This cross-sectional observational study aims to evaluate the validity and reliability of the Four Square Step Test (FSST) in individuals with rheumatoid arthritis. In addition, the study will investigate the associations between FSST performance and clinical parameters including disease activity, quality of life, pain, and muscle strength.

All assessments will be conducted according to a pre-defined standardized protocol. The order of measurement tools will be randomized to minimize potential bias, and sufficient rest intervals will be provided between tests to prevent fatigue and performance effects.

Participants will include individuals aged 18-65 years with a diagnosis of rheumatoid arthritis according to the ACR/EULAR 2010 classification criteria, who have been followed for at least 6 months, have had no major changes in treatment regimen in the last 4 weeks (e.g., initiation or change of DMARDs/biologics, high-dose steroid increase), can walk at least 10 meters independently, have a Mini-Mental State Examination score of 24 or higher, and are able to follow verbal instructions in Turkish to comply with study procedures.

Appropriate statistical methods will be applied to assess validity, reliability, and associations between FSST and clinical parameters.

Rheumatoid arthritis (RA) is a chronic, autoimmune connective tissue disease that affects approximately 0.5-1% of the global population. The disease is characterized by symmetrical joint pain, swelling, and restricted mobility. Synovial inflammation in its pathogenesis leads to structural damage, cartilage erosion, joint deformities, and irreversible functional impairments. Beyond joint involvement, RA significantly affects daily living activities, work capacity, and overall quality of life through its impact on the musculoskeletal system. Muscle weakness, reduced joint range of motion, and impaired proprioception occurring during the disease process particularly compromise lower extremity function and contribute to impaired postural control. Consequently, balance deficits, an increased risk of falls, and fall-related injuries are observed more frequently in individuals with RA compared to the healthy population.

Balance is generally divided into static and dynamic balance. Static balance refers to the ability to maintain the center of mass within the base of support while standing still, whereas dynamic balance denotes the ability to sustain the center of mass safely and in a controlled manner within the base of support during movement. In RA, balance impairments become especially evident in daily activities such as walking, changing direction, and climbing stairs. These difficulties elevate fall risk and adversely influence both physical independence and social participation.

Several assessment tools have been developed to evaluate balance in RA. The Berg Balance Scale (BBS), Tinetti Balance Test, Timed Up and Go Test (TUG), and Mini-BESTest are among the most widely used instruments in this field. However, many of these tools require lengthy administration, specialized equipment, or advanced clinical expertise. Such limitations can complicate rapid patient assessment, particularly in busy outpatient clinical settings.

The Four Square Step Test (FSST) is a practical assessment that can be administered within a short period of time using simple materials, measuring an individual's ability to step in multiple directions. The reliability and validity of FSST have been established in various clinical populations, including those with ankle injuries, hemophilia, cerebral palsy, Duchenne muscular dystrophy, Down syndrome, hip arthroplasty, and anterior cruciate ligament reconstruction. Nevertheless, no validity and reliability study of the FSST has yet been conducted in individuals with RA.

The primary objective of this study is to determine the validity and reliability of the Four Square Step Test (FSST) in individuals with rheumatoid arthritis. The secondary objective is to examine the relationship between FSST outcomes and clinical indicators such as disease activity, quality of life, pain and muscle strength.

First, demographic and clinical data of all participants will be collected, including age, height, weight, smoking and alcohol consumption, dominant side, use of assistive devices, age at diagnosis, comorbidities, medications, and history of falls within the past 1-2 years. Body mass index (BMI) will be calculated.

All analyses will be conducted as two-tailed, with a significance level set at α = 0.05, and results will be reported with 95% confidence intervals. The distribution of continuous variables will be assessed using the Shapiro-Wilk test. Normally distributed variables will be presented as mean ± standard deviation, while non-normally distributed variables will be reported as median values. Categorical variables will be presented as frequency (percentage).

The relationships between FSST time and the scores of the Berg Balance Scale (BBS) and Mini-BESTest will be examined using Pearson's or Spearman's correlation coefficients, depending on data distribution. The predictive ability of FSST for poor balance status will be evaluated with receiver operating characteristic (ROC) curve analysis, and results will be reported in terms of area under the curve (AUC), sensitivity, specificity, Youden's index, and optimal cutoff values.

Test-retest reliability will be determined using the intraclass correlation coefficient (ICC[3,1]) based on repeated FSST measurements performed by the same rater. Measurement error and minimal detectable change will be calculated as follows: SEM = SD × √(1 - ICC) and MDC95 = SEM × 1.96 × √2. Agreement between repeated measures will be illustrated using Bland-Altman plots (mean bias and ±1.96 SD limits).

Comparisons of FSST times between clinical subgroups (e.g., disease activity level, history of falls) will be performed using independent-samples t-test/ANOVA if normality assumptions are met, or Mann-Whitney U/Kruskal-Wallis tests otherwise. Effect sizes will be reported as Hedges' g, r, and η²/partial η².

Reporting will follow the STROBE guidelines for observational cross-sectional studies.

The sample size was calculated using the G*Power software with α = 0.05 and 1-β = 0.80. Previous studies have reported that the FSST demonstrates moderate to high levels of validity in different clinical populations. For example, in patients with hip osteoarthritis and total hip arthroplasty, correlations between FSST and other balance scales were found to range between r = 0.60 and r = 0.70. In individuals with total knee arthroplasty, FSST was reported to correlate with the TUG at r = 0.65 and with the HSS at r = -0.40. Furthermore, studies conducted in populations with hemophilia and Parkinson's disease indicated excellent reliability of FSST and modified FSST, with ICC values approximately between 0.96 and 0.99. Such studies have typically been conducted with sample sizes ranging from 17 to 28 participants.

In light of these findings, it was determined that including approximately 40 participants in the present study would be sufficient to ensure adequate statistical power for the planned validity analyses.

Inclusion Criteria:

• Age between 18 and 65 years
• Diagnosis of rheumatoid arthritis according to ACR/EULAR 2010 criteria
• No major changes in treatment regimen (e.g., initiation or modification of DMARDs/biologics, high-dose steroid increase) within the last 4 weeks
• Ability to walk at least 10 meters without assistance
• Mini-Mental State Examination (MMSE) score ≥ 24
• Ability to understand and follow instructions in Turkish

Exclusion Criteria:

• Participants who cannot understand or follow verbal instructions in Turkish
• Participants with a history of neurological, vestibular, or serious orthopedic disease
• Participants whose physical participation is limited due to acute infection or systemic disease
• Participants who have had lower extremity surgery or a history of serious injury within the last 6 months

Participants aged 18-65 years with rheumatoid arthritis (RA) will be included in this study.

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.