This cluster-randomised controlled trial will evaluate the effects of two internationally recognised shoulder injury prevention programmes - the Swedish Shoulder Control programme and the Norwegian Oslo Sports Trauma Research Center Shoulder Injury Prevention programme - on shoulder function, scapular control, and injury incidence in Hungarian adolescent handball players (U16-U20). Six elite-level teams (3 male, 3 female) from a single handball academy will be randomised by cluster into two intervention arms for an 18-week intervention period. Primary outcomes are changes in objective shoulder function tests; secondary outcomes include self-reported function and weekly injury monitoring.

This study is a two-armed cluster-randomised controlled trial designed to assess the effects of two established, evidence-based shoulder injury prevention programmes in Hungarian adolescent elite handball players. Both programmes have previously demonstrated effectiveness in reducing the incidence of shoulder injuries in overhead athletes; however, it remains unclear how they compare in their ability to improve specific objective outcomes such as shoulder stability, proprioception, strength, and scapular control. The trial will be conducted at a single elite handball academy, including six teams (three male, three female) in the U16-U20 age categories. Teams will be randomised at the cluster level (team) into one of two intervention arms, each performing one of the two prevention programmes for the entire competitive half-season (18 weeks).

Unlike most injury prevention studies in adolescent athletes, this trial combines weekly self-reported monitoring of shoulder health (OSTRC-O Shoulder Module) with a comprehensive battery of objective, instrumented assessments of shoulder function, scapular control, strength, and proprioception. This dual approach allows for a more precise evaluation of programme effectiveness, capturing both perceived and measurable physical changes.

This prevention trial forms part of a larger, ongoing research programme investigating shoulder function and return-to-sport readiness following shoulder injury in overhead athletes. The same battery of objective tests (scapular dyskinesis assessment, LSST, mLSST, CKCUEST, YBT-UQ, proprioception testing, isometric strength testing) used here is also applied in the broader project, allowing the present study to generate insights that directly inform both preventive strategies and post-injury rehabilitation protocols.

The findings from this trial are expected to inform refinements of existing shoulder injury prevention protocols, enhance compliance strategies, and support sport-specific adaptations for adolescent elite handball players and other overhead athletes.

Inclusion Criteria:

• Registered competitive handball players from the participating academy team (U16, U18, U20; male and female).
• Age between 13 and 19 years at baseline assessment.
• Currently participating in regular handball training and matches (minimum 3 training sessions per week).
• Member of a team assigned to the intervention for the full study period.
• Provided written informed consent (and parental consent for participants under 18 years).

Exclusion Criteria:

• Acute shoulder injury at baseline preventing training
• Shoulder surgery in last 6 months
• Contraindication to resistance training
• Concurrent enrolment in another interventional study targeting the shoulder/upper limb.
• Refusal of informed consent (and parental consent for participants <18 years).

No individual participant data will be shared. Data sharing is not planned in order to comply with data protection regulations and to safeguard participant confidentiality.

Due to the team-based cluster randomization and the nature of the intervention, masking of participants, coaches, and outcome assessors is not feasible. Entire teams are assigned to either the Swedish or the Norwegian shoulder injury prevention programme, making the intervention easily identifiable. The physiotherapists conducting the physical performance tests are aware of team allocation because they are also involved in the organization and supervision of the intervention sessions. However, the primary outcomes are objective, performance-based measures (e.g., CKCUEST, Y Balance Test-Upper Quarter, LSST, SDT, shoulder proprioception testing), which substantially reduces the potential risk of measurement bias.