Musculoskeletal System

Types of Joints and Articulating Bones

The skeleton is a framework held together by joints. A joint is formed wherever two or more bones meet, and joints are classified by how much movement they allow.

• Fibrous (fixed) joints – allow no movement
• Cartilaginous (slightly moveable) joints – allow limited movement
• Synovial (freely moveable) joints – allow the greatest range of movement. The specification focuses on two types of synovial joint: ball and socket joints and hinge joints

Ball and socket joints allow movement in every direction – the round head of one bone fits into a cup-shaped socket on the connecting bone. The hip (femur and pelvis) and shoulder (humerus and scapula) are ball and socket joints.

Hinge joints allow movement in only one direction, due to the shape of the articulating bones. The ankle (talus, tibia and fibula), knee (femur and tibia) and elbow (humerus, radius and ulna) are hinge joints.

Planes and Axes of Movement

The body can be divided by three imaginary planes of movement, each associated with a specific axis of rotation:

• Sagittal plane – a vertical plane dividing the body into left and right halves; movement here occurs about a transverse axis (running side to side)
• Frontal plane – a vertical plane dividing the body into front and back halves; movement here occurs about a sagittal axis (running front to back)
• Transverse plane – divides the body into upper and lower halves; movement here occurs about a longitudinal axis (running top to bottom)

Joint actions are grouped by which plane/axis combination they occur in:

• Flexion, extension, hyper-extension, plantar-flexion and dorsi-flexion occur in the sagittal plane about a transverse axis
• Abduction and adduction occur in the frontal plane about a sagittal axis
• Horizontal abduction and horizontal adduction occur in the transverse plane about a longitudinal axis

Joint Actions in the Sagittal Plane

Flexion is a decrease in the angle around a joint; extension is an increase in the angle around a joint; hyper-extension occurs if the angle increases beyond 180 degrees.

At the ankle, these actions are instead called plantar-flexion (pointing the toes/pushing up onto the toes) and dorsi-flexion (pulling the toes up towards the shin).

All five syllabus joints (ankle, knee, hip, elbow and shoulder) flex and extend.

Shoulder flexion and extension are an exception to the angle rule: with the arm by the side of the body, raising the arm forward is flexion and lowering it back down is extension; moving the arm further back, past the side of the body, is hyper-extension.

Joint Actions in the Frontal Plane

Abduction is movement of a limb away from the midline of the body, e.g. raising the arms or legs out to the side.

Adduction is movement of a limb back towards the midline of the body, e.g. lowering the arms or legs back to the sides of the body.

Joint Actions in the Transverse Plane

Horizontal adduction is movement of the arm forwards across the body, starting from a position of shoulder abduction (arm held out to the side, parallel to the floor).

Horizontal abduction is movement of the arm backwards away from the body, starting from a position of shoulder flexion (arm held at 90 degrees in front of the body).

Agonist and Antagonist Muscles

A joint cannot move on its own – muscles pull on bones to create movement. When a muscle contracts, one end stays anchored while the other end pulls the bone it is attached to, causing movement.

The muscle responsible for causing a movement is the agonist. The muscle working in opposition, lengthening as the agonist shortens, is the antagonist. Together they work as an antagonistic muscle pair to produce smooth, controlled movement (e.g. when the biceps contracts as the agonist to flex the elbow, the triceps acts as the antagonist and lengthens).

Main agonist/antagonist pairs for key joint actions:

• Elbow flexion: biceps (agonist) / triceps (antagonist). Elbow extension: triceps (agonist) / biceps (antagonist)
• Ankle plantar-flexion: gastrocnemius (agonist) / tibialis anterior (antagonist). Ankle dorsi-flexion: tibialis anterior (agonist) / gastrocnemius (antagonist)
• Knee flexion: hamstrings (agonist) / quadriceps (antagonist). Knee extension: quadriceps (agonist) / hamstrings (antagonist)
• Hip flexion: iliopsoas/hip flexors (agonist) / gluteals (antagonist). Hip extension/hyper-extension: gluteals (agonist) / hip flexors (antagonist)
• Hip adduction: adductors (agonist) / tensor fascia latae and gluteus medius/minimus (antagonist). Hip abduction: tensor fascia latae and gluteus medius/minimus (agonist) / adductors (antagonist)
• Hip horizontal adduction: adductors (agonist) / tensor fascia latae and gluteus medius/minimus (antagonist). Hip horizontal abduction: tensor fascia latae and gluteus medius/minimus (agonist) / adductors (antagonist)
• Shoulder flexion: anterior deltoid (agonist) / latissimus dorsi (antagonist). Shoulder extension/hyper-extension: latissimus dorsi (agonist) / anterior deltoid (antagonist)
• Shoulder horizontal abduction: latissimus dorsi (agonist) / pectorals (antagonist). Shoulder horizontal adduction: pectorals (agonist) / latissimus dorsi (antagonist)
• Shoulder adduction: posterior deltoid/latissimus dorsi (agonist) / middle deltoid/supraspinatus (antagonist). Shoulder abduction: middle deltoid/supraspinatus (agonist) / posterior deltoid/latissimus dorsi (antagonist)

Types of Muscular Contraction

An isotonic contraction is when a muscle contracts and produces movement. There are two types:

• Concentric contraction – the muscle shortens under tension, e.g. the biceps during the upward phase of an arm curl, producing flexion of the elbow
• Eccentric contraction – the muscle lengthens under tension rather than relaxing, acting like a brake to control movement (negative work), e.g. the quadriceps lengthening under tension to control the body during landing from a jump

An isometric contraction occurs when a muscle is under tension but does not change length, so no visible movement takes place – e.g. holding the crucifix position in gymnastics, or holding a weight still mid-curl.