Synovial Joints and Movement at Joints

What Is a Synovial Joint?

Every freely movable joint in the body — at the shoulder, elbow, hip and knee — is a synovial joint. Synovial joints are the most common type of joint in the body, and they all share the same basic structure: a capsule surrounding the joint, fluid inside to reduce friction, and a range of soft-tissue structures that hold the joint together and prevent injury. Understanding these structures is important not just for anatomy, but for explaining how joints withstand the demands of sport.

Structures of a Synovial Joint

The outer shell of a synovial joint is the joint capsule. It is made of tough, fibrous connective tissue that wraps completely around the joint, holding the bones in position and containing the joint fluid. Inside the capsule, the synovial membrane lines the inner surface and produces synovial fluid — a viscous substance that coats the ends of the bones inside the joint, dramatically reducing friction and allowing bones to glide smoothly past each other. During the warm-up phase of exercise, synovial fluid becomes less viscous and spreads more effectively, which is one reason why warming up reduces injury risk.

Covering the ends of the bones inside the joint is articular cartilage, a smooth tissue that absorbs compressive forces and provides an almost frictionless surface. Ligaments are short, tough bands of fibrous tissue that connect bone to bone across the joint — the primary restraints against excessive movement. Tendons connect muscles to bone, transmitting contractile force to the skeleton to produce movement at the joint.

Types of Synovial Joint

OCR identifies two types of synovial joint and four specific joints that are directly examined. A hinge joint permits movement in one plane only — flexion and extension. The knee (femur articulating with the tibia) and the elbow (humerus articulating with the radius and ulna) are both hinge joints. A ball-and-socket joint permits movement in multiple planes. The shoulder (head of the humerus fitting into the socket of the scapula) and the hip (head of the femur fitting into the socket of the pelvis) are ball-and-socket joints.

Articulating Bones at Each Joint

The OCR specification requires you to name the articulating bones at all four joints. At the knee, the femur (thigh bone) articulates with the tibia (shin bone) — these two long bones form the hinge. At the elbow, the humerus articulates with both the radius and the ulna — three bones meet here, but the elbow is still a hinge joint, so only flexion and extension happen at it. At the shoulder, the humerus articulates with the scapula — the rounded head of the humerus sits in the shallow socket of the scapula, enabling the greatest range of movement in the body. At the hip, the femur articulates with the pelvis — the rounded head of the femur slots into the deep cup of the pelvis, creating a stable but mobile ball-and-socket joint.

Types of Movement at Joints

Flexion is a decrease in the angle at a joint — bending. Extension is an increase in the angle — straightening. Both occur at all four OCR joints. At the elbow and knee (hinge joints), flexion and extension are the only movements possible because the structure of the joint restricts movement to a single plane.

At the shoulder and hip (ball-and-socket joints), a much wider range of movements is available. Abduction is movement of a limb away from the midline — lifting the arm out to the side in a star jump. Adduction is movement towards the midline — bringing the arm back down. Both abduction and adduction are available at the shoulder and hip but not at the elbow or knee.

Rotation of the shoulder is turning the arm in or out along its long axis. A cricket bowler’s arm rotates at the shoulder during delivery. Circumduction of the shoulder is a circular movement combining flexion, abduction, extension and adduction in sequence — the arm traces a full cone shape. Throwing a javelin involves circumduction of the shoulder.