Cross-cultural adaptation, reliability, and validity of the Persian version of the Lower Limb Functional Index

Abstract

Title: Cross-cultural Adaptation, Reliability, and Validity of the Persian Version of the Lower Limb Functional Index

Purpose: The Lower Limb Functional Index (LLFI) is a regional patient reported outcome measure (PROM) used to evaluate functional status and intervention effects in patients with lower limb musculoskeletal disorders in both clinical and research settings. No Persian language (LLFI-Pr) version is available. This study’s aim was translation and cross-cultural adaptation to produce the LLFI-Pr, and to evaluate its essential psychometrics properties.

Method: Stage 1, LLFI-Pr translation and cross-cultural adaptation, were performed with double forward-backward protocol. Face and content validity were evaluated through cognitive interviews and calculating the content validity index (CVI). Stage 2, psychometric evaluation, recruited from a sample of convenience (n=307, age 47.18±11.52 years, 58.3% female) where subjects completed the LLFI-Pr and the Persian Lower Extremity Functional Scale (LEFS) criterion PROMs. Test-retest reliability used a sub-sample (n=64) at an interval of 3-7 days. Internal consistency used Cronbach’s Alpha (α); concurrent validity used Pearson’s r correlation coefficient between PROMs (LLFI-Pr/LEFS); and construct validity used exploratory factor analysis (EFA) with principal factor analysis (PFA) and Promax rotation for non-Gaussian distribution.

Results: Translation and cultural adaptation required only minor wording changes. Psychometric properties were high for test-retest reliability (ICC2.1=0.90), internal consistency (α=0.77), and good for concurrent validity (r=0.63). The EFA demonstrated a two-factor structure that explained 22.01% of total variance. Five items did not demonstrate consistent factor loading and a 20-item version compared equivalently to the 25-item score (LLFI-Pr-25/LLFI-Pr-20, r=0.97), but with reduced continuity in the general/region-specific items ratio from 60/40 to 50/50. No floor or ceiling effects were observed.

Conclusion: The LLFI-Pr is a lower limb region-specific, two-factor structure PROM where 20 items scored consistently with the full 25-item LLFI-Pr, but at a lower general/region-specific ratio. It is valid for the evaluation of Persian language populations with lower limb disorders in the research and clinical settings.