This study aims to investigate the relationship between thoracic kyphosis angle, trunk rotation mobility, and balance performance in individuals with rotator cuff pathology. The rotator cuff, consisting of the supraspinatus, infraspinatus, teres minor, and subscapularis muscles, plays a crucial role in providing both static and dynamic stability to the shoulder joint. Rotator cuff pathologies are among the most common causes of shoulder pain and functional limitation, with their prevalence increasing with age. Although shoulder pain has traditionally been considered a localized musculoskeletal problem, recent biomechanical evidence highlights the importance of the kinetic chain concept, which emphasizes the interconnected function of the upper extremity with the lower extremity, thoracic spine, and postural control mechanisms. Within this framework, thoracic spine mobility and postural stability are considered key factors in preventing pathological loading on the rotator cuff tendons.

The thoracic spine serves as a fundamental biomechanical platform for scapulothoracic joint function. Increased thoracic kyphosis has been shown to negatively affect scapular upward rotation and posterior tilt during humeral elevation, contributing to narrowing of the subacromial space and increased mechanical stress on the rotator cuff tendons. In addition, trunk rotation mobility plays an essential role in efficient energy transfer along the kinetic chain, particularly during functional movements such as reaching or throwing. Limitations in trunk rotation may disrupt this energy transfer, resulting in compensatory loading of the shoulder complex and increased injury risk. Furthermore, kinesiophobia associated with chronic pain may alter movement strategies, reduce physical activity levels, contribute to muscle atrophy, and indirectly impair postural control mechanisms.

Balance performance is maintained through the integration of visual, vestibular, and somatosensory systems. In individuals with rotator cuff pathology, impaired proprioceptive input from the shoulder girdle and increased pain-related postural sway may lead to decreased performance in dynamic balance tasks such as the Y-Balance Test. In contrast, in healthy individuals, postural control strategies have been shown to correlate with self-perception and body awareness levels. Based on this theoretical framework, the primary aim of the present study is to compare thoracic kyphosis angle, trunk rotation mobility, static balance, and dynamic balance performance between individuals with rotator cuff pathology and healthy controls, and to examine the relationships between these variables. It is hypothesized that individuals with rotator cuff pathology will demonstrate significant differences in these parameters compared to healthy individuals.

The study will be conducted at the Physical Therapy Unit of Iğdır State Hospital and will include individuals aged 18-65 years. Sample size calculation was performed using G*Power software based on a previously reported effect size (Cohen's d = 0.63) for thoracic kyphosis. With an alpha level of 0.05 and statistical power of 80%, at least 36 participants per group were required. To account for potential dropouts, a total of 80 participants will be included, consisting of at least 40 individuals with rotator cuff pathology and 40 healthy controls. All participants will provide written informed consent, and sociodemographic and clinical information will be collected through face-to-face interviews.

Thoracic kyphosis angle will be measured using a smartphone-based digital inclinometer by calculating the angular difference between T1-T2 and T12-L1 vertebral levels. Trunk rotation range of motion will be assessed using a digital goniometer. Static balance performance will be evaluated using the Single Leg Stance Test under eyes-open and eyes-closed conditions. Dynamic balance performance will be assessed using the Y-Balance Test protocol, in which reach distances in three directions are normalized according to leg length. These measurement tools have been reported to demonstrate high validity and reliability in clinical research.

In conclusion, this study seeks to demonstrate that rotator cuff pathology should not be considered solely a localized shoulder disorder but rather a condition associated with impairments in the kinetic chain, including thoracic spine mobility and postural control mechanisms. The findings are expected to highlight the importance of assessing thoracic mobility and balance performance in shoulder rehabilitation programs.

The rotator cuff consists of four muscles (m. supraspinatus, m. infraspinatus, m. teres minor, and m. subscapularis) that connect the scapula to the humerus and provide both static and dynamic stability to the shoulder joint. Rotator cuff pathologies are one of the most common causes of pain and functional limitations in the shoulder joint, and their clinical incidence increases significantly with age. While traditional clinical approaches often treat shoulder pain as a localized joint problem, current biomechanical evidence is expanding this perspective. Upper extremity functions, in accordance with the "kinetic chain" principle, are inextricably linked to the lower extremity, thoracic spine, and complex postural control mechanisms. This integration within the kinetic chain demonstrates that the shoulder joint is not merely an isolated structure, but a strategic part of the energy flow that extends from the center of the body to the distal extremities. In this context, maintaining mobility and postural stability of the thoracic spine plays a critical role in preventing pathological loading on the rotator cuff tendons.

