The answer to whether medial rotation is the same as internal rotation is a clear yes; the two terms are interchangeable synonyms used within human anatomy to describe the exact same movement. The confusion often arises because anatomical terminology frequently employs both Latin-derived words and more common English synonyms to describe structures and actions. Rotation itself is defined as the movement of a bone around its own long axis, akin to a spinning motion. In the context of the body’s limbs, this specific movement is given two names, “internal” and “medial,” which both describe the direction of the spin toward the body’s center.
Defining Directional Terminology
All movements in the body, including rotation, are described relative to a universal starting point called the anatomical position. This standard posture requires the body to be standing upright with the feet parallel, the head facing forward, and the arms hanging at the sides with the palms facing forward. This position serves as the zero point from which all motion is measured, ensuring clear communication regardless of the body’s actual orientation.
The directional terms “medial” and “lateral” are fundamental to understanding rotational movement because they establish the reference points for the rotation’s direction. Medial is defined as being closer to the midline of the body, which is an imaginary vertical line that divides the body into equal right and left halves. Conversely, lateral means being further away from this midline, or toward the sides of the body. These static directional terms are then applied to describe the dynamic motion of a limb’s rotation.
Mechanics of Internal and Medial Rotation
Internal rotation, or medial rotation, is the rotational action of a limb that brings the anterior surface of that limb closer to the body’s midline. This spinning motion occurs in the transverse plane around the long axis of the bone, effectively turning the limb inward.
A common example of this movement is seen at the shoulder (glenohumeral joint), where the upper arm bone rotates inward. If the elbow is bent at a 90-degree angle, medial rotation involves moving the hand and forearm across the body toward the stomach. Similarly, at the hip joint, medial rotation of the thigh involves the femur rotating inward, which causes the knee and toes to point toward the opposite leg. This movement is powered by a group of muscles called internal rotators, which include the subscapularis muscle in the shoulder and the tensor fasciae latae in the hip.
The Opposite Movement: External and Lateral Rotation
The opposing movement is external rotation, also known as lateral rotation. This action is the exact reverse of medial rotation, involving the spinning of a limb away from the midline of the body. Like their internal counterparts, “external” and “lateral” are interchangeable terms indicating an outward movement.
In the shoulder, external rotation of the humerus causes the anterior surface of the arm to turn away from the body’s midline. Using the bent-elbow example, this movement involves swinging the hand and forearm outward, away from the torso. At the hip, lateral rotation of the femur causes the knee and foot to turn outward, away from the center of the body. Rotational movements always occur in pairs, meaning that a limb that can medially rotate must also be able to laterally rotate.