Movements Within the Arm

In this article we will cover muscles that control movements within the arm, wrist, and hand. We will first cover movement of the forearm. This is relatively straight forward as it largely involves extension and flexion of the elbow hinge joint. Next we cover movement of the wrist. This is a bit more complex because the wrist has a much more extensive range of movement. Finally we cover movements of the hand including the fingers and thumb.

We will again rely on anatomical terminology for defining the relative positions of two portions of the anatomy and for defining the direction and type of movements generated by muscles. You may wish to review these terms in the article on Anatomical Terminology.

The article you are reading is quite lengthy and full of detail. This makes it difficult to get through this material in one sitting. It is best if you study this material in sections. Take your time and review each section individually. Ignore the rest of the material until you have a firm grasp of a specific section. We present this material beginning at the proximal end of the arm. Our presentation incrementally progresses toward the distal end of this appendage. This provides one logical order of study. Perhaps begin by studying forearm movements and then concentrate on the wrist. Next study finger movements and finally how the thumb moves. Or, reverse the order entirely. The point is you will benefit from studying this article in sections rather than as one single entity.

Because the material in this article is complex and quite detailed you may benefit from additional sources of information. The Internet is full of anatomical information. You can do an Internet search for any of the terms found in this article to find additional discussions, drawings, or more detailed information. We encourage you to undertake such explorations if you feel our explanation is inadequate or inaccurate in any way.

Forearm Movement

By Anatomography (en:Anatomography (setting page of this image)) [CC BY-SA 2.1 jp (https://creativecommons.org/licenses/by-sa/2.1/jp/deed.en)], via Wikimedia Commons

Movement of the forearm occurs via flexion or extension of the elbow joint as well as via supination and pronation. As you may recall from your skeletal system curriculum the elbow is a hinge joint and can only move in this two directions. It cannot move laterally and does not rotate.

The three muscles that provide these movements are the biceps (also called the biceps brachii), triceps (triceps brachii) and the brachialis muscles. The biceps reside on the anterior portion of the humerus while the triceps reside on the posterior aspect of the humerus. The biceps are superficial to the brachialis muscle since the brachialis originates directly on the surface of the humerus.

Biceps

The biceps muscles consist of two bundles of muscle fibers. The bundles are separately anchored via tendons to the scapula. One bundle, called the short head (pictured in green), originates at the coracoid process of the scapula. The second bundle, referred to as the long head and shown in red, has an extremely long tendon. This tendon originates on the supraglenoid tubercle of the scapula. The tendon then travels over the tip of the humerus before descending and merging with the muscle tissues in the upper arm. The long head therefore spans the shoulder joint.

The two bundles of the biceps merge in the upper arm but insert via separate tendons at the radial tuberosity on the radius bone in the forearm. The long head inserts proximal to the insertion point of the short head. The biceps are one of the few muscles in the body that spans across two joints.

The biceps function in part to flex the lower arm at the elbow joint. They also play a significant role in supination of the forearm, turning the wrist outward. The supination role of the biceps is of greater significance than the flexion role. Other muscles (discussed later) aid the biceps in both of these functions.

By Anatomography (en:Anatomography (setting page of this image)) [CC BY-SA 2.1 jp (https://creativecommons.org/licenses/by-sa/2.1/jp/deed.en)], via Wikimedia Commons

Triceps

The triceps (also called the triceps brachii) consist of three muscle bundles (fascicles) . These bundles form the long head (shown in red), lateral head (shown in yellow), and medial head (shown in green).

The triceps primarily function as extensor muscles for the elbow joint. They also play a role in pulling the arm toward the back as might happen while you are walking.

The long head of the triceps originates along the infraglenoid tubercle of the scapula. The lateral and medial bundles originate on the upper portions (toward the proximal end) of the humerus. The three bundles merge at a single tendon that inserts at the Olecranon process at the proximal end and posterior portion of the ulna.

The three fascicles have different purposes. The lateral bundle is often employed when the muscle must generate a sudden and powerful force, such as when delivering Tettsui Uchi. The long head bundle is often used to hold the elbow in a fixed position. This functionality might be employed when holding your arm in your center while performing a parry. The medial head benefits fine motor control and precision movements.

Brachialis

By Anatomography (en:Anatomography (setting page of this image)) [CC BY-SA 2.1 jp (https://creativecommons.org/licenses/by-sa/2.1/jp/deed.en)], via Wikimedia Commons

Despite what you might have heard or thought, the biceps are not the primary muscle used when flexing the elbow. That function belongs to the brachialis muscle. While the biceps are useful during elbow flexion, the brachialis muscle generates far more power than the biceps. An additional differentiation is that the biceps provide supination for forearm, but the brachialis do not.

