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Muscles of the shoulder

Attachments, innervation and functions of the muscles of the shoulder joint.

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Have you ever been to the circus? If so, you might be like me and remember hiding from scary clowns? Or you might remember the fire breathers, acrobats and jugglers. You might also remember the mix of curiosity and horror you felt when you first saw a contortionist perform. Did you ever wonder how they manage to be so bendy? Well, the shoulder joint is one of the most flexible joints in the human body and it helps contortionists twist into their unusual positions. However, increased flexibility means decreased joint stability and an increase in the risk of dislocation.

We have various ways of reducing the risk of dislocation of the shoulder joint, one of which is through our rotator cuff muscles. Never heard of them? If not, then don’t worry as we’re going to talk about them in today’s tutorial on the muscles of the shoulder joint.

Before we begin, let me give you a quick overview about what we’re going to cover in today’s tutorial. First, we’re going to look at the bones that make up the shoulder joint as they form the bony framework that our muscles attach to. Then, we’ll look at the muscles of the shoulder joint discussing their origin, insertion, function, and innervation. Finally, we’ll conclude our tutorial with some clinical notes.

So let’s get started and talk about the bones of the shoulder joint.

The shoulder joint is a synovial ball-and-socket joint and we can see it here from an anterior view highlighted in green. The ball component of this joint is formed by the head of the humerus which is a feature of the long bone in our arm, the humerus, whereas the socket component is formed by the glenoid cavity, which is part of the scapula or shoulder blade. As such, the shoulder joint is also known as the glenohumeral joint.

After that short but sweet introduction to the bones of the shoulder joint, let’s move on to have a look at the muscles of the shoulder joint.

A popular circus act that some of you might have seen is the world’s strongest man lifting insanely heavy weights or even people. One of his muscles which would have been particularly noticeable are these ones you see here – the deltoid muscles. The deltoid is the most superficial muscle of the shoulder joint, therefore, it’s the muscle that defines the round contour of our shoulders.

Let’s move on to some Kenhub illustrations. If we dissect the skin and connective tissue away from the shoulder and arm region, we can see the deltoid and associated muscles more clearly. Here we’re looking at the deltoid from an anterior perspective. Before we go on to discuss the attachment points of this muscle, it’s important to know that the deltoid has three parts which are named based on their origin.

Anteriorly, we have the clavicular part which originates from the lateral third of the clavicle, and if we change views so that we see the deltoid from a posterior perspective, we can see the acromial part which originates from the acromion of the scapula. Finally, we have the spinal part which again, true to its name, originates from the spine of the scapula. These three parts converge towards their insertion point which is the deltoid tuberosity found on the lateral surface of the shaft of the humerus.

So what does the deltoid muscle do? Well, it’s actually responsible for several movements of the shoulder joint. Each part of the deltoid performs a different function. The clavicular part flexes the arm at the shoulder joint, the acromial part is the main abductor of the arm, and the spinal part extends the arm at the shoulder joint. In order for the deltoid to perform its functions, it requires innervation. It’s innervated by the nerve you can see here highlighted in green which is the axillary nerve.

Let’s move on to have a look at the muscles I mentioned at the beginning of our tutorial, the rotator cuff muscles. Before we’ll look at each muscle individually, I’ll give you some information about these muscles as a whole. So here we have a posterior view of the shoulder joint and an anterior view of the shoulder joint which together show us the four rotator cuff muscles. Posteriorly, we have the supraspinatus, the infraspinatus, and the teres minor, and anteriorly, we have the subscapularis muscle.

Generally, the muscles of the rotator cuff originate from the scapula and insert at the humerus here. As for their functions, these muscles work together to reduce the risk of dislocation by stabilizing the shoulder joint. Essentially, they help keep the head of the humerus in its correct position within the glenoid cavity. Also, as the name suggests, the rotator cuff muscles play a major role in medial and lateral rotation of the arm at the shoulder joint.

