This randomized controlled, single-blind, quasi-experimental study primarily aims to investigate the effects of unilateral resistance exercise applied without directly loading the affected shoulder on muscle strength and upper extremity function in individuals with frozen shoulder. Additionally, the study seeks to examine the effects on joint range of motion, quality of life, and activities of daily living. The study will be conducted among individuals diagnosed with frozen shoulder at the Physiotherapy and Rehabilitation Outpatient Clinic of Malatya Battalgazi State Hospital. The data collection process is planned to be completed between February 2026 and December 2026.

Participants will be selected from the relevant population using a non-probability convenience sampling method. Individuals meeting the inclusion criteria will be enrolled after providing written informed consent. The study will be carried out in accordance with the principles of the Declaration of Helsinki. Participants will be randomly assigned to either the control or intervention group.

The study will include individuals aged 18-65 years with a diagnosis of unilateral primary or secondary frozen shoulder, presenting with more than 50% restriction in at least one of the following shoulder movements compared to the unaffected side: external rotation, abduction, or flexion, and with more than 30% limitation in joint range of motion in at least two movement planes.

Individuals with concomitant shoulder trauma (e.g., rotator cuff injury), bilateral shoulder involvement, acromioclavicular joint osteoarthritis, prior treatment for frozen shoulder, psychiatric disorders that may affect compliance with medical or physical therapy interventions, concomitant neurological conditions, or a Mini-Mental State Examination (MMSE) score below 24 will be excluded from the study.

Frozen shoulder (FS) is a clinical condition characterized by progressive shoulder pain and marked restriction in joint range of motion, substantially affecting individuals' activities of daily living and overall quality of life. The lifetime prevalence of frozen shoulder in the general population has been reported to range between 2% and 5%. The condition most commonly occurs in the fifth and sixth decades of life and may lead to significant functional limitations throughout its clinical course. Additionally, involvement of the contralateral shoulder has been observed in a considerable proportion of individuals diagnosed with frozen shoulder.

The clinical course of frozen shoulder is typically described in three stages: the inflammatory stage, during which pain and inflammation predominate; the frozen stage, characterized by pronounced joint stiffness; and the thawing stage, during which gradual improvements in shoulder mobility are observed. Pain, often more severe at night, is a dominant feature in the early stage, whereas pain may decrease in later stages despite persistent movement restrictions. During the thawing stage, pain generally becomes minimal while range of motion progressively improves.

The diagnosis of frozen shoulder primarily relies on clinical evaluation and is defined by the presence of shoulder pain accompanied by significant limitations in both active and passive glenohumeral joint range of motion. Although frozen shoulder has historically been considered a self-limiting condition, contemporary evidence suggests that spontaneous and complete recovery may not occur in all individuals, and long-term functional impairments may persist in some cases.

The shoulder joint, owing to its extensive range of motion, depends heavily on the coordinated and balanced activation of surrounding musculature for stability and function. Pain, weakness, or impaired coordination within these muscle groups may disrupt shoulder biomechanics, resulting in notable functional deficits. In frozen shoulder, pain-related protective muscle activation and movement avoidance behaviors may contribute to progressive deterioration of periarticular muscle function.

Pain and restricted mobility in frozen shoulder are not limited to local mechanical factors but may also induce central nervous system mechanisms that suppress motor output. Pain-related inhibition has been associated with reductions in maximal voluntary muscle force, altered motor unit recruitment, and decreased muscle endurance. Furthermore, reduced use of the affected upper extremity and protective immobilization behaviors may exacerbate neuromuscular dysfunction.

The primary goals in frozen shoulder management are pain reduction and restoration of shoulder joint mobility. In clinical practice, analgesic interventions, physiotherapy, and manual mobilization techniques are commonly employed and have been shown to contribute to functional improvements. However, during phases in which pain is prominent and direct mechanical loading of the affected shoulder is poorly tolerated, the applicability of conventional exercise approaches may be limited. These challenges highlight the need for alternative rehabilitation strategies aimed at preserving neuromuscular function when direct exercise of the affected extremity is not feasible.

Cross-education refers to the phenomenon whereby unilateral resistance training leads to strength gains in the homologous muscles of the contralateral, untrained limb. This effect has been extensively investigated in conditions involving unilateral immobilization, orthopedic injuries, and movement restrictions, where maintaining muscle function in the non-exercised extremity is clinically desirable. Unilateral exercise interventions have been shown to enhance contralateral strength output, with evidence suggesting that these effects are predominantly mediated by neural mechanisms rather than peripheral muscular adaptations.

The mechanisms underlying cross-education are widely considered to originate from central nervous system adaptations. Unilateral resistance training has been associated with increased excitability of contralateral motor cortical regions and modifications within motor networks responsible for movement planning and execution. These neural adaptations may facilitate improved motor output in the untrained limb independent of muscle hypertrophy. Cross-education has therefore been conceptually linked to motor learning processes involving central reinforcement of efficient muscle activation patterns.

More recently, the potential contribution of visual-motor networks, including mechanisms associated with the mirror neuron system, has been proposed. Activation of shared neural substrates during both movement execution and observation may modulate motor cortical excitability, suggesting that visually mediated strategies could influence cross-education effects.

Frozen shoulder presents a unique clinical scenario characterized by pain, restricted mobility, and central inhibition mechanisms. These factors complicate efforts to maintain muscle strength and neuromuscular function during periods when direct loading of the affected shoulder is limited. Given that cross-education enables strength-related neural adaptations without imposing mechanical stress on the affected extremity, it represents a conceptually relevant approach for this population. Consequently, cross-education may offer a promising rehabilitation strategy for preserving shoulder function and supporting recovery in individuals with frozen shoulder.

Inclusion Criteria:

• Having a diagnosis of unilateral primary or secondary frozen shoulder.
• Range of motion in external rotation, abduction, or flexion reduced by less than 50% in at least one of these directions compared to the unaffected shoulder.
• Loss of joint range of motion exceeding 30% in at least two movement planes compared to the unaffected shoulder.
• Aged between 18 and 65 years.

Exclusion Criteria:

• Presence of a concomitant shoulder injury (e.g., rotator cuff tear).
• Osteoarthritis in both shoulders or in the acromioclavicular joint.
• Previous treatment for frozen shoulder.
• Psychiatric disorders that may interfere with adherence to medical or physical therapy.
• Presence of other neurological disorders.
• A score below 24 on the Mini-Mental State Examination (MMSE).

The study will include patients aged 18-65 years with a diagnosis of unilateral primary or secondary frozen shoulder. Participants will be randomly assigned to either the control or intervention group.

Participant data will be securely stored by the investigators and may be shared with authorized personnel if deemed necessary, in accordance with applicable privacy regulations.