Possible causes of knee clicking when the foot is bent include physiologic joint popping, meniscus injury, knee free body, and chondromalacia patella. 1. Physiological joint popping: the knee joint is heavily loaded, and the pressure in the joint cavity changes during activities, resulting in a released popping sound. 2. Knee meniscus injury: meniscus is an important tissue to protect the knee joint, which can increase the contact area of the joint. When the meniscus is damaged or diseased, the popping sound may occur due to the instability of the meniscus during activities. 3. Knee joint free body: when the knee joint bone or meniscus injury, resulting in small tissue detachment, the formation of free body in the joint, joint activities, free body collision extrusion issued by the popping sound. 4. Chondromalacia patella: Chondromalacia patella can cause rough and uneven patellar joint surface, when the knee joint activity, the patella and femur joint surface or the surrounding tissues friction, resulting in rattling. It is recommended to go to orthopedics as soon as possible to further find out the cause of the disease and standardize the treatment in time.