Q.What is plane of movement?

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.

There are three axes of rotation, and -- importantly -- each axis is perpendicular to its own plane, not to itself: the Sagittal (anteroposterior) axis runs horizontally from back to front and is perpendicular to the FRONTAL plane, with abduction/adduction occurring about it; the Frontal (transverse/mediolateral) axis runs horizontally from side to side and is perpendicular to the SAGITTAL plane, with flexion/extension occurring about it; and the Vertical (longitudinal) axis runs from bottom to top and is perpendicular to the TRANSVERSE plane, with rotation occurring about it.

In practice, a movement is usually named after its dominant plane (walking is called a 'sagittal plane movement'), but at the level of an individual joint, movement actually happens in all three planes at once -- this is called tri-planar motion. Describing movement in single-plane terms is simply the most useful way to generalise a gross movement pattern, even though the full joint-level picture is more complex.