The movements that occur at joints are classified according to the action that occurs between the articulating bones.
A movement pattern of a limb or body part always has starting and finishing points. By analysing the position of the finishing point relative to the starting point, a classification of movement has been formed.
- Flexion: bending. This occurs when the angle between the articulating bones is decreased (usually in the sagittal plane away from the anatomical position). For example, bending the knee. A muscle that causes flexion is known as a ‘flexor’. In the above example, the hamstrings were the flexor muscles.
- Extension: straightening. This occurs when the angle between the articulating bones is increased (usually in the sagittal plane back to the anatomical position). For example, when straightening the leg, the angle between the femur and the tibia increases, therefore, extension has taken place. A muscle that causes extension is known as an ‘extensor’. In the above example, the quadriceps group act as the extensor muscles. Extreme extension can take place, and this usually occurs at an angle greater than 180°. This is known as hyperextension (usually in the sagittal plane taking a joint backwards beyond the anatomical position).
- Abduction: this occurs when a body part is moved away from the midline of the body or other body part (in the frontal plane). Examples include placing the arms by the sides of the body and then raising them up to the side. Spreading the fingers and lifting up a leg to the side are also examples of abduction. An easy way of remembering this is ‘abduct’ – to take away.
- Adduction: this is the opposite of abduction and occurs with movement towards the midline of the body or body part (in the frontal plane). For example, by lowering the arms back down to the sides of the body, movement towards the midline of the body has taken place, and therefore adduction has occurred. An easy way of remembering this is ‘add’ – for example, ‘adding’ the arm to the side of the body.
- Circumduction: this occurs when a circle is drawn by a body part – the bone makes the shape of a cone as it moves around. It is a combination of flexion, extension, abduction and adduction. It can only truly occur at ball and socket joints at the hip and shoulder.
- Rotation: this occurs when the bone turns about its long axis within the joint (in the transverse plane). Rotation towards the body is known as internal or medial rotation, and rotation away from the body is called external or lateral rotation.
- Pronation: this occurs at the radio-ulnar joint below the elbow and involves internal/medial rotation between the radius and humerus, as well as the crossing of the radius and the ulna. It normally occurs when the palm of the hand is moved from facing upwards to downwards (in the transverse plane). A pronated grip occurs when the palms face down (or when the thumbs are next to each other).
- Supination: this is the opposite of pronation and once again takes place at the radio-ulnar joint below the elbow joint. This movement is external (lateral) rotation between the radius and the humerus, and occurs when the radius and ulna are parallel. When the palm of the hand is turned so that it faces upwards, supination has taken place (in the transverse plane). A supinated grip occurs when the palms face up (or when the little fingers are next to each other).
- Plantarflexion: this takes place at the ankle joint, and occurs when the toes are pointed forward. This occurs in the sagittal plane.
- Dorsiflexion: this also occurs at the ankle and occurs when the foot is raised upwards towards the tibia (shin). This occurs in the sagittal plane.
- Inversion: this occurs when the sole of the foot is turned inward towards the midline of the body. This movement is internal (medial) rotation and occurs in the transverse plane. It takes place at the subtalar joint.
- Eversion: this occurs when the sole of the foot is turned outwards (laterally). This movement is external (lateral) rotation and occurs in the transverse plane. It takes place at the subtalar joint.
- Elevation: this occurs when a body part is moved upwards (usually in the frontal plane). For example, shrugging the shoulders.
- Depression: this occurs when a body part is moved downwards (usually in the frontal plane). An example would be lowering the shoulders back down.
- Horizontal flexion: this only happens at the shoulder joint and occurs in the transverse plane. When an arm is held out to the side parallel to the floor and then pulled across the chest, that would be horizontal flexion.
- Horizontal extension: this only happens at the shoulder joint and occurs in the transverse plane. Returning the arm out to the side keeping it parallel to the floor would be horizontal extension. Taking the arm backwards beyond 180º would be horizontal hyperextension.
- Lateral flexion: this occurs when the spine is flexed (i.e. angle is decreased) to the side. For example, during side bends or tilting the head to the side. This takes place in the frontal plane.
- Protraction: this takes place in the transverse plane. Examples include jutting the chin forward and rounding the shoulders forward.
- Retraction: this takes place in the transverse plane. Examples include pulling the chin back and squaring the shoulders by pulling them back.
- Opposition: the saddle joint between the first metacarpal (thumb) and the carpals allows opposition of the thumb to take place. This action occurs when the thumb touches the tips of the other fingers on the same hand.
It is extremely important to ensure that movements at all joints are kept in the correct planes throughout exercise performance in order to prevent ligament strain and potential risk of injury.