Development and Validation of the Low Sit–High Step Test for Assessing Lower-Extremity Function in Sarcopenia

<b>Objectives:</b> This study aimed to evaluate the validity, reliability, and diagnostic accuracy of the Low Sit-High Step (LS-HS) Test as an original, cost-effective, and clinically practical tool for assessing lower-extremity muscle strength and function, with a specific focus on its sensitivity in detecting early-stage sarcopenia. <b>Methods:</b> This cross-sectional study included 205 participants divided into four groups: probable sarcopenia, sarcopenia, and two control groups (young and middle-to-older adults). The LS-HS Test was compared across groups and against standard assessments to evaluate its efficacy in measuring lower-extremity function. Reliability was verified through Cronbach's alpha and ICC. Multinomial logistic regression was used to determine the test's predictive power, while ROC analysis assessed its diagnostic accuracy for sarcopenia screening. <b>Results:</b> The LS-HS scores were significantly higher in participants with probable sarcopenia and sarcopenia (<i>p</i>< 0.05). Multinomial logistic regression revealed that the LS-HS performance was a significant predictor of both probable sarcopenia and sarcopenia (<i>p</i> < 0.001). The test demonstrated excellent internal consistency (Cronbach's α = 0.938) and very high inter-rater and test-retest reliability (ICC = 0.998). ROC analysis confirmed high diagnostic accuracy in distinguishing both probable sarcopenia (AUC = 0.768) and sarcopenia (AUC = 0.704) (all <i>p</i>< 0.01). <b>Conclusions:</b> The LS-HS Test is a valid, reliable, and sensitive tool for assessing lower-extremity functional capacity. Its ability to identify early functional decline, often manifesting before significant muscle mass loss, positions it as an effective alternative to traditional assessments in routine clinical practice, particularly for the early detection and monitoring of the sarcopenia spectrum.