Possible causes of knee weakness include strenuous exercise, chondromalacia patella, soft tissue injury and nerve injury. 1. Strenuous exercise: it may be due to the accumulation of more lactic acid in the knee after strenuous exercise, resulting in the symptoms of soreness and fatigue in the knee downward. 2. Chondromalacia patella: It may be caused by long-term wear and tear of the knee joint, malnutrition of the knee joint, inflammatory reaction in the knee joint, long-term overload of the knee joint, and other factors, resulting in symptoms such as weakness and pain in the knee. 3. Soft tissue injury: it may be related to muscle, meniscus, knee joint and other soft tissue injuries, such as quadriceps, patella and other places where the injury occurs, when doing flexion and extension exercises, pulling the relevant muscles or soft tissues, it will be due to the pain of the knee joint can not be able to send force or can not be affected by the force, the emergence of the knee sleepy and weak situation. 4. Nerve injury: due to injury to the nerves innervating the knee, such as femoral nerve injury, the muscles can not be innervated, which may lead to knee weakness, if the spinal cord nerve abnormalities, such as spinal cord cervical spondylosis, lumbar intervertebral disc herniation, etc., compression of the relevant nerves, will also result in neurological knee weakness, accompanied by numbness of the limbs, pain and other manifestations. There may be other reasons for the knee downward sleepiness, it is recommended to go to the regular hospital in time to improve the relevant examination, clear the cause of the disease, under the guidance of the doctor to carry out targeted treatment or treatment.