flexion

Flexion is a movement that decreases the angle at a joint — bending the elbow brings the hand toward the shoulder, and bending the knee brings the heel toward the buttock. Its opposite, extension, increases the angle and returns the part toward anatomical position. Movement terms are named for the direction of the movement, not for the muscle, so one muscle can produce more than one of them depending on which of its ends is fixed. The pairs worth separating are flexion against extension (angle), abduction against adduction (away from the midline against toward it), and pronation against supination (the forearm palm-down against palm-up). Circumduction is the combination of all four at a ball-and-socket joint, which is how the arm draws a circle in the air. When a joint is described as flexed, the measurement is the angle that got smaller, and the muscle that did it is usually named a flexor.