehabilitative treatments.