OBJECTIVES:To investigate any differences in muscle architecture (fascicle angle, fascicle length, and muscle thickness) and muscle strength in people of different ages with and without knee osteoarthritis (OA). METHODS:This is a cross-sectional study conducted with 40 individuals with and 40 without knee OA. Four groups were analyzed, middle-aged OA group (KL II/III) aged 40-50 years (n = 20), middle-aged healthy (H) group aged 40-50 years (n = 20), older OA group (KL II/III) aged 70 years and over (n = 20), and older H group, aged 70 years and over (n = 20). Outcomes analyzed were isometric and isokinetic peak torque of knee extensors, level of physical activity, self-reported pain level, and vastus lateralis fascicle length, fascicle angle, and muscle thickness assessed by ultrasound. One-way ANOVA was used to identify differences between groups, followed by the Tukey post hoc test. RESULTS:There were no differences between the middle-aged OA group and older H group for any variables. The older OA group presented the smallest muscle architecture parameters and worst isometric and concentric peak torques compared to the other three groups (p < 0.001). In contrast, the middle-aged H group presented the largest muscle architecture parameters and was the strongest group compared to the others (p < 0.001). CONCLUSIONS:The presence of knee OA is associated with early muscular changes and seems to intensify these thigh changes that are similar to the effects of the aging process.