Rasch Analysis of the 25-Question Geriatric Locomotive Function Scale in Japanese Older Adults with Musculoskeletal Disorders: Identifying Age-Related Differences in Item Difficulty and Misfitting Items

Background Locomotive syndrome (LS), a condition characterized by diminished mobility due to musculoskeletal disorders, is a growing concern among older adults. The 25-item Geriatric Locomotive Function Scale (GLFS-25) is a common tool for LS assessment. However, its reliance on classical test theory and the inclusion of non-motor function items raise questions about its accuracy in reflecting motor dysfunction severity. This study aimed to evaluate the GLFS-25's psychometric properties using Rasch analysis, focusing on item difficulty variations between young-old (60–74 years) and old-old (75–89 years) individuals with musculoskeletal disorders (MSDs). Methods This cross-sectional study recruited 1000 outpatients (500 young-old and 500 old-old) with MSDs. Participants completed the GLFS-25. Rasch analysis was performed using Winsteps software to assess item difficulty, person ability, and item fit. Wright person-item maps were generated to visualize the distribution of item difficulty and person ability. Infit and outfit mean-square values were used to identify misfitting items. Results The mean age of participants was 73.8 ± 6.8 years. Mean GLFS-25 scores were 26.4 ± 22.3 (young-old) and 35.1 ± 23.0 (old-old). Cronbach's alpha exceeded 0.95 in both groups. Significant differences in LS severity proportions were observed between age groups (p < 0.001). Wright maps revealed a scarcity of items discriminating among low-scoring individuals, particularly in the young-old group. Items related to dressing, toilet use, and bathing were most discriminating for high-scoring individuals. Neck/upper limb pain and social engagement were identified as misfitting items across both age groups. Back/lower back/buttock pain and social interaction were misfitting in the young-old and old-old groups, respectively. Conclusions While the GLFS-25 demonstrated excellent internal consistency, Rasch analysis revealed limitations in its ability to discriminate among individuals with low LS scores, particularly in the young-old group. In addition, several misfitting items were identified, suggesting that some items may not contribute effectively to the measurement of LS.