There are many reasons for sudden weakness and tenderness of legs, which may be caused by physiological factors such as strenuous exercise and running, or pathological factors such as lumbar intervertebral disc herniation, myasthenia gravis and hypokalemia. 1. Physiological factors: running, long-distance walking, flat support, push-ups and other types of physical exercise, if excessive exercise, will lead to sudden weakness and weakness of the legs, which is a normal phenomenon, and usually will be gradually relieved after rest. 2. Herniated lumbar intervertebral discs: due to ageing, some people’s lumbar intervertebral discs will appear degenerative changes, due to the lumbar intervertebral disc herniation compression of the nerves, not only lumbar pain, lumbar pain, but also accompanied by leg weakness and tenderness. 3. Myasthenia gravis: muscle contraction weakness caused by neuromuscular junction transmission disorder. In the early stage, there may be drooping eyelids. If the condition is serious, it may cause sudden weakness and soreness of legs and weakness of limbs. 4. Hypokalemia: frequent diarrhea or other underlying diseases in the recent past may induce hypokalemia, which will lead to weakness of the whole body of the patient, resulting in the phenomenon of sudden weakness and tenderness of legs. The causes of sudden weakness and tenderness of legs are very many, it is recommended that patients go to the hospital in time to improve the examination and give standardized treatment after clear diagnosis.