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Knowledge of injury patterns, an integral step towards injury prevention, is lacking in youth handball.To investigate if an increase in weekly handball participation is associated with increased shoulder injury rates compared with decrease or a minor increase, and if a potential association is influenced by scapular control, isometric shoulder strength, or glenohumeral range of motion (ROM).Prospective cohort study with weekly reports on shoulder injury, and handball participation over 31 weeks using the SMS, Phone and Medical Examination system. Assessment of shoulder isometric rotational and abduction strength, ROM and scapular control was performed at baseline and mid-season.Danish elite youth handball.686 players were enrolled. Seven players were excluded, and data from 36 players were censored.Weekly change in handball participation categorised into (1) <20% increase or decrease (reference), (2) increase between 20–60%, (3) increase >60% relative to the weekly average amount of handball participation the preceding four weeks. Scapular control, isometric shoulder strength and ROM were included as effect measure modifiers.Any handball-related shoulder problem irrespective of the need for time loss or medical attention.An increase in handball participation by >60% was associated with greater shoulder injury rate (Hazard Ratio (HR):1.91; 95% Confidence Interval (CI) 1.00; 3.70, p=0.05) compared with the reference group. The effect of an increase in handball participation between 20% to 60% was exacerbated amongst players with reduced external rotation strength (HR:4.0; 95% CI 1.1; 15.2, p=0.04) or scapular dyskinesis (HR 4.8; 95% CI 1.3; 18.3, p=0.02). Reduced external strength also exacerbated the effect of an increase above 60% (HR: 4.2; 95% CI 1.4; 12.8, p=0.01).A large increase in weekly handball participation increases the shoulder injury rate in elite youth handball players; particularly in the presence of reduced external rotational strength or scapular dyskinesis.