Knee pain and weakness may be osteoarthritis of the knee or lumbar disc herniation, as follows: 1. Knee osteoarthritis: middle-aged and elderly patients will experience degenerative changes in the knee cartilage as they age, resulting in osteoarthritis. Patients will show symptoms of knee pain and weakness, and the range of motion of the joint will be gradually restricted. MRI and front and side x-ray of the knee can confirm joint space narrowing and articular cartilage damage; 2. Lumbar disc herniation: it can lead to loss of innervation of the muscles of the lower limbs, especially the quadriceps and biceps. Patients will show symptoms of knee pain and weakness, accompanied by lumbar pain and limited lumbar movement. CT or MRI of the lumbar spine can confirm herniated discs and nerve compression, while the patient’s knee range of motion is not affected. These are the common causes of knee pain and weakness. Prompt diagnosis and treatment can improve clinical symptoms.