Hello everyone! This is Joao from Kenhub, and welcome to another tutorial where I’m going to use to talk about and dedicate an entire tutorial to one of the most powerful and important muscles on your shoulder, and this is known as the deltoid muscle, highlighted here in green. And keep in mind, this is an anterior view, but you can also see this muscle on a posterior view and also here, highlighted in green.
Now, let’s start talking about the anterior view because, here, I want to tell you that the deltoid muscle is divided into three main parts, and each part has an origin, so a different origin. The first one is known as the clavicular part or anterior part and, as you can see here, is originating… so it’s originating from the lateral third of the clavicle.
Now, the second part can be seen both on the anterior and posterior part, and you can see here, highlighted in yellow, but I’m going to talk about it on the posterior view that is more evident, also highlighted in yellow. And this is known as the acromial or middle part of the deltoid muscle. And as you can see here, and as the name indicates, this part is originating from the acromion of the scapula.
Now, the third and last part of the deltoid muscle is known as the spinal or posterior part and, as you can see here, is originating from the inferior border of the spine of the scapula.
Now, all three parts come together, and then as you can see here on this image, they’re coming together and inserting on this area right here, and this is on the shaft of the humerus on a structure known as the deltoid tuberosity. So this is where the deltoid muscle is going to insert.
In terms of innervation, this muscle is supplied by the axillary nerve, specifically the fiber C5 and C6. And the axillary nerve is shown here, highlighted in green, and it’s important to add that this nerve is a branch of the brachial plexus, as you can see here also. I’m showing you the image of the brachial plexus where you can find the axillary nerve.
Now, in terms of vessels, blood vessels, it’s important to mention that, in its course, the muscle lies in close relation with this vein, very important vein in your arm, I’m showing you right now—and this is known as the cephalic vein which runs in the deltopectoral groove as you can see here, which is a groove that lies between the deltoid muscle and this really powerful muscle known as the pectoralis major.
Then, this cephalic vein becomes the axillary vein as you can see here, shown and highlighted also in green.
Now, it’s time for us to talk about the different movements associated to the deltoid muscle, and as you probably figured out, this muscle is going to move and also stabilize the shoulder joint. And the movements are defined by the three parts, the three parts of the deltoid muscle that we described. And these parts can either interact with one another—what is known as synergistically, meaning that they will help one another produce a certain type of movement, or they can also work against each other, and that is when we say that they work antagonistically. And this all depends on the different parts of the deltoid muscle working against each or with one another and also the position of the humerus.
Now, as you know, the deltoid muscle is the most important abductor of the shoulder joint. Now, knowing that, we’re going to look at the three main parts of the deltoid so we can describe the movements that they are associated with.
Now, the first one is the clavicular part, and this part has or produces anteversion, which is the movement of the arm and the shoulder forward. It also has internal rotation and adduction.
Now, the other part is, then, this one in yellow, and this is the acromial part. The main movement associated to this part is abduction—so when you lift your arm in a 90O movement or angle.
Now, the other part can be seen on the posterior view, and this is the spinal part. And the movements associated to this part are retroversion, which is when you move the arm and the shoulder backward. You also can associate external rotation to this part. The last movement is adduction.
It’s important to add that between a 60O and 90O of abduction, the clavicular and spinal parts of the deltoid will then help the acromial part of the muscle with abduction. This is what we call the synergistic movement, parts helping one another produce a certain type of movement.
