Possible causes of knee weakness include exertion, osteoarthritis of the knee joint, osteoporosis, chondromalacia patella and so on. 1. Exertion: If you have long time physical labor, do not pay attention to rest, overwork, the knee joint has a transient synovial edema, muscle lactic acid accumulation, the symptoms of knee joint soreness and weakness, you can rest appropriately. 2. Osteoarthritis of knee joint: Mostly seen in the elderly, due to high age, obesity, chronic strain and other degenerative changes in the knee joint, degeneration of articular cartilage, osteomalacia, manifested as knee pain, aching and weakness, fatigue, difficulty in activities and other symptoms. 3. Osteoporosis: Osteoporosis is caused by hyperthyroidism, menopause, insufficient intake of calcium and vitamins, etc., which can show symptoms such as soreness, weakness and pain in the knee joint. 4. Chondromalacia patella: because of the degenerative changes after the damage of patellar osteoarticular surface, which causes knee joint pain, weakness, activity limitation and other symptoms. There are more reasons for knee weakness, and it may also be related to hypokalemia, lumbar disc herniation and other diseases. If the symptoms continue to be unrelieved, it is recommended to consult a doctor in time to clarify the cause of the disease and carry out targeted treatment.