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HYPERMUSCLE: MUSCLES IN ACTION

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This is a multimedia interactive HTML document which will help medical students learn the muscle actions of the human body.

Each muscle action is listed in the Table of Contents. You can read about each movement as you travel down the page or you may go directly to a movement for a description by clicking on the blue title. An asterisk (*) indicates that a movie can be played at that location. Click on the blue heading to see the movie.

Table of Contents

  • Introduction
  • Actions of the Scapula
  • Actions of the Foot
  • Movements of the Hand
  • Pronation/Supination*

    Introduction

    Muscle actions are always described as departures from the standard position of the body, the anatomical position.

    Certain of these actions are general, and can be applied to several regions of the body, while others are quite specific to a single region or even a single joint.

    Flexion/Extension

    This pair of actions occurs about a transverse axis through a joint.

    The movement at a joint which decreases the angle between two adjacent body segments is know as flexion.

    The opposite action is extension, where the angle between body segments is increased.

    Abduction/Adduction

    These actions are possible about an axis arranged in an antero-posterior direction through a joint.

    A movement of a body part away from the midline, either of the body as a whole or that of the hand or foot, is termed abduction (L., to carry away).

    A movement of the body part back toward the midline (i.e., to the anatomical position) is known as adduction.

    Lateral/Medial Rotation

    Rotation occurs about a longitudinal (vertical) axis passing through a joint and through the length of the bone distal to it. A reference point must be identified on the lateral aspect of the distal bone. (The flexed forearm can indicate this reference point.)

    Lateral rotation is the opposite action. The reference point is moved posteriorly or laterally, away from the body midline.

    Medial rotation involves a torsion of the distal bone so that the reference point is moved anteriorly or medially toward the body midline.

    Circumduction

    A combination of the actions of flexion, abduction, extension and adduction, in that order, is known as circumduction .

    This is a composite action, involving two axes of rotation: the transverse axis and the antero-posterior axis, and both actions allowed at each axis.

    Actions of the Scapula

    Several of the special muscle actions occur in the movement of the scapula. The scapula is rather firmly connected to the clavicle via the acromioclavicular joint, hence these two bones move together as a single unit.

    The functional joint which allows them to move is the sternoclavicular joint, located at the superior aspect of the sternum.

    All the axes of rotation involving the scapula must therefore pass through the sternoclavicular joint, and movements of the scapula must be understood to occur at that joint.

    Elevation/Depression

    On an anteroposterior axis through the sternoclavicular joint, the scapula, with its attached musculature, is able to be raised to a more superior level, or elevated ("shrugging the shoulders").

    The opposite action, depression, involves the pulling down of the scapula to a more inferior position.

    Protraction/Retraction

    The vertical axis through the sternoclavicular joint allows the scapula to be moved forward, an action termed protraction (as in "hunching the shoulders").

    The action opposite to this would be one of retraction, or movement of the scapula backwards (as in "squaring the shoulders").

    Rotation

    The scapula is also capable of being rotated upward and downward around a reference point at the tip of the shoulder. This action occurs around an oblique angle passing through the sternoclavicular joint and the scapula bone just below the shoulder joint.

    The scapula can be rotated upward around this axis, allowing a full upward swing (abduction) of the upper limb.

    Or it can be rotated downward, pushing the inferior angle of the scapula close to the midline of the body.

    Special Actions of the Foot

    Two specialized actions are allowed for the foot: one pair at the ankle joint and the other between bones of the foot itself.

    The actions of the ankle are plantar flexion and dorsiflexion; those of the foot are version and inversion .

    Dorsiflexion/Plantar Flexion

    A transverse axis through the ankle joint allows a pair of actions similar to flexion and extension at the wrist joint.

    The analogous action to wrist flexion is one that would tip the sole of the foot downward, increasing the angle between foot and leg. The usual term for the increase in such an angle would be extension, but in order to emphasize the relation between foot and hand, this action is instead termed plantar flexion .

    The action similar to extension at the wrist would be a tipping of the upper surface (dorsum) of the foot toward the anterior surface of the leg. However, this would decrease the angle between the body segments, and action usually termed flexion. Since the term, plantar flexion, has been used for the opposite action, this is now referred to as dorsiflexion .

    Eversion/Inversion

    These special actions of the foot occur as a result of movement between two major joint complexes of the foot. The resulting angle of rotation is oblique, from the medial side of the heel to the lateral side of the mid-foot. Inversion is the action of turning the sole of the foot inward, towards the opposite foot. Eversion is the movement of turning the sole of the foot outward, away from the midline.

    Movements of the Hand

    In the hand and foot, a specific application is utilized for the general terms abduction and adduction .

    In addition, movement of certain of the metacarpals (or metatarsals) occurs at their joint with carpal bones of the wrist (or tarsals of the foot). This action is termed opposition .

    Abduction/Adduction

    The actions of abduction and adduction were previously described with regard to movements relating to the body as a whole. However, they may also be used to describe actions of the fingers and toes away from (abduction) or toward (adduction) a reference point.

    Opposition

    Opposition is a special action of the hand, whereby the thumb and little finger are brought around to touch the fingertips.

    This action occurs at the joints between the carpal bones and the metacarpals of the thumb and little finger.

    Pronation/Supination

    Another special action occurs in the forearm, where the radius and ulna are arranged in such a way as to allow the crossing of the distal end of the radius over the ulna. An oblique angle of rotation occurs here, passing through the head of the radius (proximally) and the styloid process of the ulna (distally). The specialized movements occurring here are pronation and supination.

    Pronation is the movement of crossing the radius over the ulna. This movement results in the dorsal surface of the hand turning forward, or prone.

    Supination is the opposite action, wherein the radius is uncrossed and the palmar surface of the hand is returned to the anatomical position, or supine (forward).

    Copyright 1994 University of Michigan Learning Resource Center. For classroom use only. No part of this lesson can be reproduced for commercial use. For more information e-mail wwysor@umich.edu.