Muscle Actions

Contraction, Relaxation, and Antagonist muscles

Muscles act across joints to change the position of the joint or can serve to move or alter the shape of a soft anatomical structure.  Muscles only have two actions: contraction or relaxation.  Muscles cannot lengthen voluntarily, instead, they rely on antagonistic muscles to contract and thus to move the joint back to its position while the muscle that initiated the action relaxes. 

Muscle contraction is controlled and graded. If you slowly flex your elbow, the speed and force will depend upon the number of fibers that are contracting due to neural stimulation; this is a graded contraction.  If all fibers of the biceps contracted at once and unopposed, the result would be a spastic movement (indicative of a clinical deficit or damage somewhere within the motor control pathway). 

In many cases when a load is being moved, muscles antagonistic to the motion may also act to allow the muscle to work slowly against the load without damaging its muscle fibers.  If viewed in slow motion, this is the principle of “Tai Chi” in which slow, ballet type movements give one exercise, but at low impact since “agonists” and “antagonists” are getting a workout in symphony.

Movements - General Actions

As we covered in lectures, joints allow for certain types of movements around the joint and constrain other motions due to the bony architecture of the joint or ligaments 'checking' the motion.  There are a number of general concepts of how we discuss movements.

Flexion - If you decrease the angle across a joint, we refer to this movement as flexion.  Making a fist flexes the fingers.  Curling up the toes flexes the toes.  Reducing the angle across the elbow joint flexes the upper limb at the elbow.  

Extension - The opposite of flexion is extension, in which the angle of the joint is increased (i.e. opened). Unclenching your first extends the fingers. Straightening your arm extends at the elbow joint.

Abduction - This is the term for when the motion at the joint moves away from the centerline of the body (or in the special case of hands/feet from the centerline of the hand/foot). For example, lifting your arm out to the side is abduction of the shoulder.

Adduction - This is the opposite of abduction, in which you move the structure back towards the centerline of the body (or hand/foot centerline in those special cases).  For example, bringing your arm back to your side from a lifted position is adduction.

Rotation - This is the generic term for rotating (pivoting) around a joint. This can be medial rotation also known as internal rotation (rotating the anterior part of the distal bone inwards towards the midline), or lateral rotation also known as external rotation (turning the anterior part of the distal bone outwards away from the midline ).

Circumduction - Circumduction is the orderly combination of movements so that the distal structure traces a circle in space while the proximal structure remains fixed in place. The term is usually used for the the upper or lower limb where the hand or foot move in a circle and the limb itself traces a cone (executed by a sequence of flexion, abduction, extension and adduction or the reverse). The upper torso can also perform circumduction relative to the pelvis (a motion of twisting the torso so that the head traces a circle).

Movements - Special Cases

Most of the special cases on movements will be addressed during the Skin, Bones and Musculature course in MS2.

Spine - Movements around the spine can seem confusing since the spine is already at the midline.

So, we term bending forwards flexion of the spine and bending backwards extension of the spine. If you bend to the left or right that is termed lateral flexion (we can't use the term abduction/adduction since the spine is the midline). Similarly, for rotation we would simply say 'left rotation' or 'right rotation'.

Fingers - There are several special motions and frames of reference we use with the fingers and thumb. The frame of reference for the fingers is the middle finger (digit 3), around which movements of the digits are referenced.

Adduction (fingers) - This is the term when moving fingers towards the middle finger in the plane of the palm (i.e. bringing fingers together).

Abduction (fingers) - The opposite of adduction, this is moving fingers away from the middle finger parallel to the plane of the palm (i.e. spreading the fingers apart).

Flexion/Extension (fingers) - These movements are as you would expect from the generic use of flexion and extension. Flexion is curling the fingers in towards the palm and extension stretching them away from the palm.

Thumb - During development the thumb rotated such that it is at 90 degrees to the rest of the fingers (i.e. the fingernail of the thumb when the hand is at rest is at right angles to the fingernails of the fingers). Thus, movements are referenced to the thumb from that rotated 'at rest' position.

A good way to think about thumb movements is 'follow the fingernail', the movement terms are then similar to the reference frame of how the fingernail moves when moving the fingers in flexion/extension (bending fingernail in/out) or abduction/adduction (moving parallel to the plane of the fingernail).

Flexion - This is curling the thumb inward towards the palm (parallel to the plane of the thumb).

Extension - This is stretching the thumb out away from the palm (parallel to the plane of the thumb).

Abduction - This is moving the thumb away from the palm parallel to the plane of the fingernail of the thumb.

Adduction - This is moving the thumb towards the palm parallel to the plane of the fingernail of the thumb.

Opposition - This is the 'party trick' of the primate species...the opposable thumb. The thumb movement which brings the tip of the thumb into contact with the tip of a finger is called opposition.

Wrist/Forearm - There are also specific terms for rotation of the forearm and movements of the wrists.

Supination - This is rotation of the forearm such that the palm is in line with the anterior forearm (i.e. 'palm up'). The anatomic position has the forearm fully supinated.

Pronation - This is rotation of the forearm such that the palm faces posterior (i.e. 'palm down').

Ulnar deviation - This is a 'side to side' motion of the hand in the ulnar direction. In the anatomic position this is adduction of the wrist, but the term ulnar deviation is also in common use.

Radial deviation - This is a 'side to side' motion of the hand in the radial direction. In the anatomic position this is abduction of the wrist, but the term radial deviation is also in common use.

Feet - There are also special motions for the feet (similar to the hand there is a center toe frame of reference for the toes, but since we cannot abduct/adduct the toes very efficiently these are rarely of practical use).

Dorsiflexion - This is a motion at the ankle joint of lifting the top of the foot, i.e. the dorsal side. Thus, the term 'dorsiflexion'.

Plantar flexion - This is a motion at the ankle joint of planting the foot, i.e. the plantar side. Thus, the term 'plantar flexion'. A common way to remember this is if you plant your foot on the floor the motion is plantar flexion.

Inversion - This motion is tilting the foot such that the plantar surface of the foot points towards the midline.

Eversion - This motion is tilting the foot such that the plantar surface of the foot is pointed away from the midline.