Knee weakness: 1, may be caused by knee diseases, such as meniscal injury, knee ligament injury, especially cruciate ligament injury, can lead to secondary muscle atrophy of the thigh muscles and the muscles around the knee joint, as well as its own injury caused by the knee joint stability, the performance of the knee weakness. 2, there are also brain hemorrhage, cerebral infarction, peripheral nerve injury, lumbar spine disease, can lead to a decrease in nerve innervation of the lower limbs or inability to innervate, causing muscle weakness, resulting in knee weakness. The lumbar spine disease can lead to a decrease in the ability of the nerves to innervate the lower limbs or inability to innervate, causing muscle weakness, resulting in the knee can not be strong. 3, there is also the thigh muscle weakness itself, mainly due to a decrease in the strength of the quadriceps muscle, common thigh muscle atrophy, such as trauma can cause prolonged reduction in activity; or prolonged braking leading to muscle wasting atrophy, muscle dystrophy; Or patients with post-polio can cause thigh muscle atrophy, decreased muscle strength, and weakness in knee extension, resulting in symptoms of not being able to make the knee strong.