The goal of this clinical trial is to test the effect of a novel assessment and treatment strategy for young adults with severely disabling non-traumatic shoulder instability. The main question it aims to answer is if this treatment will substantially change the patients shoulder function to a more stable and controllable state and thereby give a higher quality of life. Participants failure in active shoulder muscle function will be assessed and exercises will be selected individually. Patients will be asked to perform the exercises twice per day. At clinical visits to a specialized physiotherapist the exercises will be upgraded to restore full dynamic stability throughout shoulder movement range. The active treatment period will be 4 months. Researchers will compare two groups. One group will start directly after a baseline assessment is fulfilled. The other group will receive the treatment with a delay of 4 months to evaluate if the selected exercises can substantially change the shoulder stability compared to no treatment, the natural course of the condition.

Impaired shoulder function and pain results in decreased quality-of-life, reduced working capacity and affect both society and individuals with high costs. Young adults with non-traumatic shoulder instability are one of the most difficult patient groups to treat. Surgery has previously been the treatment offered for these patients. Today, surgery is more and more questioned and physiotherapy and now active exercises are suggested to be the first - line of treatment.

A novel physiotherapy treatment strategy has been developed during the recent years.

The intervention is based on an entirely updated mode to assess and restore shoulder muscle function. The development has been initiated by new data from EMG- studies that has given a deepened understanding of the complex stabilizing muscle function of the shoulder. The hypothesis is that the main stabilizing muscles, the rotator cuff has by franc dislocations, subluxations and pain lost its rapidness, precision, and endurance to keep the humeral head stabilized and centered within the glenoid fossa.

The hypothesis is that with restored, full rotator cuff function, i. e. a rotator cuff able to perform its stabilizing role and the mover role into the extremes of the range of motion, where the fibers act from their most shortened to their most lengthened positions and during concentric as well as eccentric muscle action, will regain the sense of shoulder stability, pain-free shoulder function and higher quality-of-life.

The purpose of this study is to evaluate the effect of this new active exercise-based intervention in patients with non-traumatic shoulder instability.

Methods: A total of 100 patients randomly be allocated into two different groups, Early start and Delayed start. Both groups will receive the same 16-week physiotherapy supervised exercise program, however the latter group with a delay of four months.

The primary outcome is assessment of neuromuscular control during active rotation movements documented with video. The range of motion is measured, and sway and steadiness are graded. The secondary outcomes include: 1) active and passive range of motion; 2) shoulder pain at rest using an 11-graded numeric rating scale; 3) Pain during motion, and 4) Apprehension during motion using Patient Specific Functional Scale; 5) Quality-of-life will be assessed using Western Ontario Shoulder Instability Index (WOSI) Patients will be assessed by one of two independent investigators at baseline (second baseline if Delayed Start), 6 weeks, 4 and 12 months. At 4 and 12 months also Patient's Satisfaction of being treated with the novel strategy will be assessed.

The statistical analysis will assess change over time and differences between groups using ANOVA and Tukey's PostHoc-test as well as Kruskal- Wallis and Mann-Whitney-U test with the significant level was set at 0.05.

Inclusion Criteria:

• young adults, non traumatic as well as non-controllable, positional, functional shoulder instability according to Moroder et al. demonstrating a lack of ≥ 20%, active, dynamic control compared with the total passive range of motion in shoulder rotation with or without previous shoulder stabilizing surgery
• Understanding the Swedish language well, in speaking, in reading and writing.

Exclusion Criteria:

• Other significant shoulder pathologies, patients diagnosed with severe systemic connective tissue, congenital anatomic abnormalities, or psychiatric diagnoses according to previous medical records
• <60% adherence to assigned exercises

Investigator: 6 physiotherapists are care providers, responsible for the Intervention . Two independent investigators does not know who is the actual care provider. However, all patients recieve the same intervention.

Outcome assessor: each participant is given a research code. The results from the assessments are entered in the data base by a research assistant, not involved in the study on any level.

Individuals are not identifiable. The outcome assessor are planning for all statistics used and is not in other ways involved in the study.