The Effect of Cross Training on Clinical and Functional Outcomes in Frozen Shoulder Rehabilitation

Overview

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.

Detailed description

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.

Interventions

• Other Standard Physical Therapy Group
  The program will be applied to both groups twice weekly for eight weeks, with the experimental group additionally receiving cross-education. The control group will undergo a 40-minute standard physical therapy protocol per session, progressively structured to reduce pain and improve shoulder range of motion. During weeks 0-3, passive and active-assisted range of motion exercises, scapular mobilization, stretching, and joint mobilizations with distraction will be performed within pain limits. Be
• Other Cross-Education Group
  In the experimental group, each session will last 40 minutes, comprising 20 minutes of standard physical therapy and 20 minutes of cross-education. The standard physical therapy content will be identical to that of the control group. The cross-education program will aim to increase the internal rotation, external rotation, flexion, and abduction strength of the unaffected shoulder during immobilization. Resistance exercises will be performed at 70-80% of the one-repetition maximum (1RM) of the

Primary outcome measures

• Shoulder Pain and Functional Disability [Time frame: Baseline and post-intervention (8 weeks later )]

Secondary outcome measures (7)

• Numeric Pain Rating Scale (NPRS) [Time frame: Baseline and post-intervention (8 weeks later )]
• Muscle Strength [Time frame: Baseline and post-intervention (8 weeks later )]
• Joint Range of Motion Assessment [Time frame: Baseline and post-intervention (8 weeks later )]
• Upper Extremity Function [Time frame: Baseline and post-intervention (8 weeks later )]
• Quality of Life (QoL) [Time frame: Baseline and post-intervention (8 weeks later )]
• Activities of Daily Living (ADL) [Time frame: Baseline and post-intervention (8 weeks later )]
• Reaction Time Measurement [Time frame: Baseline and post-intervention (8 weeks later )]

Eligibility criteria

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).

Criteria are shown verbatim from the registry (in English). Final eligibility is always assessed by the study center.

Healthy volunteers: No

Study design

Observational model

Other

Study locations

Center list to be confirmed — check the primary protocol.

Publications

• Saldiran TC, Tanhan A, Celikel R, Dincer E. Reliability of a Visual-Cognitive Technology in Measuring Reaction Times: Associations with Age, Activity Level, and Grip Strength. J Manipulative Physiol Ther. 2025 Jul-Dec;48(6-9):789-799. doi: 10.1016/j.jmpt.2025.10.054. Epub 2025 Nov 13. PMID 41236463
• Loubani K, Kizony R, Milman U, Schreuer N. Hybrid Tele and In-Clinic Occupation Based Intervention to Improve Women's Daily Participation after Breast Cancer: A Pilot Randomized Controlled Trial. Int J Environ Res Public Health. 2021 Jun 2;18(11):5966. doi: 10.3390/ijerph18115966. PMID 34199448
• Mena-Del Horno S, Balasch-Bernat M, Louw A, Luque-Suarez A, Rodriguez-Brazzarola P, Navarro-Ledesma S, Murillo C, Duenas L, Lluch E. Is there any benefit of adding a central nervous system-focused intervention to a manual therapy and home stretching program for people with frozen shoulder? A randomized controlled trial. J Shoulder Elbow Surg. 2023 Jul;32(7):1401-1411. doi: 10.1016/j.jse.2023.02.13 PMID 37001795
• Duzgun I, Turgut E, Eraslan L, Elbasan B, Oskay D, Atay OA. Which method for frozen shoulder mobilization: manual posterior capsule stretching or scapular mobilization? J Musculoskelet Neuronal Interact. 2019 Sep 1;19(3):311-316. PMID 31475938
• Yasaci Z, Celik D. Does Integration of Graded Motor Imagery Training Augment the Efficacy of a Multimodal Physiotherapy Program for Patients With Frozen Shoulder? A Randomized Controlled Trial. Clin Orthop Relat Res. 2025 Apr 1;483(4):707-716. doi: 10.1097/CORR.0000000000003252. Epub 2024 Sep 17. PMID 39436270
• Lluch-Girbes E, Duenas L, Mena-Del Horno S, Luque-Suarez A, Navarro-Ledesma S, Louw A. A central nervous system-focused treatment approach for people with frozen shoulder: protocol for a randomized clinical trial. Trials. 2019 Aug 13;20(1):498. doi: 10.1186/s13063-019-3585-z. PMID 31409380
• Hendy AM, Lamon S. The Cross-Education Phenomenon: Brain and Beyond. Front Physiol. 2017 May 10;8:297. doi: 10.3389/fphys.2017.00297. eCollection 2017. PMID 28539892
• Lee M, Carroll TJ. Cross education: possible mechanisms for the contralateral effects of unilateral resistance training. Sports Med. 2007;37(1):1-14. doi: 10.2165/00007256-200737010-00001. PMID 17190532

Identifiers

NCT: NCT07510737 · 1004

Primary sources (government registries)

• ClinicalTrials.gov (US NIH)

View this study on ClinicalTrials.gov ↗