The synovial joints are mainly the brachioradialis, interphalangeal joints, and patellofemoral joints in the elbow joint. The synovial joints are characterized by a synovial head with a synovial shape, the corresponding femur has an articular fossa to fit, and can only be used for flexion and extension activities on the coronal axis. The stability of the joint mainly depends on the joint capsule, periarticular ligaments, and periarticular muscle protection, such as the medial patellofemoral joint ligaments, lateral supportive bands, and the medial head of the quadriceps muscle, which can protect and maintain the patella in the normal sliding on the scooter, as well as complete the flexion and extension activities. In the elbow joint, the ulnar joint has a thick joint capsule and ulnar collateral ligament to strengthen the protection, so that it can complete the elbow flexion and extension activities. The interphalangeal joint relies on the joint capsule and the collateral ligaments on both sides to protect, stabilize, and maintain flexion and extension activities.