Is It Time to SIMPLI-FAI? Development of the SIngle-Leg Squat Movement Performance scaLe for Individuals With FemoroAcetabular Impingement Syndrome Codesigned With End Users

OBJECTIVE: To develop and assess the validity and reliability of the “SIngle-leg squat Movement Performance scaLe for Individuals with FemoroAcetabular Impingement syndrome” (SIMPLI-FAI) through a codesign approach with end users. DESIGN: Codesigned instrument development and measurement properties assessment based on the COnsensus-based Standards for the selection of health Measurement Instruments (COSMIN) checklist. METHOD: The scale’s theoretical ground and structure were established. Seven physiotherapists (end users) participated in content validity assessment through online surveys and focus group discussions. The single-leg squat movement quality of 45 individuals with femoroacetabular impingement (FAI) syndrome post–hip arthroscopy and 15 asymptomatic individuals was evaluated by 2 physiotherapists using the scale. Interrater and intrarater reliabilities were assessed using the intraclass correlation coefficient (ICC), minimal detectable change, and standard error of measurement. Internal consistency was evaluated using Cronbach’s alpha. RESULTS: SIMPLI-FAI comprises 6 questions with dichotomous answers (yes/no) regarding overall and segmental movement quality. It had good internal consistency (alpha = 0.88) and good interrater (ICC = 0.84) and intrarater reliability (ICC = 0.90) for individuals with FAI syndrome. When asymptomatic individuals were included, SIMPLI-FAI had good internal consistency (alpha = 0.81), moderate interrater reliability (ICC = 0.68) and good intrarater reliability (ICC = 0.79). CONCLUSION: SIMPLI-FAI can be used for assessing single-leg squat movement quality in patients with FAI syndrome, exhibiting adequate content validity, reliability, and internal consistency through a codesign approach. This is the first scale systematically developed for this end, and future studies incorporating SIMPLI-FAI in the clinical assessment of patients with FAI syndrome are required to demonstrate the clinical validity of the new instrument. JOSPT Open 2025;3(3):286-295. Epub 7 May 2025. doi:10.2519/josptopen.2025.0050