Shoulder instability (SI) is a complex impairment, and identifying biomarkers that differentiate subgroups is challenging. Children and adolescents with SI (irrespective of etiology) have differences in their movement and muscle activity profiles compared to age- and sex-matched controls (2-tailed). There are limited fundamental movement and muscle activity data for identifying different mechanisms for SI in children and adolescents that can inform subgrouping and treatment allocation.Young people between 8 and 18 years were recruited into 2 groups of SI and age- and sex-matched controls (CG). All forms of SI were included, and young people with coexisting neurologic pathologies or deficits were excluded. Participants attended a single session and carried out 4 unweighted and 3 weighted tasks in which their movements and muscle activity was measured using 3-dimensional (3D) movement analysis and surface electromyography (sEMG). Statistical parametric mapping was used to identify between-group differences.Data were collected for 30 young people (15 SI [6 male, 9 female] and 15 CG [8 male, 7 female]). The mean (standard deviation) age of the participants was 13.6 years (3.0). The SI group demonstrated consistently more protracted and elevated sternoclavicular joint positions during all movements. Normalized muscle activity in latissimus dorsi was lower in the SI group and had the most statistically significant differences across all movements. Where differences were identified, the SI group also had increased normalized activity of their middle trapezius, posterior deltoid, and biceps muscles but decreased activity of their latissimus dorsi, triceps and anterior deltoid muscles compared with the CG group. No statistically significant differences were found for the pectoralis major across any movements. Weighted tasks produced fewer differences in muscle activity patterns compared with unweighted tasks.Young people with SI may adapt their movements to minimize glenohumeral joint instability. This was demonstrated by reduced variability in acromioclavicular and sternoclavicular joint angles, adoption of different movement strategies across the same joints, and increased activity of the scapular stabilizing muscles, despite achieving similar arm positions to the CG. Young people with SI demonstrated 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 normalized activity of the posterior scapula-stabilizing muscles. Existing methods of measurement may be used to inform clinical decision making; however, further work is needed to evaluate the prognostic and clinical utility of derived 3D and sEMG data for informing decision making within SI.