First, meniscus injury, which can lead to weakness in the legs, can also lead to quadriceps atrophy, which can lead to weakness in both knees. Second, chondromalacia patellae and patellar tendonitis, etc., can lead to the symptoms of weakness in both knees. Third, osteoarthritis of both knees. When osteoarthritis of both knees develops to a more serious degree, there can be atrophy of the quadriceps muscle, resulting in a weakening of the quadriceps muscle strength, resulting in weakness of both knees. Fourth, degenerative changes in osteoporotic bone and knee joints can cause walking knee weakness.