Rotator cuff-related shoulder pain (RCRSP) is one of the most common musculoskeletal disorders affecting the shoulder and is frequently associated with pain, reduced muscle strength, and impaired upper extremity function. Although previous studies have primarily focused on structural and functional changes around the shoulder, little is known about the potential adaptations occurring in the distal upper extremity, particularly the forearm muscles and tendons.

Altered upper extremity use, pain-related muscle inhibition, and compensatory movement patterns associated with prolonged shoulder pain may lead to morphological changes in the forearm musculature and tendons. However, the relationship between symptom duration and these structural adaptations has not yet been investigated.

The aim of this cross-sectional study is to evaluate forearm muscle and tendon thickness using portable ultrasonography in individuals with RCRSP and to investigate whether these parameters differ according to symptom duration. In addition, hand grip strength, pain intensity, and upper extremity function will be assessed to explore their associations with ultrasonographic findings.

The findings of this study are expected to provide new insights into distal upper extremity adaptations associated with RCRSP and may contribute to the development of more comprehensive rehabilitation strategies that address not only the shoulder but also the forearm musculature.

Rotator cuff-related shoulder pain (RCRSP) is one of the leading causes of shoulder pain and disability in adults. In addition to local impairments such as pain, muscle weakness, and reduced range of motion, individuals with RCRSP frequently demonstrate altered upper extremity movement patterns and reduced functional performance. These adaptations may extend beyond the shoulder joint and influence distal segments of the upper extremity.

Previous research has primarily focused on structural changes in the rotator cuff muscles and tendons, scapular musculature, and shoulder biomechanics. However, the potential morphological adaptations of the forearm muscles and tendons remain largely unexplored. Persistent pain, reduced upper limb use, altered neuromuscular control, and compensatory motor strategies associated with prolonged shoulder symptoms may contribute to changes in forearm muscle and tendon morphology. Understanding these adaptations may improve the current understanding of the regional effects of RCRSP.

Musculoskeletal ultrasonography provides a valid, reliable, non-invasive, and accessible method for evaluating muscle and tendon morphology. The increasing availability of portable ultrasound devices has expanded the feasibility of assessing musculoskeletal structures in both clinical and research settings. Despite these advantages, ultrasonographic evaluation of forearm muscles and tendons has rarely been performed in individuals with RCRSP.

This cross-sectional study aims to investigate the association between symptom duration and forearm muscle and tendon thickness measured by portable ultrasonography in individuals with RCRSP. The study also seeks to explore whether structural changes in the forearm are accompanied by alterations in hand grip strength, pain intensity, and upper extremity function. The findings may contribute to a better understanding of distal musculoskeletal adaptations associated with prolonged shoulder pain and provide evidence for incorporating forearm assessment into rehabilitation programs for individuals with RCRSP.

Inclusion Criteria:

RCRSP Group Adults aged 18-65 years. Clinical diagnosis of rotator cuff-related shoulder pain (RCRSP). Unilateral shoulder pain lasting at least 6 weeks. Pain localized around the anterolateral shoulder, aggravated by arm elevation and/or overhead activities.

Positive findings on at least three clinical tests suggestive of RCRSP (e.g., painful arc, Hawkins-Kennedy test, Neer test, Jobe/Empty Can test, resisted external rotation).

Ability to understand the study procedures and provide written informed consent.

Healthy Control Group Adults aged 18-65 years. No current shoulder pain. No history of shoulder pain, shoulder surgery, or upper extremity musculoskeletal disorders during the previous 12 months.

Ability to provide written informed consent.

Exclusion Criteria:

Previous shoulder surgery on either upper extremity. History of shoulder fracture, shoulder dislocation, or major upper extremity trauma.

Full-thickness rotator cuff tear or other shoulder pathology requiring surgical management.

Clinical signs of adhesive capsulitis. Cervical radiculopathy or other neurological disorders affecting the upper extremity.

Systemic inflammatory, rheumatologic, or neuromuscular diseases. Peripheral nerve entrapment (e.g., carpal tunnel syndrome, cubital tunnel syndrome).

Previous surgery or significant injury involving the forearm, wrist, or hand. Corticosteroid injection into the affected shoulder within the previous 3 months.

Current pregnancy. Inability to complete study assessments or provide informed consent.

### Study Population Description

The study population will consist of adult individuals aged 18-65 years, including both patients with rotator cuff-related shoulder pain (RCRSP) and healthy control participants. Participants with RCRSP will be recruited based on clinical diagnosis confirmed by physical examination and positive clinical tests consistent with rotator cuff-related shoulder pain. Healthy controls will be recruited from the community and hospital staff and will have no history of shoulder pain or upper extremity musculoskeletal disorders.

Patients with RCRSP will be further categorized according to symptom duration into subgroups for comparative analysis. All participants will undergo a single assessment session including ultrasonographic evaluation of forearm muscle and tendon thickness, hand grip strength measurement, and standardized functional and pain assessments.

Participants will be recruited from outpatient physiotherapy and orthopedic clinics of the participatin