Hello everyone! It's Megan from Kenhub here, and welcome to our tutorial about the main muscles of the upper limb. The focus of today's tutorial will be on the function of these muscles starting with the muscles of the shoulder joint and working our way down towards the muscles of the hand. The upper limb is capable of a wide range of movement. In fact, the shoulder joint is the most mobile joint in the human body. Our upper limbs also allows to grasp and manipulate objects whereas our lower limbs are built for support and locomotion. The muscles of the upper limb can be classified according to innervation, origin, function or topography and as I just mentioned, we'll discuss these muscles in relation to their function as well as the origin.
So we'll begin this tutorial with the muscles of the shoulder joint where we'll focus on five muscles in particular. The name of the first muscle we'll talk about is derived from its location. It's located superior to the spine of the scapula and is therefore called the supraspinatus muscle. This muscle is located on the posterior aspect of the scapula so we're currently looking at a posterior view of the shoulder joint. The function of the supraspinatus muscle is to facilitate abduction of the shoulder joint. Abduction refers to the movement of a body part – in this case the upper limb – away from the midline of the body. As we go through this tutorial, I'll explain movement terms such as abduction but if you'd like more information on the types of movement found in the human body, we have a really useful article available on our website.
The next muscle we can see is the infraspinatus muscle which as you may have guessed is located inferior to the spine of the scapula. This muscle facilitates the external or outward rotation of the shoulder joint. Moving to the anterior aspect of the scapula, we can see this muscle highlighted in green which is the subscapularis muscle. The subscapularis muscle functions to facilitate internal or inward rotation of the shoulder joint. Another muscle of the shoulder joint is the teres minor muscle. This muscle's functions include external rotation and weak adduction of the shoulder joint. Adduction is the movement of a body part such as the upper limb towards the midline of the body.
The four muscles we've seen so far form a group known as the rotator cuff so let's quickly go through them again. The muscles of the rotator cuff are the supraspinatus muscle, the infraspinatus muscle, the subscapularis muscle and the teres minor muscle. The rotator cuff as the name suggests plays a major role in the internal and external rotation of the shoulder joint and upper limb, however, its main function is to stabilize the head of the humerus within the shoulder joint.
The largest muscle of the shoulder joint is the deltoid muscle and we can see it here from an anterior perspective. This muscle has several functions. The anterior portion facilitates flexion and internal rotation of the shoulder joint. Flexion at the shoulder joint results in the upper limb moving anteriorly and upwards. The middle part is responsible for abduction. The deltoid is actually the most important abductor of the shoulder joint. And finally, the posterior portion of this muscle is responsible for extension and external rotation of the shoulder joint. During extension at the shoulder joint, the upper limb moves posteriorly and downwards.
Now that we've covered the main muscles of the shoulder joint, let's move on to look at the muscles of the arm. I'd like to emphasize that anatomically, the arm is not a collective term for the upper limb but rather refers specifically to the region between the shoulder joint and the elbow joint. The arm can be further divided into the anterior compartment and the posterior compartment. We'll begin with the muscles of the anterior compartment using images that show the arm from an anterior perspective.
The muscle we can see here highlighted in green is the coracobrachialis muscle. This muscle facilitates the flexion and adduction of the arm at the shoulder joint. Another muscle of the anterior compartment is the biceps brachii muscle. Its name comes from the Latin "bi-" meaning two and "caput" meaning head. So as its name suggests, the biceps brachii has two heads. It has a long head which we can see here and a short head which we can see here. At the shoulder joint, the long head facilitates abduction and internal rotation whereas the short head facilitates adduction. At the elbow joint, this muscle supinates the forearm and also flexes the supinated arm. Supination is the medial rotation of the forearm resulting in the palm of the hand facing anteriorly or superiorly if the elbow is flexed whereas flexion at the elbow joint involves bending the upper limb and decreasing the angle between the arm and forearm.
The final muscle of the anterior comportment of the arm is the brachialis muscle. This muscle is actually the strongest flexor of the elbow joint even stronger than the biceps brachii muscle.
