The common cause of knee tenderness and weakness at night is patellar chondromalacia, which is a chronic cumulative injury caused by abnormal wear and tear of the patella and the femoral epicondyle when the patella is out of its normal running path. Clinical manifestations include knee pain, softness and weakness, increased pain when running and jumping, no pain when walking on a flat road and obvious pain when going up or downstairs, positive patellar pressure pain test and positive patellar grinding test during physical examination, and patella damage or degeneration of patellar cartilage as shown by MRI images of the knee joint. Treatment may include ultrashort wave therapy, pulsed magnetic therapy, etc., which can reduce the inflammatory response at the cartilage injury. If the symptoms worsen, arthroscopic surgery can also be chosen to restore the flatness of the patellar articular surface, and postoperative strength training of the quadriceps should be strengthened to enhance joint stability.