There are many causes of thigh muscle weakness, such as thigh muscle injury, lumbar disc herniation, osteoporosis, osteosarcoma, cerebral infarction and so on. 1. Thigh muscle injury: without adequate preparation for long-distance running, mountain climbing and other strenuous exercise, the thigh muscles will be damaged and cause the symptoms of soreness and weakness. 2. Lumbar disc herniation: when the lumbar spine undergoes degenerative changes, it will compress the sciatic nerve around the lumbar spine, causing pain and weakness in the innervated area, which in turn will cause soreness and weakness of the thigh muscles. 3. Osteoporosis: this disease will lead to calcium deficiency in the body, and severe calcium deficiency will cause muscle spasm and soreness and weakness. 4. Osteosarcoma: after the occurrence of tumor in bone tissue, it will cause the destruction of bone quality and the damage of surrounding soft tissues, which will result in the weakness of thighs. 5. Cerebral infarction: the embolism of blood vessels in the brain causes the blood circulation in the brain tissue area to be obstructed, resulting in ischemia and hypoxia of the brain tissue, which leads to the damage of the nerve function and the symptom of soreness and weakness of the thighs. As there are many causes of thigh weakness, it is recommended to go to the hospital in time when the symptom occurs, in order to clarify the cause of the disease and target treatment.