The brachialis muscle originates on the anterior distal half of the humerus beginning near the deltoid insertion point. This large origin area enables the brachialis to generate substantial flexion force. The brachialis generates about two-thirds of the elbow flexion forces while the biceps generate about one-third of the associated forces.

The large tendon of the brachialis inserts both at the coronoid process and the tuberosity of the ulna.

Anatomically the brachialis muscle is found deep to (further from the surface of the skin) the biceps. Therefore, it can be difficult to discern them visually. However, a large and well-developed brachialis muscle will push the biceps muscle in a superficial direction, making the biceps look larger and more pronounced.

By Grant, John Charles Boileua (An atlas of anatomy, / by regions 1962) [Public domain], via Wikimedia Commons

The diagram at right depicts many origin and insertion points for various muscles associated with the scapula and humerus. The areas shown in red represent muscle origins. Areas shaded in blue show muscle insertion locations. As you can see the brachialis muscle has a very large origin surface on the humerus, allowing the muscle to exert powerful flexion force across the elbow joint.

Brachioradialis

The brachioradialis muscle rests in the forearm and is a posterior forearm muscle (even though much of the muscle if seen on the anterior surface). It originates along much of the lateral lower one-third of the humerus and inserts into the styloid process of the radius near the wrist. This muscle assists with elbow flexion. It is not as powerful as the brachialis or biceps, but it is a fast reflex muscle that proves useful during rapid movement of the elbow.

How this muscle behaves depends a great deal on the arm’s current position. In addition to its role as a flexor it also provides both pronation and supination of the forearm. If the arm is both extended and rotated in pronation, then the brachioradialis plays a more substantial role in flexion since the muscles of the upper arm are not aligned well to support flexion. If the forearm is in pronation then the brachioradialis will cause supination when contracted. If the arm is in supination then the muscle will cause pronation during contraction.

If you lift weights you may notice that the larger muscles (brachialis and biceps) are primarily involved during the early part of elbow flexion, but the brachioradialis becomes more involved after the first third of the elbow flexion.

Anconeus

The anconeus muscle is a small muscle spanning the elbow joint. It originates on the lateral epicondyle of the humerus and inserts at the olecranon of the ulna. The muscle provides minor assistance during extension of the elbow. Its primary benefit is in keeping the elbow stabilized during extension. It serves to abduct the ulna during pronation. This latter function allows any finger of the hand to become the focus of elbow rotation.

Wrist Movement

The human wrist can move in four distinct directions. It can move in flexion, extension, abduction, and adduction. This is possible due to the skeletal structure of the wrist and by the many muscles found in the forearm. We will cover these various muscles and how each affects movements of the wrist.

By OpenStax [CC BY 4.0 (http://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons

The muscles in the forearm exist in distinct layers. There are three such layers. The superficial layer is closest to the skin. The deep layer is closest to the bone. An intermediate layer can exist between the superficial and deep layers.  There are a great many muscles involved in movements of the wrist and hand. Subsequent discussions will often rely on the adjacent diagram which depicts these various muscles. You can click on the diagram to view a larger image in a pop-up window. If the larger image does not display then you may need to enable pop ups for this website in your browser.

Wrist Flexion

The wrist moves in flexion via three muscles found on the anterior compartment (palmar side) of the forearm. The muscles involved in flexion are the flexor carpus radialis, flexor carpus ulnaris, and palmaris longus. Each of the muscles originates from the medial epicondyle of the humerus via the common flexor tendon on the distal end of the humerus and insert on the palm side of the carpals and metacarpals of the hand.

The flexor carpus ulnaris also has a point of origin at the olecranon of the ulna and has a lengthy attachment along the dorsal side of the ulna. It has three tendons that insert into the pisiform and hamate carpals as well as the fifth metacarpal bone.

The flexor carpus radialis inserts at the second and third metacarpal bones. In addition to its role as a flexor it also provides abduction of the wrist. The palmaris longus inserts into the fibrous tissues of the palmar aponeurosis. The palmaris longus muscle is a vestigial muscle. The muscle is not present in a significant portion (perhaps 15% on average) of the general population. The muscles absence has no bearing on grip strength or wrist flexion.

The diagram at right shows many of the origin and insertion points on the ulna and radius for muscles in the forearm and hand.

