Left leg weakness may be physiological reasons, lumbar disc herniation, cerebrovascular disease and other reasons. 1. Physiological reasons: poor posture, often do stilt leg action or sleep at night on the left side of the limb pressure is likely to occur in the left leg weakness. 2. Lumbar intervertebral disc herniation: the degeneration of the intervertebral disc nucleus pulposus protrudes and compresses the left nerve root, and the weakness of the left leg, and some of them have numbness, and the symptom is aggravated by coughing, sneezing, and other actions that increase the abdominal pressure. 3. Cerebrovascular disease: hemiplegia caused by cerebral infarction and cerebral hemorrhage can be characterized by weakness of the left leg, gait abnormality, decreased sensation and decreased muscle power. If the symptoms are not alleviated by resting, it is necessary to go to the hospital in time.