Axis and Planes

To describe movements precisely, biomechanics treats each joint as an axis and pairs it with a plane. In kinematics the limbs or segments of the body are assumed to rotate about the joints, with no sliding (translational) movement. While this is not strictly true, it is a usable approximation of real joint motion. The joint serves as an axis — a line around which something can rotate — and associated with every axis is a plane, like a sheet of paper held perpendicular to the axis, in which the rotational movement takes place.

Directional terminology you need first

Before studying the planes and axes, it helps to know the standard anatomical directions (always described from the anatomical position — standing upright, facing forward, palms forward):

• Anterior / Ventral — towards the front of the body.
• Posterior / Dorsal — towards the back of the body.
• Superior / Cranial — towards the head or upper part of the body.
• Inferior / Caudal — towards the lower part of the body.
• Medial — towards the midline of the body (the inner side).
• Lateral — away from the midline of the body (the outer side).

Why describing movement this precisely matters

A coach who simply says "the bowler's arm came through fast" has described nothing a biomechanist can measure or compare. Saying instead that the shoulder rotated internally about the vertical axis while the trunk flexed in the sagittal plane gives an exact, repeatable description that can be filmed, measured and compared between two different bowling actions, or the same bowler's action on two different days. This is exactly why video and motion analysis (part of the importance of biomechanics discussed in section 8.2.3) always describes a movement by its plane and axis, never by everyday language alone.

The anatomical position — the fixed starting reference

All of this directional language is only meaningful if everyone agrees on a single starting reference. That reference is the anatomical position: standing fully upright, facing forward, feet together, arms hanging at the sides with the palms turned forward. Every direction above, and every plane and axis described in the next three sub-sections, is defined relative to a body standing in this position, regardless of which direction the body is actually facing during the real sporting action.

A worked example …