Wrist Extension

There are far more muscles involved in wrist extension than wrist flexion. A total of nine different muscles play a part in wrist extension versus the three muscles involved in flexion.

Wrist extensor muscles are all found in the posterior compartment (dorsal side) of the forearm. The muscles are found in a superficial and a deep layer.

Muscles in the superficial layer include:

  • Extensor carpi radialis longus
  • Extensor carpi radialis brevis
  • Extensor digitorum
  • Extensor digiti minimi
  • Extensor carpi ulnaris

The extensor carpi radialis longus muscle originates largely along the lateral distal edge of the humerus (with a few other less substantial origin points on the humerus as well). The extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi and extensor carpi ulnaris all originate from a common tendon at the lateral distal end of the humerus. You can find these various attachments on the earlier image showing the humerus and scapula origin and insertion points.

The extensor muscles also have various origin points on the ulna and on the radial collateral ligament. All of these muscles serve to move the wrist in extension. Some have other functions as well, such as finger extension and moving the wrist in abduction.

The extensor carpi radialis long and brevis muscles insert on the second and third metacarpal bones, respectively. These muscles serve both to provide wrist extension and wrist abduction.

The extensor digitorum muscle inserts via four tendons into the distal phalanges of each of four fingers. These connections support finger extension and aid during extension of the wrist.

The extensor digiti minimi inserts at the base of the proximal phalanx of the little finger.

The extensor carpi ulnaris inserts at the base of fifth metacarpal bone. The muscle supports extension and adduction of the wrist.

Wrist extensor muscles found in the deep layer include:

  • Abductor pollicis longus
  • Extensor pollicis longus
  • Extensor pollicis brevis
  • Extensor indicis

The abductor pollicis longs muscle originates on both the radius and ulna near the mid-point of the length of these two bones. It inserts into the lateral base of the first metacarpal bone and primarily works to both abduct and extend the thumb. It provides minor support for wrist extension.

The extension pollicis longus and brevis muscles originate on the ulna and are again primarily used to support extension of the thumb. They also provide assistance with wrist extension.

That leaves extensor indicis. This muscle originates on the posterior surface of the ulna. It inserts at the extensor hood of the index finger providing both extension of index finger and additional support for wrist extensions.

There are not the only muscles in these layers (as discussed later). These are only the muscles in these layers that support wrist extension.

Wrist Abduction and Adduction

Wrist abduction involves moving the wrist so the thumb moves toward the radial side of the forearm. If you extend your arm as though to shake someone’s hand, then abduction would occur if you move wrist such that the thumb moves closer to your shoulder. Abduction involves moving the wrist such that the little finger moves toward the ulnar side of the forearm. This is the motion you use when you wish to employ a Haito Uchi.

Muscles supporting wrist abduction are the extensor carpi radialis longus, extensor carpi radialis brevis muscles and abductor pollicis longus muscles. The extensor carpi ulnaris and flexor carpus radialis muscles support wrist adduction.

Hand Movement

The hand is one of the most intricate structures in the human body. Muscles originating from the humerus, ulna, radius, and within the hand itself all play a role in hand movements. Muscles that originate outside (proximal to) the hand are extrinsic muscles. Intrinsic muscles are those muscles originating within the hand itself.

By OpenStax [CC BY 4.0 (http://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons

Within the hand there are four intrinsic compartments, each containing muscles that perform related functions. The four compartments are:

  • Thenar
  • Hypothenar
  • Lumbricals
  • Interossei

We will cover the muscles in these compartments as we cover various hand movements in the following sections. There are two other muscles in the hand that are not part of any of these compartments. We will cover these muscles last.

Finger Movement

We earlier mentioned that the extensor digitorum muscle enable extension of the four fingers (not the thumb).

The flexor digitorum profundus muscle provides flexion of the fingers. This deep muscle originates along the upper three-quarters of the anterior and medial surfaces of the ulna. Four fascicles from this muscle terminate in four long tendons that insert into the distal phalanges of each finger. These long tendons travel down the lower third of the forearm, through the wrist and palm, and through the fingers before inserting on the palmar side of the distal phalanx of each finger. Because these tendons are so long they require some form of constraint so they pull along their length rather than popping up under stress. The tendons travel beneath the transverse carpal ligament (which forms the roof of the carpal tunnel) ensuring the tendons remain tight against the wrist and palm during flexor digitorum profundus contraction.

Thumb Movement

We also mentioned that the abductor pollicis longs muscle supports thumb movement (providing extension and abduction) along with the extensor pollicis longus and brevis muscles (providing extension and weak abduction).

