This study evaluates the addition of a high dose of simple home-based elastic band strengthening exercises to usual care in patients with subacromial impingement syndrome. Half of the participants receive instructions in simple home-based elastic band strengthening exercises in addition to usual care, while the other half receive usual care.

The SEXSI trial is a pragmatic, assessor and participant blinded, randomized, controlled, superiority trial, with a two-group parallel design. Patients with subacromial impingement syndrome will be randomized to either usual care or a home-based intervention consisting of progressive high volume resistance training in addition to usual care using a 1:1 allocation. The primary end-point will be change in the Shoulder Pain and Disability Index (SPADI) 16 weeks after baseline.

The overall frame for the trial is a main clinical effectiveness-part and an embedded mechanistic part. The clinical effectiveness-part will be reported in the main trial paper, and include the following outcomes: SPADI (primary outcome), Abduction strength, External rotation strength, Abduction ROM, Pain last week, QoL-index, QoL-VAS, Global impression of change and PASS. Missing outcome data will be imputed using multiple imputations based on the following variables: All previous scores in the relevant outcome, Age, Gender and Allocation.

A full trial protocol will be published and made publicly available. A constrained Linear Mixed Model (cLMM) will be applied for all continuous outcomes with Time Frame 16 weeks (including the primary outcome), to compare the change from baseline to 16 weeks in the intervention group (IG) to that in the control group (CG). The outcome at 16 weeks will be included as dependent variable, treatment group (IG or CG) as main effect and both baseline score and any additional follow up measurements as repeated measurements. The covariance structures will be selected based on the MAICE procedure. Binary outcomes will be reported as proportions with corresponding 95% confidence intervals, and will be compared using Chi-squared tests, and Odds Ratios computed from random effects logistic regression models. All of these analyses will be conducted as intention to treat (ITT) analyses, including all randomized participants, regardless of protocol adherence, meaning that all participants will be analysed as randomized. In the primary trial report, all collected outcomes will be listed, and it will be stated that the below-mentioned variables - belonging to the mechanistic part - will be reported in secondary publications.

The mechanistic part will be reported in secondary papers with a clear reference to the primary trial and trial registration, and it will hold the label "secondary analyses from a pragmatic randomized controlled trial" in the title. The following outcomes will be reported:

Outcomes regarding pain sensitization (temporal summation of pain, conditioned pain modulation, pain pressure threshold and pain catastrophizing) will be reported in a subsequent paper where we also plan to investigate:

• the modifying effects of pain sensitization on the effectiveness of the add-on intervention
• the dose-response relationship between objectively monitored adherence to the add-on intervention and change in SPADI, shoulder abduction strength and external rotation strength
• the dose-response relationship between pain sensitization and change in SPADI, shoulder abduction strength and external rotation strength, and to what degree this is mediated through adherence to the intervention.

Outcomes regarding scapula dyskinesia and scapula dysfunction will be reported in another subsequent paper, were we also plan to investigate the modifying effects of scapula dyskinesia and scapula dysfunction on the effectiveness of the intervention on changes in SPADI, abduction strength and external rotation strength.

Inclusion Criteria:

• Convened for the first time to the clinic, for examination of their current shoulder disorder
• Shoulder disorder lasting at least three months
• Living in Capitol Region of Denmark
• Not pregnant
• Do not permanently use strong pain medication
• Able to understand spoken and written Danish
• ≥3 positive of the five diagnostic test for subacromial impingement syndrome (Hawkins-Kennedy test, Neer's test, pain-full arc, Resisted External Rotation test and Jobe's test)
• Offered a rehabilitation plan due to a medically justified need for general rehabilitation after discharge from the hospital under the Danish Health Act § 140.
• Completed Shoulder Pain And Disability Index (SPADI) questionnaire on the day of the medical examination

Exclusion Criteria:

• A radiologically verified new or previous fracture related to the shoulder joint, including the scapula
• Clinically suspected Glenohumeral osteoarthritis
• A clinically suspected luxation or sub-luxation of the glenohumeral, acromioclavicular or sternoclavicular joint
• A clinically suspected labral lesion, complete tear of the rotator cuff, frozen shoulder or other competing diagnoses (i.e. rheumatoid arthritis, cancer, neurological disorders, fibromyalgia, psychiatric illness)