Weakness of the legs after running may be due to muscle atrophy, knee disease, excessive activity, anemia. 1. Leg muscle atrophy: elderly patients with leg muscle atrophy, weakened strength, so when running there will be symptoms of leg weakness, belonging to the degenerative changes. Usually pay attention to strengthen the leg muscle strength training, appropriate activities, can make the clinical symptoms have been significantly relieved. null2. Knee joint disease: running leg weakness, usually indicates that the knee joint lesions, such as meniscus injury, joint free body, articular cartilage damage, etc., resulting in the leg muscles can not force, the patient is prone to fall. Through MRI examination, the specific lesions and extent of the joints will be found, and timely arthroscopic surgery can be used to remove the lesions and relieve the clinical symptoms of leg weakness. 3. Over-activity: Patients who run for too long or exercise too much will lead to lactic acid buildup after exercise, resulting in lactic acid buildup in the leg muscles and causing leg weakness. 4. Anemia: due to the low oxygen content in the blood of patients with anemia, the lack of oxygen in the body after running will cause weakness in the legs and even weakness in the whole body. Weakness of legs after running may also be caused by lumbar disc herniation and cardiovascular diseases.