Background: Affordable screening for knee extensor weakness (KEW) is needed in primary care. We piloted a pragmatic sex-specific serial algorithm that first uses a low-cost body-composition indicator and then a smartphone measure.
Methods: In a cross-sectional pilot, 60 community-dwelling older adults (30 women, 30 men) were assessed (age, mean±SD: women 65.90±4.29 year; men 66.33±5.37 year). Bioimpedance analysis (BIA) provided the muscle mass-to-body weight ratio (MM/BW). A smartphone-video recorded sit-to-stand test velocity (m/s). Reference strength was knee extensor strength normalized to body weight (KES/BW). “Lower strength” was sex-specific 25th percentile (women ≤ 0.325; men ≤ 0.370). Cut-point derivation, sex-specific ROC curves were used. Step-1 targeted high sensitivity using MM/BW cut-points (women < 0.634; men < 0.676). Step-2 targeted high specificity using velocity cut-points (women < 0.385 m/s; men < 0.375 m/s). The serial rule was positive only if both steps were positive. Diagnostic indices were calculated against KES/BW. Results: Prevalence of lower strength was 28.3% (17/60). The two-step algorithm produced 14/60 positives with sensitivity 0.76 (13/17), specificity 0.98 (42/43), PPV 0.93 (13/14), and NPV 0.91 (42/46). False positives were 1/43 normals, indicating a very low false-positive rate. Conclusions: A sex-specific two-step workflow that screens with BIA-derived MM/BW and confirms with smartphone velocity identified probable KEW in older adults with excellent specificity and good sensitivity while conserving resources. Larger, multi-site validation with predefined external cut-points, test–retest reliability, and implementation outcomes is recommended.

Authors:
Weerasak Tapanya, University of Phayao, Thailand
Noppharath Sangkarit, University of Phayao, Thailand

About the Presenter(s)
Dr. Weerasak Tapanya is Assistant Professor of Physical Therapy at the University of Phayao, Thailand. His work links movement science, biomechanics, ergonomics, and rehab to optimize exercise, reduce injury risk, and support healthy aging.

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