There are three main reasons for a sudden weakness in the knee when walking on the leg: the first is seen in meniscus injuries. After a meniscus injury, a sudden painful sensation occurs due to the irritation of the injured end when flexing and extending the joint, resulting in a sudden weakness of the knee. The second type of injury is due to patellofemoral instability or a significant strain of the patellar tendon and quadriceps muscle. When walking, sudden bending and straining of the knee can cause violent strains on the patellar tendon, patella and quadriceps. If there is a significant injury, this can lead to further stimulation of the injured area, a sudden increase in pain, and also tenderness. The third type, synovial crease syndrome, is a condition in which the synovial crease of the knee is stuck between the knee joints and the joints squeeze this area, resulting in a significant painful sensation and a sudden tenderness.