The thoracic spine serves as a fundamental biomechanical platform for scapulothoracic joint functions. The literature indicates that an increased thoracic kyphosis angle hinders scapular uprotation and posterior tilt during humerus elevation, and this plays a key role in the etiology of rotator cuff pathologies. This increase in thoracic kyphosis angle is known to trigger scapular protraction, narrowing the subacromial space and increasing mechanical stress on the rotator cuff tendons. In accordance with kinetic chain principles, trunk rotation ensures efficient energy transfer from the lower extremity to the upper extremity, especially in activities such as throwing or reaching. However, it is thought that limitations in trunk rotation mobility disrupt energy transfer in the kinetic chain, increasing compensatory loads on the shoulder complex and predisposing to injury risk. In addition to these biomechanical changes, kinesiophobia developing due to pain can negatively affect static and dynamic balance performance by altering individuals' movement strategies. High kinesiophobia scores, frequently encountered in chronic shoulder pain and assessed with the Tampa Kinesiophobia Scale (TKS), are argued to restrict the individual's physical activity level, leading to muscle atrophy and indirectly to impairments in postural control mechanisms. Balance is maintained through the integration of visual, vestibular, and somatosensory systems. In individuals with rotator cuff injuries, impaired proprioceptive input from the shoulder girdle and increased pain-induced postural oscillations can lead to decreased performance in dynamic measurements such as the Y-Balance Test. On the other hand, it is reported that physical performance and postural control strategies in healthy individuals show a strong correlation with self-perception and bodily sensation levels measured by the Body Awareness Questionnaire (BAS).

In light of all this literature information, the aim of this study is to evaluate thoracic kyphosis angle, trunk rotation mobility, static and dynamic balance performance in healthy individuals and individuals with rotator cuff pathology, and to comparatively examine the relationship between these parameters.

The hypotheses of the study are as follows:

H0: There is no significant difference in thoracic kyphosis, trunk rotation, and balance scores between individuals with rotator cuff pathology and healthy controls.

H1: Individuals with rotator cuff pathology show a significant difference in thoracic kyphosis, trunk rotation, and balance scores compared to healthy controls.

Materials and Metod The study will be conducted at the Physical Therapy Unit of Iğdır State Hospital on healthy individuals aged 18-65, both with and without rotator cuff pathology. The research will commence after ethical committee approval and will be completed within 6 months.

Sample size was calculated using G*Power software. This analysis was based on the effect size (Cohen's d = 0.63) of the thoracic kyphosis measurement parameter from a previous study. The analysis, performed with α = 0.05 and 80% power, determined that at least 36 individuals should be included in each group. Considering the possibility of exclusion or withdrawal, it was decided to include a total of 80 individuals in the study, with at least 40 in each group. Patients included in the study according to the criteria will be informed verbally and in writing about the study, and a consent form will be obtained from volunteer patients.

Inclusion criteria: Having been diagnosed with unilateral rotator cuff pathology (impingement syndrome, tendinopathy, or partial tear) by a specialist physician through clinical examination and/or radiological imaging (USG/MR); being between 18 and 65 years of age; having shoulder pain that has lasted for at least 3 months; having a positive result on at least two of the Neer, Hawkins-Kennedy, and Empty Can tests; having pain of at least 3/10 on the Pain Numerical Rating Scale (PMR) during rest or activity; having sufficient cognitive ability to follow simple instructions and perform the tests; having a Mini Mental State Test score ≥24; having the ability to provide signed informed consent and agreeing to participate in the study voluntarily. For inclusion in the control group: The experimental group consisted of healthy individuals matched for age, gender, and dominant arm, with no history of shoulder, neck, or upper back injury, no painful symptoms reported in any of these areas within the last 12 months, negative shoulder-specific tests, full range of motion on physical examination, and cognitive capacity to perform the tests. Exclusion criteria included: previous surgery on the shoulder or thoracic region; full-thickness rotator cuff tears requiring surgical indication; adhesive capsulitis, shoulder instability, labrum tears, or calcific tendinitis; cervical radiculopathy or neurological diseases affecting the upper and lower extremities; uncontrolled diabetes, inflammatory rheumatic diseases (rheumatoid arthritis, etc.), or malignancy; severe osteoporosis in the thoracic region, structural scoliosis or advanced postural deformity affecting trunk rotation, unstable vertebral fractures, or active infection. Individuals who had received steroid (cortisone) or PRP injections to the shoulder area within the last 3 months, those with vestibular or visual impairment, those using medications that may affect balance, and those with a lower extremity injury within the last 6 months were excluded from the study. Participants were asked about their dominant arm, age, height, weight, education level, and the side of the shoulder with the pathology.

