Prolonged left leg weakness may be caused by lumbar spine lesions, osteoporosis, osteoarthritic diseases, cerebrovascular lesions and other factors. It may also be caused by non-disease factors. 1. Lumbar spine lesion: when lumbar spine lesion occurs, such as lumbar disc herniation or lumbar spinal stenosis, etc., it can compress the nerve root, which will produce the symptom of left leg weakness. 2. Osteoporosis: When the body lacks calcium for a long time or loses too much calcium, it may cause osteoporosis, which may lead to bone and joint pains as well as the symptoms of left leg pain and weakness. 3. Osteoarthritic diseases: If there are osteoarthritic diseases in the left lower limb, such as degenerative knee joints or inflammation of the knee joints, etc., it is easy to cause the symptoms of left leg pain and weakness, and it even affects the range of motion of the joints. 4. Cerebrovascular lesions: cerebral infarction, cerebral hemorrhage and other diseases that compress the central nervous system may lead to weakness in the left leg. 5. Non-disease factors: usually if excessive exercise, or prolonged weight-bearing labor, easily triggered by leg muscle fatigue, resulting in the phenomenon of weakness of the left leg, which is not caused by disease factors, and can be relieved after rest. When the symptoms of left leg weakness occur, pay attention to rest, so as not to aggravate the symptoms. If the symptoms persist or tend to worsen, you need to consult a doctor in a timely manner to clarify the cause of the disease and then targeted treatment under the guidance of a professional physician.