Knee pain with tenderness usually has three causes: First, injury to the meniscus of the knee joint. After meniscal injury, the tibia and femur will squeeze and stimulate the meniscus during knee activities, especially when going up and down stairs, the knee joint will produce more obvious pain, the body will produce self-protective reactions, and the knee joint will become soft. Second, see the patellofemoral arthritis and patellar tendon damage, in the knee flexion and extension due to the patellar tendon injury is stretched, or patellofemoral joint stimulation, will produce obvious pain, but also lead to the knee soft situation. The synovial crease syndrome is a painful condition that occurs when the synovial crease is squeezed by the bones of the knee, and may cause sudden pain and tenderness in the knee joint during activities such as flexion and extension and when walking.