Background: Access to field screening for lower-limb weakness is limited by equipment, cost, and time. We piloted a pragmatic serial algorithm that combines a low-tech tape measure with a smartphone sit-to-stand signal to detect knee extensor weakness (KEW) in community-dwelling older women.
Methods: In this cross-sectional pilot (N=60; female-only, age, mean±SD = 67.02±5.03 year), participants underwent calf circumference (Calf), smartphone sit-to-stand video recordings (mean power), and reference knee extensor strength (KES), expressed as KES normalized to body weight (KES/BW). “Low strength” was defined by the within-sample lowest quartile of KES/BW. Using ROC coordinates, Step-1 Calf aimed for high sensitivity; Step-2 smartphone power aimed for high specificity. The serial rule was positive only if both steps were positive. Diagnostic indices were calculated against the reference.
Results: Step-1 Calf ≤30.25 cm achieved sensitivity 0.911 and flagged 14/60 women for Step-2 testing. Step-2 smartphone power ≤2.9183 (W/kg) yielded specificity 0.867 and 20/60 positives. The combined two-step algorithm produced 12/60 positives with sensitivity 0.73 (11/15), specificity 0.98 (44/45), positive predictive value 0.92, and negative predictive value 0.92 (prevalence 25%). The approach reduced smartphone testing burden to 23% of the sample while maintaining a very low false-positive rate.
Conclusions: A simple serial algorithm using Calf followed by smartphone-derived power identified older women with probable KEW with excellent specificity and good sensitivity. This low-cost, rapid procedure appears feasible for primary care and community programs in resource-limited settings. Larger, sex-inclusive validation with predefined external cut-points and test–retest reliability is recommended.

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

About the Presenter(s)
Dr. Noppharath Sangkarit is a Physical Therapy lecturer at the University of Phayao (Ph.D., Human Movement Science, Khon Kaen). Research: pediatric PT, postural/trunk control, early motor development, motion analysis, early interventions.

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