Weak legs may be due to lumbar spinal stenosis, calcium deficiency, malnutrition, hypokalemia and so on. 1. Lumbar disc herniation compresses the nerves, and if it takes a long time, it will cause muscle atrophy and weakness of the legs. 2. Calcium deficiency can cause weak legs, and in some cases, leg cramps. 3. Malnutrition, if poor health combined with tuberculosis, anemia, hypoproteinemia, heart disease can also appear weak legs. 4. Hypokalemia: prolonged fasting, diarrhea or sweating can cause hypokalemia, which can lead to weak legs and symptoms can be relieved by potassium supplementation. If the symptoms are more serious, you need to go to the hospital to check the cause of the disease.