Axes Of Motion
Wrist motion is typically thought of as operating in two axes. Motion in a semicircle from the thumb to the pinky is called wrist deviation:
The names for the two directions of the deviation movement are derived from the bones in the forearm, the radius and the ulna. Movement toward the radius is radial deviation, and movement toward the ulna is ulnar deviation. For this reason, wrist deviation is sometimes referred to as radial-ulnar deviation, or simply RUD.
Perpendicular to the deviation axis is the flexion-extension axis:
Movement in the direction of the palm is wrist flexion, and movement in the direction of the knuckles is wrist extension. Together, this axis of movement is referred to as wrist flexion-extension, or abbreviated as FE.
By combining these two planes of motion, the wrist can move parallel to the strings, perpendicular to the strings, or anywhere in between. In actual practice, because of the tilted forearm position required to switch strings efficiently, the most common wrist motions used in picking technique are in fact the “in-between” ones that require some amount of each motion working together.