Preliminary Psychometric Properties of the S-Series Functional Scales (SFS-S, KFS-S, LBFS-S): A Pilot Feasibility Study

BackgroundMusculoskeletal (MSK) disorders are a leading cause of disability. Existing Patient-Reported Outcome Measures (PROMs) are often long and structurally inconsistent across different joints. We developed the S-Series: a unified system of 6-item, ICF-based, 0-24 point scales. ObjectiveTo report the preliminary feasibility and psychometric properties of the S-Series (SFS-S for Shoulder, KFS-S for Knee, and LBFS-S for Low Back) from a pilot study. MethodsA cross-sectional pilot study was conducted on a convenience sample of 74 participants (22 Knee, 23 Shoulder, 29 Low Back), including healthy controls and symptomatic patients. All participants provided informed consent. Participants completed the relevant S-Series scale and its gold-standard comparator. Feasibility (completion time, ease of understanding), internal consistency (Cronbachs alpha), convergent validity (Spearmans rho), and discriminative validity (AUC, sensitivity, specificity) were assessed. ResultsFeasibility was excellent. All participants (100%) reported that the S-Series items were "easy to understand" or "very easy to understand." The average completion time for the entire research packet (which included the 6-item S-Series scale plus its comparator) ranged from 7.1 to 9.7 minutes. Internal consistency was excellent (alpha = 0.92-0.94). Convergent validity was strong (rho = -0.89 to -0.91). Discriminative validity was excellent for KFS-S (AUC = 0.98) and SFS-S (AUC = 0.97), both demonstrating 100% sensitivity. The LBFS-S also showed good discriminative validity (AUC = 0.84) with 85.7% sensitivity and 87.5% specificity. ConclusionThese preliminary pilot data suggest the S-Series functional scales are feasible, reliable, and valid tools for assessing MSK function. The strong psychometric performance supports the rationale for a large-scale validation study, which is currently underway (Hue University IRB Approval No. H2025/627).