OBJECTIVES: Enhanced muscle echo intensity (EI) with ultrasound imaging and a higher extracellular water-to-intracellular water (ECW/ICW) ratio with segmental-bioelectrical impedance spectroscopy (S-BIS) represent muscle quality loss. This study aimed to clarify quadriceps muscle degeneration characteristics, focusing on muscle quality changes in patients with knee osteoarthritis (OA). METHOD: Forty-one women with knee OA (mean age, 71.4±6.0 years) and 27 healthy women (mean age, 75.6±4.9 years) participated. Ultrasonography was used to evaluate the muscle thickness (MT) and the EI of each quadriceps compartment. The ECW/ICW ratio was obtained by S-BIS. MT, EI, and ECW/ICW ratio differences between the two groups were tested using univariate analysis of variance, adjusting for age and body mass index. Logistic regression analyses were performed with the group as the dependent variable, and the MT and EI of the vastus medialis (VM) and the ECW/ICW ratio as independent variables. RESULTS: Patients with knee OA had a significant decrease in VM MT, enhanced VM, and vastus intermedius EIs and a higher ECW/ICW ratio compared with healthy participants. Logistic regression analysis showed that the VM EI (odds ratio [OR], 1.19; 95% confidence interval [CI], 1.06-1.35) and the ECW/ICW ratio were independently associated with knee OA (OR, 1.19; 95% CI, 1.00-1.42). CONCLUSIONS: VM EI and the ECW/ICW ratio, rather than VM MT, characterised quadriceps muscle degeneration in patients with knee OA. Therefore, enhanced EI and a higher ECW/ICW ratio are helpful clinical signs for detecting muscle degeneration in patients with knee OA. Key Points •Echo intensity (EI) of the vastus medialis and the extracellular-to-intracellular water (ECW/ICW) ratio significantly increased in patients with knee osteoarthritis OA). •Enhanced EI and a higher ECW/ICW ratio are useful clinical signs for detecting muscle degeneration in patients with knee OA.