Sore knees and weak legs may be caused by lumbar intervertebral disc herniation, osteoporosis, hypokalemia and so on. 1. Lumbar intervertebral disc herniation: it is a disease in which the nucleus pulposus protrudes from the damaged annulus fibrosus under the action of lesions or external factors, leading to irritation and compression of lumbar spinal nerves, which can cause symptoms such as weakness, lumbar pain, radiating pain and numbness of one side of the lower limbs, and so on. 2. Osteoporosis: It is a kind of disease that reduces the amount of bone tissue per unit volume, mostly related to the decrease of hormone level, malnutrition, musculoskeletal wastage, and so on. Osteoporosis can cause the symptoms of weakness of waist and knees, or accompanied by other symptoms such as low back pain and hunchback. 3. Hypokalemia: serum potassium is lower than 3.5mmol/L, which is called hypokalemia, mostly caused by insufficient intake of potassium, excessive loss of potassium due to massive vomiting, diarrhea, etc. It can lead to fatigue and weakness, and may be accompanied by nausea, abdominal distension and other symptoms. Weakness of waist and knees may also be caused by other diseases, which need to be examined and treated in time to avoid symptoms affecting daily life.