In human anatomy and kinesiology, the study of movement requires precise terminology to categorize the body’s vast range of motion. Anatomical movements are broadly grouped into categories like angular, gliding, and rotational. Rotational movement represents a specific and functionally important class of motion that enables complex coordination and dexterity. This article defines and illustrates the mechanics and applications of rotation within the human musculoskeletal system.
Defining Rotational Movement
Rotational movement describes the spinning motion of a bone around its own central axis, known as the longitudinal axis. This imaginary line runs vertically through the middle of the moving bone or structure. The motion is a simple “twist” or swivel occurring within a joint, where the bone turns like a spindle.
Rotation does not necessarily involve increasing or decreasing the angle between two bones. This spinning motion occurs in the transverse plane, which is perpendicular to the longitudinal axis of rotation. The anatomical design of certain joints allows for this pure, isolated twisting motion.
Types of Rotation: Internal vs. External
Rotational movements are categorized based on the direction the anterior surface of the limb moves relative to the body’s midline.
Internal rotation, also known as medial rotation, is the movement of a limb that turns its anterior surface toward the body’s midline. For example, internal rotation at the shoulder turns the palm inward toward the torso. In the hip, this movement causes the foot and toes to turn inward.
External rotation, or lateral rotation, is the opposite movement, turning the anterior surface of the limb away from the midline. This action can be seen when a person turns their leg outward at the hip. If the arm is bent at the elbow, external rotation causes the hand to move away from the abdomen.
Where Rotation Occurs: Key Joints and Actions
Rotational movements are primarily facilitated by two specific types of synovial joints: ball-and-socket and pivot joints. These joints are structurally designed to permit the necessary spinning motion.
Ball-and-socket joints, such as the glenohumeral (shoulder) and coxal (hip) joints, allow rotation because the rounded head of one bone fits into the cup-like socket of another. The humerus and femur rotate around their own longitudinal axes at these joints. An action like reaching into a back pocket requires internal rotation of the shoulder, while a baseball pitcher’s wind-up involves substantial external rotation.
Pivot joints are uniaxial, meaning they only allow movement around one axis, and that movement is exclusively rotation. A prime example is the atlanto-axial joint between the first two cervical vertebrae, the atlas (C1) and the axis (C2). This joint allows the head to rotate from side to side, enabling a person to shake their head “no.”
Another pivot joint is the proximal radioulnar joint in the forearm. The movements of pronation and supination are specialized forms of rotation here. Pronation is the rotation of the forearm to turn the palm downward, while supination turns the palm upward, such as when carrying a bowl of soup.
Distinguishing Rotation from Other Joint Actions
Rotational movement is fundamentally different from the angular movements that make up the majority of body motion. Angular movements, which include flexion and extension, involve changing the angle between two bones at a joint. Flexion decreases the angle, while extension increases it.
In contrast, rotation is a spinning motion that moves the limb around its axis, not away from it. This distinction means that rotation can occur without any change in the angle of the joint.
Rotation must also be differentiated from circumduction, a complex movement often mistaken for rotation. Circumduction is a sequential combination of four distinct angular movements: flexion, adduction, extension, and abduction. The result is that the distal end of the limb traces a circle, while the proximal end acts as a pivot, creating a cone shape. Rotation is a pure twist, whereas circumduction is a composite, conical movement.