Weakness and heaviness in legs may be caused by excessive exercise, potassium and sodium deficiency, lumbar disc herniation, spinal cervical spondylosis and other factors. 1. Excessive exercise: Excessive physical exercise and excessive labor may cause muscle pain and numbness, resulting in weakness and heaviness in the legs. 2. Lack of potassium and sodium: Potassium and sodium ions are very important in the human body, when the human body is deficient in potassium and sodium, it will easily lead to weakness and heaviness in legs. 3. Herniated lumbar intervertebral disc: As the patient’s nerves are compressed by the protruding nucleus pulposus, it is easy to cause numbness and weakness of the innervated limbs. In this case, the patient will have leg pain and lack of energy when walking. 4. Spinal cord cervical spondylosis: When the patient is subjected to various kinds of strain, load, trauma, etc., it is easy to lead to spinal cord cervical spondylosis, with symptoms such as numbness and heaviness in the lower limbs. With the development of the disease, there may be difficulty in walking, tightness of the muscles of the lower limbs, inability to walk quickly, and even incontinence. Patients who experience weakness and heaviness in their legs should go to the hospital in time to complete the relevant examinations and receive proper treatment under the guidance of the doctor.