Moving to the posterior compartment of the arm, we see this three-headed muscle here which is the triceps brachii. As we saw with the biceps brachii which is a two headed muscle, the triceps brachii is given its name because it has three heads. Here we can see the long head and the lateral head and between them we can see the medial head. This muscle functions as an extensor of the elbow which is essentially the act of increasing the angle between the arm and forearm. The long head of the triceps brachii also assists in the extension of the shoulder joint. A muscle that can be considered as both the muscle of the arm and forearm is the anconeus muscle, however, we'll not discuss it further in this tutorial as this muscle is often morphologically and functionally classed as a continuation of the triceps.
So far we've covered the main muscles of the shoulder joint in the arm so we can now travel distally to look at the muscles of the forearm or the region between the elbow joint and the wrist. Again, the forearm can be divided into an anterior compartment and a posterior compartment. These muscles can then be further classified as superficial or deep muscles. Let us begin with the anterior compartment of the forearm starting with the superficial muscles.
The muscles of the anterior compartment are flexors and the one we can see highlighted in green here is the pronator teres. This muscle is a weak flexor of the elbow joint and a strong pronator of the forearm. Pronation is the lateral rotation of the forearm resulting in the palms facing posteriorly or downwards if the elbow is flexed. The second of the superficial flexors of the forearm we'll look at is the flexor carpi radialis muscle. This muscle is a flexor and abductor of the wrist joint. So if you remember I said that flexion decreases the angle between two body parts and as we can see in this image, flexion of the wrist joint reduces the angle between the forearm and the hand.
Also found in the anterior compartment of the forearm, we have the flexor carpi ulnaris which we can see here highlighted in green. As the name suggests, it facilitates the flexion as well as adduction of the wrist joint. The final muscle of the superficial flexors of the forearm we'll look at is the flexor digitorum superficialis. The flexor digitorum superficialis facilitates flexion at the proximal interphalangeal joints of the hand which are these joints here. Note that although the palmaris longus muscle is also one of the superficial flexors of the forearm, it's absent in about fifteen percent of the population, therefore, will not classify as one of the main muscles of the upper limb.
Now that we've seen some of the superficial flexors of the forearm, let's move on to look at the deep muscles of the anterior forearm starting with this muscle here, the flexor digitorum profundus. The flexor digitorum profundus facilitates flexion at the distal interphalangeal, proximal interphalangeal and wrist joints. The deepest muscle of the anterior forearm and the one we can see here highlighted in green is the pronator quadratus muscle. This muscle functions to pronate the forearm. So flexor digitorum profundus flexes phalanges two to five but what muscle flexes the thumb? It's the flexor pollicis longus that's responsible for flexion of the thumb at the metacarpophalangeal and distal interphalangeal joints as well as opposition at the saddle joint. Opposition is essentially the act of pinching your fingers together and involves touching the pad of any one of your fingers with the thumb of the same hand.
Now let's turn to the posterior aspect of the forearm again starting with the superficial muscles. The muscles of the posterior compartment are extensors and the one we can see here highlighted in green is the extensor digitorum. This muscle facilitates extension of the metacarpophalangeal joints of digits two to five as well as extension of the wrist joint. Another superficial extensor of the forearm is the extensor carpi ulnaris muscle. As you can see, this muscle has its insertion at the base of the fifth metacarpal, therefore, it only acts to extend and adduct the wrist joint and doesn't facilitate movement of the fingers.
Moving laterally, we can see the extensor carpi radialis longus muscle. This muscle facilitates the extension and abduction of the wrist joint. Also located on the lateral aspect of the posterior forearm is the extensor carpi radialis brevis muscle seen here. This muscle like the one we just discussed facilitates the extension and abduction of the wrist joint. Please note that brachioradialis and extensor digiti minimi are also superficial muscles of the posterior forearm, however, here we're not considering them as main muscles of the upper limb.
