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Motion, the process of movement, is described using specificanatomical terms. Motion includes movement oforgans,joints,limbs, and specific sections of the body. The terminology used describes this motion according to its direction relative to theanatomical position of the body parts involved.Anatomists and others use a unified set of terms to describe most of the movements, although other, more specialized terms are necessary for describing unique movements such as those of the hands, feet, and eyes.

In general, motion is classified according to theanatomical plane it occurs in.Flexion andextension are examples ofangular motions, in which two axes of a joint are brought closer together or moved further apart.Rotational motion may occur at other joints, for example theshoulder, and are described asinternal orexternal. Other terms, such aselevation anddepression, describe movement above or below the horizontal plane. Many anatomical terms derive fromLatin terms with the same meaning.

Motions are classified after theanatomical planes they occur in,^[1] although movement is more often than not a combination of different motions occurring simultaneously in several planes.^[2] Motions can be split into categories relating to the nature of the joints involved:

• Gliding motions occur between flat surfaces, such as in theintervertebral discs or between thecarpal bones of thewrist, and themetacarpal bones of thehand.^[1]
• Angular motions occur oversynovial joints and causes them to either increase or decrease angles between bones.^[1]
• Rotational motions move a structure in arotational motion along a longitudinal axis, such as turning the head to look to either side.^[3]

Apart from this motions can also be divided into:

• Linear motions (ortranslatory motions), which move in a line between two points.Rectilinear motion is motion in a straight line between two points, whereascurvilinear motion is motion following a curved path.^[2]
• Angular motions (orrotary motions) occur when an object is around another object increasing or decreasing the angle. The different parts of the object do not move the same distance. Examples include a movement of the knee, where the lower leg changes angle compared to the femur, or movements of the ankle.^[2]

The study of movement in the human body is known askinesiology.^[4] A categoric list of movements and themuscles involved can be found atlist of movements of the human body.

The prefixhyper- is sometimes added to describe movement beyond the normal limits, such as inhypermobility,hyperflexion orhyperextension. Therange of motion describes the total range of motion that a joint is able to do.^[5] For example, if a part of the body such as a joint is overstretched or "bent backwards" because of exaggerated extension motion, then it can be described ashyperextended. Hyperextension increases the stress on theligaments of a joint, and is not always because of a voluntary movement. It may be a result of accidents, falls, or other causes of trauma. It may also be used in surgery, such as in temporarily dislocating joints for surgical procedures.^[6] Or it may be used as apain compliance method to force a person to take a certain action, such as allowing a police officer to take him into custody.

These are general terms that can be used to describe most movements the body makes. Most terms have a clear opposite, and so are treated in pairs.^[7]

Flexion and extension are movements that affect the angle between two parts of the body. These terms come from theLatin words with the same meaning.^[a]

Flexion is a bending movement thatdecreases the angle between a segment and its proximal segment.^[9] For example, bending theelbow, or clenching a hand into a fist, are examples of flexion. When a person is sitting down, the knees are flexed. When a joint can move forward and backward, such as the neck and trunk, flexion is movement in the anterior direction.^[10] When the chin is against the chest, the neck is flexed, and the trunk is flexed when a person leans forward.^[10] Flexion of the shoulder or hip is movement of the arm or leg forward.^[11]

Extension is the opposite of flexion, a straightening movement thatincreases the angle between body parts.^[12] For example, when standing up, the knees are extended. When a joint can move forward and backward, such as the neck and trunk, extension is movement in the posterior direction.^[10] Extension of the hip or shoulder moves the arm or leg backward.^[11] Even for other upper extremity joints – elbow and wrist, backward movement results in extension. The knee, ankle, and wrist are exceptions, where the distal end has to move in the anterior direction for it to be called extension.^{[13][page needed]}

For thetoes, flexion is curling them downward whereas extension is uncurling them or raising them.^[14]

Abduction is the motion of a structure away from the midline while adduction is motion towards the center of the body.^[15] The center of the body is defined as themidsagittal orlongitudinal plane.^[3] These terms come fromLatin words with similar meanings,ab- being the Latin prefix indicating'away',ad- indicating'toward', andducere meaning'to draw or pull'.^[b]

Abduction is a motion that pulls a structure or partaway from the midline of the body, carried out by one or moreabductor muscles. In the case of fingers and toes, it is spreading the digits apart, away from the centerline of the hand or foot.^[15] For example, raising the arms up, such as whentightrope-walking, is an example of abduction at theshoulder.^[11] When the legs are splayed at the hip, such as when doing astar jump or doing asplit, the legs are abducted at the hip.^[3]

Adduction is a motion that pulls a structure or parttowards the midline of the body, or towards the midline of a limb, carried out by one or moreadductor muscles. In the case of fingers and toes, it is bringing the digits together, towards the centerline of the hand or foot. Dropping the arms to the sides, and bringing the knees together, are examples of adduction.^[15]

Adduction of thewrist is also known asulnar deviation which moves the hand towards theulnar styloid (or, towards thelittle finger). Abduction of the wrist is also calledradial deviation which moves the hand moving towards theradial styloid (or, towards thethumb).^[15][17]

Elevation and depression are movements above and below the horizontal. The words derive from the Latin terms with similar meanings.^[c]

Elevation is movement in a superior direction.^[19] For example, shrugging is an example of elevation of the scapula.^[20]

Depression is movement in an inferior direction, the opposite of elevation.^[21]

Rotation of body parts may be internal or external, that is, towards or away from the center of the body.^[22]

Internal rotation (medial rotation orintorsion) is rotation towards the axis of the body,^[22] carried out byinternal rotators.

External rotation (lateral rotation orextorsion) is rotation away from the center of the body,^[22] carried out byexternal rotators.

