Planes of Movement and Terms of Position
It's easy to become confused when trying to describe locations in the body. Terms that make sense in everyday life, like "beside", "under", and "on top of", are simply too fuzzy when talking about a three-dimensional human body. For example, consider a phrase like "an inch below the navel". Is the speaker looking for a point an inch toward the feet, or toward the spine? For clarity, we need points of reference.
Anatomical Neutral Position
No matter whether the body is upright, lying down, or curled up in a ball, anatomists always speak as if it's in a standard neutral position, so the head is considered the top of the body even when it's hanging upside down. Not to be confused with an ordinary standing position, anatomical neutral is a position in which the body is standing upright, slightly out-toeing, with palms facing forward.
Planes of Reference
The body can be divided by planes, like slices, running either vertically or horizontally.
Sagittal planes slice the body lengthways from front to back, producing a full-body profile. There are an infinite number of places to make a sagittal plane, but the one in the very middle slicing down through the tip of the nose, the navel, and the tailbone is called the midsagittal or median plane, or the midline. Movement in a sagittal plane is called flexion or extension.
Coronal or frontal planes slice lengthways from side to side, producing gingerbread men. Movement in a coronal plane is called abduction, adduction, or lateral flexion.
Transverse or horizontal planes slice across the body lopping off the head, sawing the torso in half at the waist, or slicing the legs into round steaks. Movement in a transverse plane is called rotation.
Terms of Position and Direction
A number of terms are useful when describing the location of a particular point on the body in relation to the rest of the body.
Superior: toward the head, higher up (also called cranial or rostral)Inferior: toward the feet, lower down (also called caudal, toward the buttocks)Anterior: toward the front (also called ventral)Posterior: toward the back (also called dorsal)Medial: toward the midlineLateral: away from the midline, toward the sidesSuperficial: toward the surfaceDeep: away from the surface, more inside the bodyProximal: of a limb, closer to the body's midline (usually, closer to the shoulder or hip)Distal: of a limb, further from the body's midline (usually, closer to the fingers or toes)Ipsilateral: on the same sideContralateral: on the opposite sideDirections of Movement
Flexion, generally speaking, closes a joint: bending the elbow in a bicep curl, or scrunching the body down in an airline seat in preparation for a crash. Extension, generally speaking, opens a joint: straightening the knee or arching the neck. Sometimes it's more useful to think of flexion as a movement in a sagittal plane that takes part of the body forward from anatomical neutral (reaching the arm forward is flexion of the shoulder), and extension as a sagittal movement that takes part of the body backward (reaching the arm back is shoulder extension).
Adduction brings part of the body closer to the midline ("adding" it to the middle), while abduction takes part of the body away from the midline ( as if it has been ruthlessly abducted, by aliens, probably). These movements are in the frontal plane, moving only sideways, snow-angel-style. However, the hands and feet have their own midline so spreading the fingers is abduction, even though the ring and pinky are moving toward the whole body's midline.
Lateral flexion is sidebending of the spine a quizzical tilt of the head, still in the frontal plane.
Rotation is a transverse twist of the spine, shoulder, hip, or other joint. It's not to be confused with circumduction, in which a joint smoothly moves through flexion, abuduction, extension and adduction in a circular motion.
There are also more specific terms of movement for different structures - pronation, supination, inversion, eversion, nutation, and counternutation, to name a few. These are outside the scope of this article.
Building Confidence with Anatomical Language
Anatomical terms may seem unnecessarily complex and formal, but they make anatomy study much easier to understand. Learning terminology is a great place to start thinking about anatomy.