There are many reasons for downstairs knee weakness, including cervical spondylosis, lumbar intervertebral disc herniation, and knee joint diseases. 1. Cervical spondylosis: a series of neurological reflex symptoms caused by cervical degenerative disc disease, and its secondary hyperplasia, spinal stenosis, herniation, and so on, thus compressing or stimulating the nerve roots, spinal cord, etc. For example, in nerve root cervical spondylosis, the muscles innervated by the compressed or damaged nerve roots will be affected. For example, in nerve root cervical spondylosis, the muscles innervated by the compressed or damaged nerve roots will suffer from motor dysfunction, and the patient will experience knee weakness when walking down the stairs. 2. Lumbar intervertebral disc herniation: It is mainly caused by degenerative changes in the lumbar intervertebral disc, trauma, strain and cold. The fibrous ring of the intervertebral disc ruptures and the disc protrudes, stimulating the spinal nerve root and producing corresponding neurological symptoms. Such as numbness, weakness and abnormal sensation of the lower limbs, and in severe cases, it may cause difficulty in walking, weakness, unwillingness to take steps, and the need to sit down or squat down to rest. 3. Knee joint disease: such as knee joint ligament injury, mainly due to trauma, strenuous exercise and other reasons. Mainly manifested as localized pain, movement down the stairs when the symptoms are serious, may appear swelling and pain, weakness and other manifestations. In addition to the above reasons, cerebrovascular disease, knee synovitis and other diseases can also cause the above symptoms. It is recommended to go to the hospital to find out the cause of the disease, and carry out regular treatment under the guidance of the doctor.