In our study, after obtaining informed consent from all individuals, their sociodemographic information and disease-related assessments will be recorded through face-to-face interviews. The degree of thoracic kyphosis of the participants will be measured using a smartphone equipped with the 'Angle Meter' software, a digital inclinometer, and calibrated gyroscope and accelerometer sensors. During measurements, participants will stand upright with their feet shoulder-width apart and their arms freely at their sides. The short edge of the phone will be parallel to the spine line; the first measurement will be taken at the T1-T2 vertebral level, and the second measurement at the T12-L1 level. The angular difference between these two points will be recorded as the thoracic kyphosis angle in degrees (°). To ensure standardization of the measurements, all assessments will be repeated three times by the same researcher, and the averages will be taken. Smartphone inclinometer applications have been reported to show a high correlation (95% CI) with radiological measurements such as the Cobb angle and manual inclinometers. Trunk Active Range of Motion (ROM) will be assessed using a digital goniometer. Participants' trunk rotation angles will be recorded in standard measurement positions. Digital goniometers have been shown to have higher accuracy than universal manual goniometers in shoulder measurements and provide reliable results in clinical studies (ICC > 0.90). Participants' static balance performance will be assessed separately for both the right and left lower extremities using the Single Leg Stance Test (SLST). Measurements will be performed in two different conditions: Eyes Open (EA) and Eyes Closed (EC). Participants will be asked to hold their hands crossed over their chest and their non-tested leg bent at the knee and raised off the ground. The time the balance position is maintained will be recorded in seconds; the test will be terminated if the supporting leg shifts, the raised leg touches the ground, or the arm position changes. Measurements will be repeated three times for both legs and both eye positions, and the best scores will be used in the analysis. The single-leg stance test has been shown to have high validity and reliability (ICC>0.90) in distinguishing differences in neuromuscular control between healthy and pathological groups. Participants' dynamic balance performance will be evaluated according to the standard Y-Balance Test protocol. During the test, the participant will stand on one leg in the center of the platform and attempt to reach the furthest point they can in three directions with their free leg without touching a hard surface or losing their balance: Anterior, Posteromedial, Posterolateral. Measurements will be repeated three times for each leg, and the maximum distances reached will be recorded in centimeters (cm). To ensure standardization of the measurements, the distance reached will be divided by the participant's leg length (distance between the Anterior Superior Iliaca and the Medial Malleolus) to calculate the "normalized reach" (%). The test will be considered invalid if the supporting leg moves out of place, balance is lost, or the outstretched leg touches the ground hard. The Y-Balance test has high sensitivity in identifying neuromuscular control deficits along the kinetic chain. While it is a basic balance indicator in healthy individuals, it is a valid tool (95% CI) for measuring the impact of upper extremity dysfunction on lower extremity stability in individuals with shoulder pathology.

Inclusion Criteria for rotator cuff pathology group:

• Have been diagnosed with unilateral rotator cuff pathology (impingement syndrome, tendinopathy, or partial tear) by a specialist physician through clinical examination and/or radiological imaging (USG/MR);
• have been between 18 and 65 years of age; have shoulder pain that has lasted for at least 3 months;
• have a positive result on at least two of the Neer, Hawkins-Kennedy, and Empty Can tests;
• have pain of at least 3/10 on the Pain Numerical Rating Scale (PMR) during rest or activity;
• have sufficient cognitive ability to follow simple instructions and administer the tests; have a Mini Mental State Test score ≥24;
• be able to provide signed informed consent and voluntarily agree to participate in the study.

Inclusion Criteria for healthy group:

• No history of shoulder, neck, or upper back injury and no painful symptoms reported in any of these areas within the last 12 months,
• negative shoulder-specific tests (Neer, Hawkins, etc.), full range of motion on physical examination, and cognitive capacity to perform the tests.

Exclusion Criteria:

• Having undergone previous surgery in the shoulder or thoracic region;
• Full-thickness rotator cuff tears requiring surgical indication; adhesive capsulitis, shoulder instability, labrum tears, or calcific tendinitis;
• cervical radiculopathy or neurological diseases affecting the upper and lower extremities;
• uncontrolled diabetes, inflammatory rheumatic diseases (rheumatoid arthritis, etc.), or malignancy;
• severe osteoporosis in the thoracic region, structural scoliosis or advanced postural deformity that may affect trunk rotation, unstable vertebral fractures, or active infection;
• having received steroid (cortisone) or PRP injections in the shoulder region within the last 3 months;
• vestibular or visual impairment, those using medications that may affect balance, those with a lower extremity injury within the last 6 months.

Rotator cuff pathology (impingement syndrome, tendinopathy, or partial tear)