Purpose <p>The infrapatellar fat pad (IFP) is a source of pain in patients with knee osteoarthritis (OA). Changes in pressure due to knee joint movement may increase IFP stiffness and potentially cause pain. This study aimed to investigate changes in IFP stiffness during knee flexion in knee OA patients.</p> Methods <p>Sixteen patients with knee OA and 14 healthy elderly controls participated in this study. IFP stiffness was measured with the patient in the supine position at maximum knee extension, 45°, 90°, and maximum flexion using ultrasound elastography. Stiffness was measured at the superficial layer of the IFP, located beneath the patellar tendon. Additionally, the knee pain score for knee OA patients was assessed using the numeric rating scale and the Intermittent and Constant Osteoarthritis Pain (ICOAP).</p> Results <p>Two-way analysis of variance showed a significant main effect for group and joint angle, as well as an interaction effect. IFP stiffness increased with knee flexion. Knee OA patients exhibited significantly higher IFP stiffness at both maximum knee extension and flexion than the control group. A significant correlation was found between IFP stiffness in knee OA patients at 45°, 90°, and maximum knee flexion and ICOAP intermittent pain.</p> Conclusion <p>These results suggest that knee movement affects IFP stiffness, with increased stiffness observed in knee OA patients. Increased stiffness in OA patients may contribute to knee pain, particularly at mid-to-deep flexion angles.</p>