Adaptation of the Upper Limb Functional Index (ULFI) to a Polish version and validation in patients with upper limb musculoskeletal disorders.

Background: The Upper Limb Functional Index (ULFI) is a robust, widely used tool for assessing the functional status of the upper limbs (ULs) and the effectiveness of interventions in patients with musculoskeletal disorders (MSDs). This study aimed to translate and culturally adapt the ULFI into Polish (ULFI-PL) and evaluate its psychometric properties and practical characteristics.

Methods: A total of 127 patients (54% female, $\overline{x}$  = 45.07 ± 14.97 years) with various ULMSDs and symptom durations completed the ULFI-PL, the shortened Disabilities of the Arm, Shoulder, and Hand questionnaire-Polish version (QuickDASH-PL), the Polish version of the World Health Organization Quality of Life-BREF (WHOQOL-BREF-PL), Numeric Pain Rating Scale (NPRS), and the seven-point Global Rating of Change (GRC) scale. The internal consistency, construct validity, and factor structure were assessed in all the participants; the test-retest reliability and measurement error were evaluated in a subgroup (n = 112, 2-3-day interval); and the responsiveness and interpretability were evaluated in another subgroup (n = 56, 8-week interval, after physiotherapy).

Results: The ULFI-PL demonstrated a good internal consistency (α = 0.77) and high construct validity, supported by the confirmation of five out of six a priori hypotheses (83.33%). A confirmatory factor analysis (CFA) revealed a unidimensional structure. It also demonstrated a high test-retest reliability (r = 0.85). The measurement error was calculated using the standard error of measurement (SEM = 4.75%) and the minimal detectable change (MDC₉₅ = 13.17%). The ULFI-PL showed a high responsiveness after physiotherapy, which was confirmed by the effect size (ES = 2.08) and the standardized response mean (SRM = 1.88). The minimal clinically important difference (MCID) for the ULFI-PL was 28.29% (95% CI: 24.96-31.63).

Conclusions: The ULFI-PL is a reliable, valid, and responsive tool for assessing the upper limb function in Polish-speaking patients with ULMSDs and is suitable for use in both clinical and research contexts. The results are consistent with previous studies on the original English version and other language adaptations.