Internal and external rotators make up therotator cuff, a group of muscles that help to stabilize theshoulder joint.

• 
  Thelotus position ofyoga, demonstratingexternal rotation of the thigh at the hip.
• 
  Rotating the arm away from the body isexternal rotation.
• 
  Rotating the arm closer to the body isinternal rotation.

• Anterograde andretrograde flow refer to movement of blood or other fluids in a normal (anterograde) or abnormal (retrograde) direction.^[23]
• Circumduction is a conical movement of a body part, such as aball and socket joint or theeye. Circumduction is a combination of flexion, extension, adduction and abduction. Circumduction can be best performed at ball and socket joints, such as thehip andshoulder, but may also be performed by other parts of the body such as fingers, hands, feet, and head.^[24] For example, circumduction occurs when spinning the arm when performing a serve intennis or bowling acricket ball.^[25]
• Reduction is a motion returning a bone to its original state,^[26] such as a shoulder reduction followingshoulder dislocation, or reduction of ahernia.

• 
  The swinging action made during atennis serve is an example ofcircumduction

Dorsiflexion andplantar flexion refer to extension or flexion of the foot at the ankle. These terms refer to flexion in direction of the "back" of the foot, which is the upper surface of the foot when standing, and flexion in direction of the sole of the foot. These terms are used to resolve confusion, as technicallyextension of the joint is dorsiflexion, which could be considered counter-intuitive as the motion reduces the angle between the foot and the leg.^[27]

Dorsiflexion is where the toes are brought closer to theshin. This decreases the angle between the dorsum of the foot and theleg.^[28] For example, when walking on theheels the ankle is described as being in dorsiflexion.^[27] Similarly, dorsiflexion helps in assuming a deepsquat position.^[29]

Plantar flexion or plantarflexion is the movement which decreases the angle between the sole of the foot and the back of the leg; for example, the movement when depressing acar pedal or standing on tiptoes.^[27]

• 
  Aballerina, demonstratingplantar flexion of the feet
• 
  Dorsi and plantar flexion of the foot

Palmarflexion and dorsiflexion refer to movement of the flexion (palmarflexion) or extension (dorsiflexion) of the hand at the wrist.^[30] These terms refer to flexion between the hand and the body'sdorsal surface, which inanatomical position is considered the back of the arm; and flexion between the hand and the body'spalmar surface, which in anatomical position is considered the anterior side of the arm.^[31] The direction of terms are opposite to those in the foot because ofembryological rotation of the limbs in opposite directions.^[10]

Palmarflexion is flexion of the wrist towards thepalm andventral side offorearm.^[30]

Dorsiflexion ishyperextension of thewrist joint, towards thedorsal side of forearm.^[30]

• 
  Praying Hands byAlbrecht Dürer, demonstratingdorsiflexion of the hands.

Pronation and supination refer generally to theprone (facing down) orsupine (facing up) positions. In the extremities, they are the rotation of the forearm or foot so that in thestandard anatomical position the palm or sole is facing anteriorly when in supination and posteriorly when in pronation.^[32] As an example, when a person is typing on a computer keyboard, their hands are pronated; when washing their face, they are supinated.

Pronation at theforearm is a rotational movement where the hand and upper arm are turned so the thumbs point towards the body. When the forearm and hand are supinated, the thumbs point away from the body. Pronation of the foot is turning of the sole outwards, so that weight is borne on the medial part of the foot.^[33]

Supination of the forearm occurs when the forearm or palm are rotated outwards. Supination of the foot is turning of the sole of the foot inwards, shifting weight to the lateral edge.^[34]

• 
  Supination and pronation of the foot
• 
  Supination and pronation of the arm

Inversion and eversion are movements that tilt the sole of the foot away from (eversion) or towards (inversion) the midline of the body.^[35]

Eversion is the movement of thesole of the foot away from themedian plane.^[36]Inversion is the movement of thesole towards themedian plane. For example, inversion describes the motion when anankle is twisted.^[28]

• 
  Example showing inversion and eversion of the foot
• 
  Eversion of the right foot
• 
  Inversion of the right foot

Unique terminology is also used to describe the eye. For example:

• Aversion is an eye movement involving botheyes moving synchronously and symmetrically in the same direction.^[37]
• Torsion is eye movement that affects the vertical axis of the eye,^[38] such as the movement made when looking in to the nose.

• Occlusion is motion of themandibula towards themaxilla making contact between the teeth.^[39]
• Protrusion andretrusion are sometimes used to describe the anterior (protrusion) and posterior (retrusion) movement of the jaw.^[40]

• 
  Examples showing protrusion and retrusion.
• 
  Elevation and depression of the jaw.

Other terms include:

• Nutation andcounternutation^[d] are movement of thesacrum defined by the rotation of thepromontory downwards and anteriorly, as with lumbar extension (nutation); or upwards and posteriorly, as with lumbar flexion (counternutation).^[42]
• Opposition is the movement that involves grasping of the thumb and fingers.^[43]
• Protraction andretraction is an anterior (protraction) or posterior (retraction) movement,^[44] such as of the arm at the shoulders, although these terms have been criticised as non-specific.^[45]
• Reciprocal motion is alternating motions in opposing directions.^[46]
• Reposition is restoring an object to its natural condition.^[47]

• 
  Nutation at left, counternutation at right
• 
  An example ofopposition
• 
  Example ofopposition of the thumb and index finger

• Anatomical terms of location
• Anatomical terms of muscle
• Anatomical terms of bone
• Anatomical terms of neuroanatomy

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• Seeley, Rod R.; Stephens, Trent D.; Tate, Philip (1998).Anatomy & Physiology (4th ed.). WCB/McGraw-Hill.ISBN 0-697-41107-9.
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