Preliminary Psychometric Properties of the S-Series Functional Scales (SFS-S, KFS-S, LBFS-S): A Pilot Feasibility Study
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Background
Musculoskeletal (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.
Objective
To 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.
Methods
A 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 (Cronbach’s alpha), convergent validity (Spearman’s rho), and discriminative validity (AUC, sensitivity, specificity) were assessed.
Results
Feasibility 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.
Conclusion
These 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).
