Background Shoulder-instability is a complex impairment and identifying biomarkers which differentiate subgroups is challenging. Robust methods of measurement and movement protocols for improving our current understanding of muscle activity mechanisms, which may inform subgrouping and treatment allocation are needed. Hypothesis Null hypothesis: there are no differences between the movement and muscle activity of young-people with shoulder instability (irrespective of aetiology) and age- and sex-matched controls (two-tailed). Methods Young-people between eight to 18 years were recruited into two groups of shoulder-instability (SI) or and age- and sex-matched controls (CG). All forms of SI were included and young-people with co-existing neurological pathologies or deficits were excluded. Participants attended a single session and carried out four unweighted and three weighted tasks in which their movements and muscle activity was measured using 3D-movement analysis and surface electromyography. Statistical parametric mapping was used to identify between group differences. Results Data was collected for 30 young-people (15 SI (6M:9F) and 15 CG (8M:7F)). The SI cohort had mean (SD) age, height and weight values of 13.9 years (2.9), 163.0 cm (15.7) and 56.6 kg (17.5) respectively. The CG had age, height and weight values of 13.3 years (3.1), 160.6 cm (16.8) and 52.4 kg (15.1) respectively. The SI group demonstrated consistently more protracted and elevated sternoclavicular joint positions during all movements. Normalised muscle activity in Latissimus dorsi had the most statistically significant between group differences across all movements. The SI group also had increased normalised activity of their middle trapezius, posterior deltoid and biceps muscles whilst activity of their latissimus dorsi, triceps and anterior deltoid were comparatively decreased. Discussion Young-people with SI may constrain their movements to minimise glenohumeral joint instability. This was demonstrated by reduced variability in joint angles, adoption of different movement strategies across the sternoclavicular and acromioclavicular joints and increased activity of the scapular stabilising muscles, despite achieving similar arm positions to the CG. Conclusion Young-people with shoulder instability have consistent differences in their muscle activity and movement patterns. Consistently observed differences at the shoulder girdle included increased sternoclavicular protraction and elevation accompanied by increased normalised activity of the posterior scapula stabilising muscles and decreased activity of the posterior humeral mobilising muscles. Existing methods of measurement may be used to inform clinical decision making, however, further work is needed evaluate the prognostic and clinical utility of derived 3D and sEMG data for informing decision making within shoulder instability and associated subgroups.
