In kinematics, the limbs and body segments are treated as rotating about the joints, with the joint itself serving as an AXIS (a line about which rotation occurs) and an associated PLANE (like a sheet of paper perpendicular to that axis) in which the rotation actually takes place. To describe this precisely, the chapter uses a shared directional vocabulary: Anterior/Ventral (towards the front), Posterior/Dorsal (towards the back), Superior/Cranial (towards the head/upper body), Inferior/Caudal (towards the lower body), Medial (towards the midline), and Lateral (away from the midline).

There are three planes of motion, each dividing the body a different way. The Sagittal (median) plane lies vertically and divides the body into right and left halves; flexion and extension happen in this plane (kicking a football, walking, jumping, squatting). The Frontal (lateral/coronal) plane also lies vertically but divides the body into anterior and posterior parts; abduction and adduction happen here (jumping jacks, a cartwheel). The Transverse (horizontal) plane divides the body into superior and inferior parts; rotation happens here (a golf swing's hip rotation, a discus twist, a skater's spin). The point where all three planes intersect is defined as the body's centre of gravity. …