When a patient has sudden weakness in the thighs, most of them are due to lesions of the nerves that innervate the thighs, such as when a patient has lumbar disc herniation, due to the nucleus pulposus protruding and compressing the nerve roots, it will lead to impaired absorption of nutrients by the muscles in the thighs in the area innervated by the nerves, resulting in local muscle weakness, soreness, tenderness, reduced skin sensation, difficulty walking and other clinical phenomena. The correct diagnosis can be made by electromyography or by taking CT films or MRI films of the lumbar spine. Patients suffering from myasthenia gravis in the thigh area or having severe localized fasciitis can also affect the patient’s thigh muscle strength with weakness due to painful stimulation.