The knee joint is the most complex and functionally demanding joint in the body and the most susceptible to disease and injury. The vast majority of people will suffer from a knee injury or illness during their lifetime. To avoid, reduce or alleviate these problems, we need to start with the prevention, treatment and rehabilitation of knee injuries and diseases. First, the basic structure and function of the knee joint The human knee joint is simply composed of three bones, four sets of ligaments and two washers. The three bones are the upper thigh bone (femur), the lower leg bone (tibia), and the front knee bone (patella), all of which have a layer of cartilage on the surface that significantly reduces friction between the bones when moving. The thigh bone and calf bone constitute the femur-tibial joint, which transmits gravity when a person is standing and bearing weight. There is a groove in front of the thigh bone, and the kneecap forms the patellofemoral joint, and the kneecap moves along this groove during knee movement. The patellofemoral joint is subjected to a greater load during semi-squatting activities. The four groups of ligaments include the anterior cruciate and posterior cruciate ligaments, which form a cross in the middle of the knee joint, and the medial and lateral ligament structures located on either side of the knee joint. These four groups of ligaments connect the thigh and calf bones together and maintain the relative stability of these two bones during knee motion. Injury to any of these four groups of ligaments can cause instability in the joint and have an impact on daily activities and sports. In the joint space formed by the femur and tibia, there is a piece of cartilage on the inside and outside that acts as a gasket to protect the knee joint. This cartilage is called the meniscus because it is shaped like a crescent in most people. Second, the prevention of knee injuries and illnesses Knee injuries and illnesses occur either with developmental abnormalities or with improper exercise or rehabilitation care. At different ages, there are different types and characteristics of the onset of the disease. Therefore, the prevention of knee injuries needs to recognize these characteristics and thus be targeted. 1, the characteristics of adolescent knee injuries and protective measures Adolescent joint cartilage, ligaments, meniscus are in the best condition, knee injuries are mainly related to improper sports. One of the most frequent is the anterior cruciate ligament injury. Compared to professional athletes, the incidence of ACL injuries is higher in the general sports population. This is mainly due to the lack of warm-up and self-protection of the average sportsperson. The stability of the human knee depends on the integrity of the ligaments and the coordination of the muscles. Inadequate warm-up, when muscles and ligaments are needed to protect the knee joint in concert, the muscles do not respond well and fail to provide timely protection, which can cause injury to the ACL due to excessive stress. In addition, the lack of awareness of ACL protection and lack of understanding of protection techniques by the average sports enthusiast is also a major cause of ACL injuries. ACL injuries are most likely to occur during jump landings and sharp turns, when the foot lands first and is relatively fixed, but the upper body continues to rotate, resulting in a load that exceeds the ACL’s ability to withstand. Although ACL injuries are inevitable, research has shown that some conscious training can help reduce the incidence of ACL injuries. These include proper jump landing techniques (especially soft landing techniques with forefoot, knee and hip flexion), proprioceptive and neuromuscular training, and training to avoid extreme joint valgus during jump landings and squats. It is the nature of adolescents to enjoy sports, but there are some congenital or developmental abnormalities that make some people unsuitable for certain sports. For example, patients with unstable kneecaps are not suitable for sports that require half-squats, and patients with meniscal disc deformities are not suitable for prolonged weight-bearing exercises. Patients with congenital ligamentous laxity are not suitable for sports that require sharp stopping and turning movements. Therefore, for young people who like to play sports, it is important to go to the doctor for examination and consultation to understand the characteristics of the main structures of the knee joint and to have an idea of the appropriate and unsuitable sports or movements. 2, the characteristics of middle-aged knee injuries and protective measures people in middle age, often realize the importance of recreational sports, will consciously do some fitness exercises. However, clinically, we often encounter patients who do not exercise well, a movement of the knee joint pain instead; also often encounter deep squat training caused by medial meniscus injury, practice horse stance squat crotch caused patellar cartilage damage, a transient excessive exercise caused synovial inflammation patients. This is related to the structural state and relative overuse of the joints in middle-aged people. The articular cartilage and meniscus of the middle-aged have degenerated to some degree. The cruciate ligaments are not only degenerating, but they are also being rubbed by the adjacent areas of enlarged bone, which decreases their strength. Inappropriate training, even for recreational sports, can be counterproductive and cause injury. Middle-aged people should pay attention to the following points when training: First, do not advocate middle-aged people to carry out bodybuilding training. Bodybuilding training is for certain muscle mass training, the overall function of the limb does not help much, but will cause damage to the end of the tendon and cartilage damage. For example, resistance knee extensions, whether done on an exercise machine or at home with weights on the ankles, do increase the strength and fullness of the quadriceps, but they do cause pain behind the patella (caused by worn patellar cartilage) and pain below the patella (caused by strain on the quadriceps and patellar tendons). Simple comprehensive training such as running and appropriate bouncing can increase the overall function of the lower extremity without causing damage. Secondly, it is not advisable for middle-aged people to perform half-squats and climbing exercises. In the knee joint half squat, especially the single knee weight-bearing half squat, the patellofemoral joint to withstand three times the weight of the stress, the more half squat activities, the faster the patellofemoral joint degeneration. Sports involving half squat activities include taijiquan, mulanquan, table tennis, etc. Taijiquan, in particular, is more damaging to middle-aged and elderly joints. In addition, going up and down stairs, going up and down hills, and climbing mountains also involve bending the knee to bear or exert force, when the patellar cartilage is also subjected to excessive stress. Those who take climbing hills or stairs as a kind of training often lose more than they gain, and end up having difficulty climbing hills and stairs very early. Third, middle-aged people need to train gradually. Often people do not exercise for years, and suddenly go to practice for a few hours on a whim, resulting in joint pain for months or even years. Middle-aged people because of cartilage degeneration, the cartilage’s ability to withstand limited. Therefore, all kinds of exercise should follow the principle of gradual progress, to give the various structures of the joints a chance to adapt. In terms of training, middle-aged people are first suitable for small load training of the joint, such as sitting knee extension and flexion activities, cycling (including small load fitness bicycle), swimming, etc.; followed by running, bouncing and other holistic training. 3, the characteristics of the elderly knee injuries and protective measures The degeneration of the joints is inevitable when people grow old. This degeneration is manifested firstly in the wear and tear of the articular cartilage, secondly in the degeneration and damage of the meniscus and synovial hyperplasia. Bone growth, as previously described, is a manifestation of joint degeneration on X-rays, but bone growth in the knee joint is not the main cause of joint pain at all. Joint pain is mainly caused by the wear and tear of the cartilage and the exposed friction of the subchondral bone. The aging of the joints is a natural process that varies from person to person, but all we have to do and can do is to slow down the aging of the joints and reduce the symptoms of aging. For older knee joints, not exercising residual cartilage without nutrition will accelerate degeneration, while overtraining will accelerate wear and tear on the joint. So the key is to get the right degree. The state of joint cartilage is different for each elderly person, and so is the appropriate type and intensity of exercise. Medical examination and exercise counseling are also necessary. In terms of joint protection, the first contraindication for middle-aged people is also a contraindication for the elderly. Sitting knee extension and flexion activities, stationary bicycles and swimming are suitable for the elderly, but prolonged walking and running are not. The exercise of flexing the knee and hip and rocking the knee joint back and forth is contraindicated for the elderly, as this exercise will increase the wear and tear of the joint cartilage and cause a sudden increase in pain.