Association Between Muscle Quality Assessed by the 5-Repetition Sit-to-Stand Test and Falls in Community-Dwelling Older Adults in Japan: A Cross-Sectional Study

<b>Background:</b> Falls in older adults are a major barrier to healthy longevity. Recent studies suggest that muscle quality is associated with fall risk. This study aimed to determine whether a functional muscle quality index (MQI) using the 5-repetition sit-to-stand test (5R-STS) reflects fall risk in community-dwelling older adults. <b>Methods:</b> This cross-sectional study included 137 community-dwelling older adults (≥65 years) in Japan. Lower limb skeletal muscle mass (SMM) was measured using the BIA method, and muscle function was assessed using the 5R-STS. The MQI was calculated as "(5R-STS (s)/SMM (kg)) × 10". Fall history was collected using a self-administered questionnaire, and binary logistic regression analysis including gait speed and physical frailty was performed. <b>Results:</b> Participants were divided into fallers (n = 36; age = 78.2 ± 5.6) and non-fallers (n = 101; age = 76.9 ± 5.3). Significant differences were found between the groups in gait speed (<i>p</i> = 0.01), TUG (<i>p</i> < 0.01), 5R-STS (<i>p</i> < 0.01), and MQI (<i>p</i> < 0.01). Binary logistic regression identified MQI (OR = 1.28; <i>p</i> < 0.01) and gait speed (OR = 0.14; <i>p</i> = 0.02) as explanatory variables for fall history. The results of the evaluation using the receiver operating characteristic (ROC) curve showed that the cutoff value for MQI to distinguish fall history was 8.04 s/kg, and the cutoff value for gait speed was 1.21 s. <b>Conclusions:</b> The MQI using the 5R-STS shows promise as an indicator of fall risk in older adults. Further longitudinal studies are needed to clarify the causal relationship.