So, the first muscle of the rotator cuff we’ll talk about is the supraspinatus. As the name suggests, this muscle originates from the supraspinous fossa of the scapula which is a shallow depression in the body of the scapula above its spine. It then inserts here at the greater tubercle of the humerus.

In terms of function, this muscle initiates abduction of the arm at the shoulder joint, then the deltoid takes over. As a rotator cuff muscle, it also stabilizes the shoulder joint. The supraspinatus muscle is innervated by the suprascapular nerve which arises from the superior trunk of the brachial plexus.

Inferior to the spine of the scapula, we find the infraspinatus muscle. This muscle originates from the infraspinous fossa of the scapula and like the supraspinatus, it inserts here at the greater tubercle of the humerus. Contraction of this muscle causes lateral rotation of the arm at the shoulder joint and it also stabilizes the shoulder joint. This muscle is innervated by the suprascapular nerve which we can see here highlighted in green.

The next muscle of the rotator cuff we’re going to talk about is the teres minor muscle. This muscle originates from the infraspinous fossa and the lateral border of the scapula and inserts at the greater tubercle of the humerus. This muscle is responsible for lateral rotation of the arm at the shoulder joint and it also stabilizes the shoulder joint. As for innervation, teres minor is innervated by the axillary nerve.

To see the last muscle of the rotator cuff, we need to change to an anterior view of the shoulder joint. Here we see the subscapularis muscle which originates from the subscapular fossa. Unlike its buddies, this muscle inserts at the lesser tubercle of the humerus as you can see here. Contraction of the subscapularis results in medial rotation of the arm at the shoulder joint and it also stabilizes the shoulder joint. When it comes to the innervation of the subscapularis muscle, this muscle is innervated by the upper and lower subscapular nerves, which are branches of the posterior cord of the brachial plexus.

And there you have it, the muscles of the rotator cuff. In this image, we can see the scapula from a lateral perspective which allows us to see all four of these muscles. Posteriorly, we have the supraspinous, the infraspinatus and the teres minor, and anteriorly, we have the subscapularis muscle. A useful tool to help you remember the four members of this group is the phrase “The head of the humerus SITS in the glenoid cavity”, with SITS being a handy little mnemonic for you to use here.

Earlier, we talked about the teres minor muscle and if you have a teres minor, then you must have a teres major. Here we see this muscle from a posterior perspective and we can see that it originates from the inferior angle of the scapula. It then goes on to insert at the crest of the lesser tubercle of the humerus. This muscle is responsible for medial rotation of the arm at the shoulder joint and extension of the arm at the shoulder joint.

Now we’re going to talk about the innervation of the teres major muscle. It’s innervated by the subscapular nerve which we can see here highlighted in green.

Before we move on, I’d like to mention three anatomical spaces that are defined by some of the muscles we’ve already talked about. First, we have the triangular space which I’ve outlined for you here. The superior border of the space is formed by the inferior border of teres minor, the inferior border is defined by the superior border of teres major, and the lateral border is formed by the medial border of the long head of the triceps brachii muscle. The triangular space transmits the circumflex scapular vessels.

Next, we have the quadrangular space. Again, I’ve outlined the space for you so that you can see it nice and clearly. The superior border of the space is formed by the inferior border of teres minor and the inferior border is defined by the superior border of teres major. The lateral border is formed by the surgical neck of the humerus and the medial border is defined by the lateral border of the long head of the triceps brachii muscle. The quadrangular space transmits the axillary nerve and the posterior circumflex humeral artery.

Finally, we have the triangular interval. As we can see in this illustration, the superior border of this space is formed by the inferior border of the teres major, the lateral border is formed by the surgical neck of the humerus, and the medial border is defined by the lateral border of the long head of the triceps brachii muscle. The triangular interval transmits the profunda brachii artery and the radial nerve.

