Supplementary Material for: Ultrasound-Derived Anterior Thigh Muscle Thickness as a Marker of Physical Frailty in Geriatric Outpatients

Background: Physical frailty in older adults is frequently associated with adverse outcomes, yet most assessment tools rely on subjective or time-consuming measures. This study explores ultrasound-derived anterior thigh muscle thickness (ATMT) as an objective marker for frailty. Methods: This cross-sectional study included 641 patients aged 65 years and older. Comprehensive geriatric evaluations and anterior thigh muscle thickness (ATMT) measurements by ultrasound were performed. Frailty was diagnosed using the Fried Frailty Phenotype (FFP). The association between ATMT and frailty was examined using multivariate logistic regression analysis. Results: Participants’ mean age was 75±6.4 years; 62.9% were women, and 12% were frail. Frail individuals had significantly lower ATMT compared to non-frail counterparts (p<0.001). ATMT remained independently associated with frailty after adjusting for confounders (β=0.94, 95% CI: 0.90–0.99, p=0.033). The area under the ROC curve for ATMT predicting frailty was 0.660 (95% CI: 0.62–0.69; p<0.001). Sex-stratified analyses revealed higher accuracy in men (AUC=0.757) than in women (AUC=0.599). The optimal ATMT cut-off was ≤30.5 mm, with a negative predictive value of 90.3%. Conclusions: Ultrasound-derived ATMT offers a simple, radiation-free, and objective method for frailty screening, showing stronger discriminative ability in men and moderate accuracy in women. It may be used as a practical screening measure in combination with other geriatric assessment tools in outpatient settings.