The thenar compartment comprises three small muscles at the base of the thumb. The opponens pollicis is a deep muscle and the largest of the three thenar muscles. It originates at the tubercle of the trapezium and the emerges from the transverse carpal ligament. It inserts at the along the entire lateral edge of the first metacarpal of the thumb. The muscle functions to pull the thumb into opposition, allowing the thumb to touch each of the other fingers of the hand. A firm grip is dependent on this muscle’s action.

The abductor pollicis brevis muscle originates on the scaphoid tubercle and the transverse carpal ligament. It inserts into the lateral side of the proximal phalanx of the thumb. As the name suggests, this muscle abducts the thumb.

The flexor pollicis brevis originates from superficial and a deep locations. The deep origins include the tubercle of the trapezium, the capitate, and the trapezoid. The superficial origin emerges from the transverse carpal ligament. The muscle inserts at the base of the proximal phalanx of the thumb. It flexes the metacarpal/phalanges joint of the thumb. It is what allows you to bend the end of your thumb. In addition it supports both adduction and abduction of the thumb providing support for opposition movements of the thumb (moving the thumb toward the fingers).

The adductor pollicis muscle has two points of origin. One is along the shaft of the metacarpal for the middle finger (called the transverse head). The second point of origin (the oblique head) is at the capitate, the base of the metacarpals of the ring and middle fingers, the transverse carpal ligament, and the sheath of the flexor carpi radialis tendon. The two fascicles of this muscle insert into the base of the proximal phalanx of the thumb. This muscle functions to adduct the thumb.

Little Finger Movement

Muscles in the hypothenar compartment are the opponens digit minimi, abductor digiti minimi, and flexor digiti minimi brevis. This compartment has functionality similar to the thenar compartment, except that the muscles affect the little finger and not the thumb.

The opponens digit minimi is the deeper of the three muscles. It originates at the hamate carpal bone and the transverse carpal ligament. It inserts into the medial margin of the metacarpal for the little finger. When contracted it rotates the metacarpal to pull the little finger inward (palmar) and in opposition to the thumb.

The abductor digiti minimi is the most superficial of the hypothenar compartment muscles. It originates from the pisiform carpal bone, the pisohamate ligament, and the transverse carpal ligament. It functions to abduct the little finger.

The flexor digiti minimi brevis sits laterally to the abductor digiti minimi. Its originates from the hamate carpal bone and the transverse carpal ligament. It inserts into the proximal end of the first phalanx of the little finger. This muscle causes flexion of the MCP joint in the little finger. This is the action you undertake when you begin wrapping your little finger around a Jo.

Additional Finger Movements

There are four muscles in the lumbricals. These are deep muscles. The muscles are bipennate (except the muscle for the index finger, which is unipennate), originating between and from the dorsal side of two adjacent metacarpal bones. The muscles insert at the extensor hood between the proximal and middle phalanges of a finger. Two lumbrical muscles attach on either side of the middle finger. One lumbrical muscle inserts on the index finger and another on the ring finger. There are no lumbrical muscles attached to the ring finger or thumb. The muscles flex the MCP joint and extend the IP joint of their respective fingers.

The interossei are muscles that attached to the metacarpal bones of each finger. There are two muscles attached to each metacarpal. The dorsal interossei muscle of each finger originates along the lateral and medial surface of the associated metacarpal bone. It inserts into the extensor hood and the proximal phalanx of each finger. The dorsal interossei abduct finger at the MCP joint.

There are three unipennate palmar interossei muscles, each situated between two adjacent metacarpal bones. These muscles attached to the index, ring, and little finger of the hand. There is no palmar interossei muscle for the middle finger. Each palmar interossei muscle originates along the palmar lateral or medial length (depending on the finger) of the associated metacarpal. They insert into the side and base of the proximal phalanx and extensor hood of each associated finger. These muscles allow adduction of the associated finger.

Additional Hand Muscles

The palmaris brevis muscle is a small thin muscle on the palmar and ulnar side of the hand. It serves to wrinkle the skin on the palm of the hand to improve grip. It originates from the transverse carpal ligament and the thick  interconnecting tissues of the palmar aponeurosis. It emerges from the opposite side of the transverse carpal ligament from the flexor pollicis brevis. It inserts on the dermis of the skin along the medial margin of the hand. Pull your little finger toward your thumb and observe the palm of your hand. The wrinkling of your palm is largely the work of the palmaris brevis.

 

 

 

 

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