Okay, now, we’re going to talk about a few muscles that aren’t found in the shoulder region, but act on the shoulder joint. The first one we’ll talk about is the pectoralis major which is a muscle of the pectoral region. Like the deltoid, it’s made up of three parts which are named based on their origin. First, we have the clavicular part which originates from the medial half of the clavicle, next is the sternocostal part which originates from the sternum and the second to sixth costal cartilages, and finally we have the abdominal part which originates from the anterior layer of the rectus sheath. These three parts come together to insert at the crest of the greater tubercle of the humerus.

So what does the pectoralis major muscle do? Well, it’s actually responsible for several movements. Contraction of this muscle causes flexion, adduction and medial rotation of the arm at the shoulder joint. The innervation of pectoralis major is supplied by two nerves – the medial pectoral nerve and the lateral pectoral nerve – which are direct branches of the brachial plexus.

Next, we’re going to look at some muscles of the arm starting with the coracobrachialis muscle. The coracobrachialis is a muscle of the anterior compartment of the arm and it originates from the coracoid process of the scapula. From here, it extends distally along the humeral shaft to insert along the anteromedial surface of the humerus. This muscle is responsible for flexion of the arm at the shoulder joint, and as for innervation, coracobrachialis is innervated by the musculocutaneous nerve, which we can see here highlighted in green.

Another muscle of the anterior compartment of the arm we’re going to talk about is the biceps brachii. As the name suggests, this muscle has two heads, both with different points of origin. The long head originates from the supraglenoid tubercle of the scapula and the short head originates from the coracoid process of the scapula. Both heads unite to form one large muscle belly which inserts at the radial tuberosity.

So what movement does the biceps brachii perform? This muscle assists with flexion of the arm at the shoulder joint, and like coracobrachialis, the biceps brachii is innervated by the musculocutaneous nerve.

The last muscle we’re going to talk about today is the latissimus dorsi muscle which we can see here from a posterior perspective. The latissimus dorsi is a muscle of the back and it originates from the spinous processes of T7 to T12, the iliac crest, the thoracolumbar fascia, and the ninth to twelfth ribs. The fibers of this muscle then converge to insert at the intertubercular sulcus of the humerus.

As for the functions of this muscle, the latissimus dorsi is responsible for medial rotation, adduction and extension of the arm at the shoulder joint. By performing these three movements, the latissimus dorsi earns its title as one of the swimming muscles of the human body. This muscle is innervated by the thoracodorsal nerve, which is a branch of the brachial plexus.

Now that we’re familiar with the muscles of the shoulder joint, let’s get clinical.

Today’s clinical notes are going to feature the rotator cuff muscles which stabilize the shoulder joint by keeping the head of the humerus within the glenoid cavity. The most common injuries of the rotator cuff muscles include strains, tendinitis, and bursitis.

Strains ore tears of the rotator cuff are caused by overuse or acute trauma such as a sports injury or car accident. These types of injury result in an immediate and intense pain and may require surgical intervention. Tendinitis refers to inflammation of the tendons of the rotator cuff muscles and is most commonly caused by overuse. This type of injury is common in individuals who frequently reach upwards such as painters or tennis players.

Lastly, we have bursitis which causes inflammation of the bursae. Specifically, it causes inflammation of the subacromial and subdeltoid bursae which we can see here highlighted in green. Treatment of individuals with rotator cuff injuries usually takes a conservative approach. This includes application of heat or ice packs to reduce swelling, physiotherapy to improve the strength and range of motion, administration of cortisone and other antiinflammatory medications such as ibuprofen, as well as rest. If symptoms don’t improve, then surgery may be required.

Before we bring our tutorial to a close, let’s quickly summarize what we’ve learned today.

We started our tutorial with the bones of the shoulder joint which are the humerus and the scapula. We then talked about the muscles of the shoulder joint discussing their origin, insertion, function, and innervation. We also looked at some muscles that are not found in the shoulder region, but act on the shoulder joint. Finally, we finished up our tutorial with some clinical notes about injuries of the rotator cuff muscles.

So that brings us to the end of our tutorial on the muscles of the shoulder joint. I hope you enjoyed it. Thanks for watching and see you next time.

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