Now let's look at the deep extensors of the forearm. The first muscle we see here is the supinator muscle. As the name suggests, this muscle facilitates supination of the forearm. Another deep extensor of the forearm is the extensor pollicis longus muscle. The extensor pollicis longus muscle facilitates the extension of the thumb at the carpometacarpal and interphalangeal joints. It also partially abducts the first metacarpal. The last of the deep extensors of the forearm we'll look at is the extensor pollicis brevis muscle. It functions to extend the thumb at the metacarpophalangeal and carpometacarpal joints. For the purposes of this tutorial, we'll not be discussing the abductor pollicis longus or the extensor indices which are also deep extensors of the posterior compartment of the forearm.
Finally, let's talk about the muscles of the hand. These muscles can be divided into four categories – the thenar muscles, the hypothenar muscles, the interossei muscles and the lumbrical muscles. First we'll focus on the thenar muscles which are located on the radial side of the palm of the hand. There are four of these muscles, one of which is the flexor pollicis brevis muscle and we can see it here highlighted in green. This muscle has both a deep and superficial part. The collective function of the flexor pollicis brevis is the flexion and opposition of the carpometacarpal joint of the thumb.
Next we'll take a look at the adductor pollicis muscle which we can see here highlighted in green. This muscle has two heads – an oblique head and a transverse head. Adductor pollicis facilitates the adduction and opposition of the thumb. Another thenar muscle of the hand is opponens pollicis. This muscle as the name suggests functions to oppose the thumb. The last muscle considered to be a thenar muscle is the abductor pollicis brevis. Again, the function of this muscle is pretty self-explanatory. It acts to abduct the thumb.
The next group of muscles we'll talk about is the hypothenar muscles of the hand starting with the abductor digiti minimi muscle. This muscle facilitates the abduction, flexion and extension of the little finger. The muscle we can now see highlighted in green is also hypothenar muscle. It's called flexor digiti minimi. This muscle is also sometimes referred to as flexor digiti minimi brevis. Flexor digiti minimi facilitates the flexion of the little finger at the metacarpophalangeal joint. In many cases, this muscle is very small or may even be completely absent. Another muscle found in the hypothenar group is the palmaris brevis. This muscle functions to tense the skin of the palm on the ulnar side of the hand during a grip action and it also deepens the hollow of the palm. One of the deepest and strongest type of thenar muscles is the one we can see here, the opponens digiti minimi. This muscle facilitates flexion, adduction and lateral rotation of the little finger at the carpometacarpal joint.
Now let's talk about the interossei muscles. The interossei muscles of the hand are found between the metacarpal bones in both the palmar and dorsal sides of the hand. Here, we can see the palmar interossei muscles. They are comprised of three muscles which arise from the metacarpals of the index, ring and little finger. All three muscles facilitate the adduction of the fingers at the metacarpophalangeal joint. I'd like to note here that when we talk about adduction and abduction of the fingers, the fingers are moving towards or away from the midline of the hand rather than the midline of the body.
On the dorsal aspect of the hand, we find the dorsal interossei muscles. They're comprised of four muscles and are the most superficial of the interosseus muscles. All the dorsal interosseus muscles facilitate the abduction or spreading out of the fingers at the metacarpophalangeal joint.
You'll be relieved to hear that we've reached the last muscle group of this tutorial, the lumbrical muscles of the hand. These four muscles are crucial to finger movement in that they link the extensor tendons to the flexor tendons. The lumbrical muscles facilitate flexion at the metacarpophalangeal joints and extension at the interphalangeal joints of each finger.
Before we conclude our tutorial, let's quickly summarize what we covered today. We started with the muscles of the shoulder joint then we looked at the muscles of the arm which can be divided into anterior and posterior compartments followed by the muscles of the forearm which can also be divided into anterior and posterior compartments and further classified as superficial or deep. Finally, we looked at the muscles of the hand which can be divided into four groups – the thenar, hypothenar, interossei and lumbrical muscles. Unfortunately, there are lots of muscles to remember, however, their locations and names can rather be useful in deducing their functions. For example, supinator acts to supinate the forearm, muscles on the anterior aspect of the upper limb are generally flexors and muscles on the posterior aspect are generally extensors. We also have lots of quizzes on our website that will help you remember these muscles and their functions.
So that brings us to the end of our tutorial on the main muscles of the upper limb. I hope you enjoyed it and thank you for listening.
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