This randomized controlled clinical trial aims to evaluate the effectiveness of a standardized shoulder stabilization exercise program in amateur volleyball players diagnosed with atraumatic shoulder instability. Shoulder pain is one of the most prevalent musculoskeletal complaints, affecting up to two-thirds of the general population at some point in life and frequently leading to long-term symptoms, functional limitations, sleep disturbances, and high socio-economic costs. Atraumatic glenohumeral instability is particularly common among athletes performing overhead movements, such as volleyball players, who are repeatedly exposed to high-velocity external and internal rotation actions during serves and spikes.

The Derby Shoulder Instability Rehabilitation Programme (DSIRP) is a structured, reproducible protocol integrating strengthening, proprioceptive, neuromuscular control, plyometric, and speed-oriented exercises. Although widely used in clinical practice, evidence supporting its superiority over the natural progression of the condition remains limited. More than 80% of physiotherapists in the United Kingdom do not use specific standardized protocols for atraumatic shoulder instability, highlighting the need for high-quality clinical trials that evaluate structured rehabilitation approaches.

This study will recruit amateur volleyball players aged 20 to 30 years with a clinical diagnosis of atraumatic shoulder instability lasting more than 3 months. Participants will be randomized (1:1) into two parallel groups: an experimental group performing the DSIRP for 8 weeks (three 25-minute sessions per week), and a control group following a "wait-and-see" approach. Outcome measures will be assessed at baseline, 1 month, and 2 months (post-intervention). The primary outcomes include pain intensity measured with the Numeric Pain Rating Scale and shoulder disability measured with the Spanish version of the Shoulder Pain and Disability Index. Secondary outcomes include external rotation strength, grip strength, shoulder range of motion, and health-related quality of life assessed with the SF-12 questionnaire. All measurements use validated instruments with high reliability for clinical and research settings.

Statistical analysis will follow an intention-to-treat approach using mixed-model repeated-measures ANOVA or non-parametric alternatives when appropriate. Effect sizes and Bonferroni correction for multiple comparisons will be applied. An independent biostatistician blinded to allocation will conduct the analysis. The study follows CONSORT and TIDieR guidelines for reporting and intervention description.

This trial seeks to determine whether a structured, progressive stabilization protocol offers clinically meaningful improvements in pain, disability, strength, mobility, and quality of life compared with no intervention in young volleyball players with atraumatic shoulder instability. Results may support the implementation of standardized rehabilitation strategies in physiotherapy practice for this condition.

Atraumatic shoulder instability is a prevalent condition among athletes who perform repetitive overhead movements, particularly volleyball players. The biomechanical demands of serving, spiking, and blocking expose the glenohumeral joint to repeated high-velocity rotational forces, which may contribute to impaired neuromuscular control, altered proprioception, and insufficient dynamic stabilization. These factors can lead to recurrent symptoms of pain, weakness, and functional limitation, often persisting for extended periods and interfering with sports performance and daily activities.

Although physiotherapy is widely recognized as a first-line approach for managing atraumatic shoulder instability, clinical practice frequently lacks standardized and reproducible rehabilitation protocols. Existing programs vary significantly in content, dosage, and progression criteria, limiting consistency across clinicians and settings. The Derby Shoulder Instability Rehabilitation Programme (DSIRP) is one of the few structured intervention models specifically designed for this condition. It integrates strengthening, proprioception, plyometric exercises, and motor control tasks, with progression based on clearly defined functional milestones. However, despite its clinical use, robust comparative evidence supporting its effectiveness remains limited.

This randomized controlled trial aims to evaluate the clinical impact of an eight-week DSIRP-based intervention compared with no active treatment in amateur volleyball players aged 20 to 30 years with at least three months of atraumatic shoulder instability. The study follows CONSORT and TIDieR recommendations to ensure transparent reporting and comprehensive intervention description. Participants are randomly allocated in a 1:1 ratio to either the experimental group, which performs the structured exercise program three times per week, or the control group, which receives no intervention ("wait-and-see"). The analytical team remains blinded to group assignment.

The intervention is designed to address multiple dimensions of shoulder stability, including rotator-cuff strength, scapular control, reactive stability, and proprioceptive acuity. It progresses through phases that incorporate isometric and isotonic strengthening, closed-chain stability tasks, rhythmic stabilization, plyometric drills, and functional movement patterns relevant to overhead athletes. Session duration and frequency are standardized, but progression criteria ensure individualized advancement according to each participant's ability to meet predefined performance targets.

Outcome assessments are conducted at baseline, one month, and after the completion of the eight-week intervention. Measures span several domains relevant to shoulder instability, including pain intensity, self-reported disability, shoulder mobility, isometric strength, and overall health-related quality of life. All clinical instruments used in the study have demonstrated strong reliability and validity in previous research. The measurement protocol ensures standardized positioning and examiner consistency to reduce variability.

The statistical analysis incorporates repeated-measures models to evaluate changes over time and between groups. Parametric or non-parametric methods are selected based on data distribution, and effect sizes are calculated to quantify the magnitude of observed changes. A blinded biostatistician performs all analyses to minimize bias.

This study is designed to generate high-quality evidence on the usefulness of a structured, pathology-specific rehabilitation program for atraumatic shoulder instability in young overhead athletes. By comparing the DSIRP-based intervention with a no-treatment control condition, the trial aims to clarify whether a well-defined and reproducible exercise protocol leads to meaningful improvements in pain, function, strength, and mobility. Findings may inform clinical practice, support the adoption of standardized rehabilitation pathways, and contribute to reducing the variability that currently characterizes conservative management of this condition.

Inclusion criteria:

• Amateur volleyball players.
• Age range: 18 to 30 years.
• Clinical diagnosis of atraumatic shoulder instability.
• Symptom duration greater than 3 months.

Exclusion criteria:

• Previous surgeries on the affected shoulder.
• Previous fractures of the affected shoulder.
• History of traumatic shoulder dislocation.
• Shoulder pain originating from cervical spine pathology.
• Presence of pain in the elbow, wrist, or hand.
• Clinical diagnosis of rotator cuff-related shoulder pain or frozen shoulder.
• Medical diagnosis of cardiovascular, rheumatologic, or neurological disorders.
• Pregnancy.
• Cognitive impairment.
• Concurrent physiotherapy treatment during the study period.
• Use of analgesic or anti-inflammatory medication within 24 hours prior to study assessments.