Adduction and abduction are anatomical terms of motion pertaining to the possible movements of a joint. Using the hip joint as an example, adduction is the act of drawing the leg sideways toward the midline of the body, while abduction is the act the lifting the leg sideways away from the body. Both motions occur in a single plane of movement known as the frontal plane, and joints performing these motions include the hip as well as those found at the shoulder, wrist, and base of the fingers and thumb. Adduction and abduction are initiated by multiple muscles at each joint, with one muscle or group of muscles adducting the joint and an opposing muscle or group of muscles abducting the joint.
Always performed in the frontal plane in a side-to-side direction, these movements occur relative to anatomical position. The body is said to be in anatomical position when standing with feet together, arms at the sides, and palms facing forward. This position is used as a point of reference when describing movements.
Adduction involves pulling the limbs or extremities inward from a side-lifted position toward the vertical midline of the body, while abduction involves lifting the same parts to either side. From anatomical position, adduction includes the actions of drawing the arms in to the sides, tilting the pinky side of the hand toward the same side of the forearm, and bringing the legs together laterally, among others. Abduction includes the opposite motions: raising the arms sideways away from the body, tilting the thumb side of the hand toward the same side of the forearm, and jumping the legs apart.
Three kinds of synovial or movable joints can produce the movements of adduction and abduction by articulating in opposing directions: condyloid, saddle, and ball-and-socket joints. Condyloid joints feature adjoining bones that are elliptical in shape. The wrist joint is an example of a condyloid joint.
Saddle joints, like that seen at the carpometacarpal joint at the base of the thumb, are similar to condyloid joints but feature two saddle-shaped bony surfaces curving around each other. The hip and shoulder are ball-and-socket joints, which permit movement in nearly every direction. All three joint types allow adduction and abduction in addition to the front-to-back movements of flexion and extension and a circling movement known as circumduction.
Another distinction between adduction and abduction is the muscles performing these actions. At any joint, one muscle or set of muscles performs a given movement while the opposing movement is performed by another muscle or set of muscles. Hip adduction, for example, is initiated by five muscles found along the inner thigh that are collectively known as the adductor group. The opposing motion of abduction is made possible by another group of muscles on the back and side of the hip; these include the gluteal muscles and several deeper muscles in the posterior hip.