The aim of the study is to measure the validity and reliability of the StUD test in patients with TKA. Forty-seven patients with primary TKA included in this study. The test-retest reliability of the StUD test was measured with 1-hour interval. Validity was assessed that the 30s Chair Stand Test (30sCS), the Hospital for Special Surgery score (HSS) and Short Form-12 Quality Life Questionnaire (SF-12) were used as comparator instruments.

The study was designed as test-retest research. This study was approved by the ethics committee of Dokuz Eylul University, Izmir, Turkey. This study was conducted between 2021 and 2022 at the Dokuz Eylul University Hospital Orthopedics Department, Izmir, Turkey. Informed consent was obtained from all participants in accordance with the Declaration of Helsinki.

Sample Size The NCSS Trial and PASS 14 software (NCSS, LLC, Kaysville, Utah USA) was used to conduct the sample size. Power of 0.8 and an alpha level of 0.05 were used. Previous research on StUD in patients with knee Osteoarthritis was noted good test-retest reliability (ICC= 0.87) (G. P. L. Almeida et al., 2021). The test-retest reliability of the current study was thus hypothesized to be excellent. Assuming a %10 attrition rate, a null reliability with ICC=0.75 and expected reliability with ICC=0.90, 47 subjects would be needed.

Participant Forty-seven patients with primary TKA who were operated on by the same surgeon using the paramedian approach were contacted by phone and volunteered to participate in this study. Patients with primary TKA who are at least six months post-surgery were included. Patients were selected and invited to the study by the surgeon and physiotherapist.

Statistical Analysis All data were analyzed using the IBM SPSS Statistics (Version 23.0) software. The Kolmogorov-Smirnov/Shapiro-Wilk tests were used for the determination of the normal distribution. The difference between the pain levels before and after the trials was compared using the Paired Samples T test. The level of significance was determined p < .05. The correlations between the StUD and 30 CST, HSS, PCS12, and MCS12 were calculated to determine convergent validity. The convergent validity was analyzed based on the Spearman's correlation and was interpreted as follows: 0-0.09 = no correlation; 0.1-0.39 = weak correlation; 0.4-0.59 = moderate correlation; 0.7-0.89 = strong correlation; and 0.9-1 = perfect correlation. Paired samples' t test was used for comparing the difference between the first and second trials of StUD test. The intraclass correlation coefficient (ICC2,1) was used assess the test-retest reliability. The ICC values were interpreted as follows: poor, <0.5; moderate, 0.5-0.75; good, 0.75-0.9; excellent reliability, >0.9 (Koo & Li, 2016). The standard error of measurement (SEM) was calculated as SD×√(1-ICC) , where SD is the pooled standard deviation from the first session. Minimal detectable change (MDC) was calculated at a 95% confidence level (MDC95). MDC95 was calculated according to following formula: MDC95=SEM×1.95×√2. All data were analyzed using the IBM SPSS Statistics (Version 23.0) software

Inclusion Criteria:

• Patients with primary TKA who are at least six months post-surgery
• Patients over 40 years of age

Exclusion Criteria:

• having a body mass index (BMI) greater than 40 kg/m2,
• having had other surgery on a leg with TKA,
• having symptomatic hip OA,
• having a history of knee or hip fracture,
• having a previous orthopedic or neurological disease that prevents stepping

Patients with primary TKA who were operated on by the same surgeon using the paramedian approach were contacted by phone and volunteered to participate in this study. Patients with primary TKA who are at least six months post-surgery were included. Patients were selected and invited to the study by the surgeon and physiotherapist