What are joints?

Joints form essential connections within the skeleton, enabling movement and providing support. They consist of more than just bones, incorporating various tissues that work together to allow the body to function effectively.

A joint is any point where two or more bones meet. The bones that meet at a joint are known as articulating bones, which are shaped to fit together and facilitate smooth movement.

Synovial joints

A synovial joint is a type of joint that permits a wide range of movement and is enclosed by a joint capsule. Ball and socket joints and hinge joints are common examples of synovial joints.

Types of synovial joints

Synovial joints vary in structure, which determines the range of movement they allow.

Ball and socket joints

A ball and socket joint features the rounded end of one bone fitting into a cup-like depression in another bone. This structure enables movement in nearly every direction.

Examples of ball and socket joints and their articulating bones:

• Hip - Formed by the pelvis and femur
• Shoulder - Formed by the humerus and scapula (where the head of the humerus fits into the glenoid cavity of the scapula)

Hinge joints

A hinge joint permits movement in only one direction, similar to the action of a door hinge, allowing bending and straightening.

Examples of hinge joints and their articulating bones:

• Ankle - Formed by the tibia and talus bone
• Knee - Formed by the femur and tibia
• Elbow - Formed by the humerus, radius, and ulna

Connective tissues in joints

Connective tissues play vital roles in supporting joints, ensuring they remain stable and functional during movement. There are three main types: ligaments, tendons, and cartilage.

Ligaments

Ligaments are tough, fibrous tissues that connect bones to each other. They act like strong strings to hold bones together and restrict excessive movement.

Functions of ligaments:

• Maintain skeletal stability
• Prevent joint dislocation
• Absorb shock to protect bones and joints during impacts

Tendons

Tendons are strong cords that attach muscles to bones (or sometimes to other muscles). They transmit the force generated by muscle contractions to the bones.

Functions of tendons:

• Enable bones to move when muscles contract

Cartilage

Cartilage is a smooth, flexible tissue that covers the ends of bones at joints. It acts as a protective layer to reduce wear and tear.

Functions of cartilage:

• Cushions bones to prevent damage during movement
• Aids joint stability

Structure and function of synovial joints

Synovial joints are designed for smooth, efficient movement, with specific components that work together to minimise friction and support load-bearing.

Key components of a synovial joint

• Ligaments - Hold the bones together and provide stability
• Cartilage - Covers the ends of the articulating bones to cushion them and allow smooth gliding
• Bone shapes - The articulating bones are contoured to fit precisely, enabling fluid movement
• Synovial membrane - A lining inside the joint capsule that releases synovial fluid
• Synovial fluid - A lubricating liquid within the joint capsule that reduces friction

Types of joint movements

Joint movements describe the specific ways in which articulating bones can shift relative to each other. There are six main types, each essential for different physical tasks.

The six types of joint movements

• Flexion - Decreases the angle at a joint, like bending the arm at the elbow during a bicep curl
• Extension - Increases the angle at a joint, like straightening the leg during a leg press
• Rotation - Twists a bone around its axis in a clockwise or anticlockwise direction, such as turning a doorknob with the arm
• Circumduction - Moves a limb in a circular path, combining other movements, for example, swinging the arm in a full circle during a swimming stroke
• Abduction - Moves a limb away from the body's imaginary centre line, like raising the arms sideways in a jumping jack
• Adduction - Moves a limb towards the body's imaginary centre line, like lowering the arms back to the sides in a jumping jack

Movements allowed by different joint types

Different joints permit specific movements based on their structure.

Ball and socket joints (e.g., hip and shoulder):

• Allow flexion and extension
• Allow adduction and abduction
• Allow rotation
• Allow circumduction

Hinge joints (e.g., knee and elbow):

• Allow only flexion and extension

Joint movements in exercise

In physical activities, multiple joint movements and types combine to perform complex actions.

Examples of joint movements in specific exercises

• Pull-up - Involves extension at the elbow to hang from the bar, followed by flexion to pull the body upwards
• Serving in tennis - Uses circumduction at the shoulder for the circular arm swing, combined with extension and rotation to generate power