Synovial Joints and Types of Movement

What Is a Synovial Joint?

Every freely movable joint in the body — at the shoulder, elbow, hip, knee and ankle — 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, forming a sealed envelope. The joint capsule holds the bones in position and contains the joint fluid.

Inside the capsule, the synovial membrane lines the inner surface of the capsule. Its job is to produce synovial fluid — a viscous, egg-white-like substance that coats the ends of the bones inside the joint. This fluid dramatically reduces friction, allowing bones to glide smoothly past each other. Without synovial fluid, the grinding of bone on bone would cause rapid joint damage. 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, bluish-white tissue that absorbs compressive forces and provides an almost frictionless surface. Cartilage has no blood supply of its own — it relies on synovial fluid for nourishment. Once damaged, cartilage heals very slowly, which is why cartilage injuries (such as a torn meniscus in the knee) can end sporting careers.

Ligaments are short, tough bands of fibrous tissue that connect bone to bone across the joint. They are the primary restraints against excessive movement — without them, joints would be unstable. The anterior cruciate ligament (ACL) inside the knee, for example, prevents the tibia from sliding forward relative to the femur during landing from a jump. A ruptured ACL is one of the most serious injuries in contact sport.

Tendons do not form part of the synovial joint structure itself, but they pass close to most joints and contribute to stability. The Achilles tendon at the ankle and the patellar tendon at the knee are essential force transmitters from muscle to bone.

Bursae (singular: bursa) are small cushioning sacs filled with fluid. They sit at points where tendons or ligaments would otherwise rub against bone, such as at the front of the knee (the prepatellar bursa). Inflammation of a bursa, known as bursitis, is a common overuse injury in sports that involve repeated kneeling or shoulder work.

Types of Synovial Joint

The spec identifies two types of synovial joint relevant to GCSE PE. A hinge joint permits movement in one plane only — flexion and extension. The elbow (humerus articulating with the ulna and radius), the knee (femur articulating with the tibia) and the ankle (tibia and fibula articulating with the talus) are all hinge joints. A ball-and-socket joint permits movement in multiple planes. The shoulder (head of the humerus fitting into the glenoid cavity of the scapula) and the hip (head of the femur fitting into the acetabulum of the pelvis) are ball-and-socket joints.

Types of Movement at Synovial Joints

Flexion is a decrease in the angle at a joint — bending. Extension is an increase in the angle — straightening. Both occur at the shoulder, elbow, hip and knee. At the elbow, flexion and extension are the only movements possible because it is a hinge joint. At the shoulder and hip (ball-and-socket), a much wider range of movements is available.

Abduction is movement of a limb away from the midline of the body — 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 (ball-and-socket joints) but not at the elbow or knee (hinge joints).

Rotation of the shoulder is turning the arm in or out along its long axis. A tennis player rotates the arm at the shoulder when hitting a serve. Circumduction of the shoulder is a circular movement combining flexion, abduction, extension and adduction in sequence — the arm traces a full cone shape. A cricket bowler circumducts the shoulder as the bowling arm sweeps through a full circle in the delivery.

At the ankle, the movements are named differently. Plantar flexion is pointing the foot downward (pressing the toes away from the body). A sprinter’s powerful push-off at the end of each stride is driven by plantar flexion at the ankle. Dorsiflexion is pulling the toes upward towards the shin — the ankle bends when a gymnast lands from a vault. Both plantar flexion and dorsiflexion are hinge movements at the ankle joint.