Sudden weakness and tenderness of the knee may be caused by physiological factors such as long walking time and strenuous exercise, or it may be caused by other factors such as hypokalemia, lumbar intervertebral disc herniation, and osteoporosis. 1. Physiological factors: if the leg walking time is too long or in a short period of time suddenly carry out strenuous sports, may be in accordance with the local muscle lactic acid metabolism increases, the body may appear knee suddenly weak and soft performance, which belongs to the normal physiological phenomenon. 2. Hypokalemia: Hypokalemia may be caused by insufficient potassium intake, profuse sweating, severe nausea and vomiting, metabolic alkalosis. Patients will have weakness of limbs, abdominal distension, cardiac arrhythmia and so on, so patients will show sudden weakness and tenderness of knees. 3. Lumbar intervertebral disc herniation: Lumbar intervertebral disc herniation is considered to be caused by degeneration of intervertebral discs, intervertebral disc injuries, and abnormalities of lumbar spine development. Patients will not only have the symptom of weakness of lower limbs or knees, but also may have lumbar pain, numbness of lower limbs and other symptoms. 4. Other: Sudden weakness and tenderness of the knees may also be caused by osteoporosis, osteoarthritis, hypothyroidism and other reasons. It is recommended that patients with sudden weakness of the knees need to seek medical attention in time, if it is due to physiological factors without treatment. For diseases that cause knee weakness, you need to address the cause of the disease.