This is a tutorial on the muscles of the upper arm. The muscles of the upper arm are split into anterior and posterior compartments. The anterior compartment is the flexor compartment because these muscles flex the forearm. The posterior compartment of the upper arm is the extensor compartment because it extends the forearm.
It’s not shown on this model, but the anterior compartment is separated from the posterior compartment by intermuscular septa and also, the humerus. I’ll just flick over to a cross-section to demonstrate it.
We’re just looking at a cross section of the upper arm here. This is anterior up here, posterior down here, medial and lateral. You’ve got the anterior muscles here and the posterior muscles here.
In between the anterior and posterior compartments, you’ve got this septum. You’ve got a lateral septum and a medial intermuscular septum. This separates the anterior and posterior compartments.
This intermuscular septum is actually continuous with the deep fascia of the arm. The deep fascia surrounds all the muscles of the arm and is continuous with this intermuscular septum where it attaches to the humerus, which also separates the flexor and extensor compartments.
It’s important to know about these compartments, these fascial compartments because compartment syndrome is a condition where the pressure of the intrafascial compartment rises and puts pressure on important structures like nerves and blood vessels. It will often need to be treated with an emergency fasciotomy to release the rising pressure in the compartments and prevent any neurovascular damage.
You’ve got three muscles in the anterior compartment. You’ve got the biceps brachii, the coracobrachialis and the brachialis muscle. The muscles of the anterior compartment are innervated by the musculocutaneous nerve.
I’ll just take a look at these muscles. I’ll just get rid of the deltoid muscle and we can have a look at this muscle here, which is the biceps brachii.
This muscle is the most superficial of the anterior compartment. It’s called the ‘biceps’ because it has two heads, a long and a short head. I’m just going to get rid of the pectoralis major so we can take a look at this muscle.
The long head of the muscle runs up through the intertubercular groove on the humerus and inserts on the supraglenoid tubercle of the scapula, so that little protuberance just superior to the glenoid cavity. This long tendon runs up through the intertubercular groove over the head of the humerus and inserts onto the supraglenoid tubercle. The short head of the biceps brachii inserts onto the coracoid process of the scapula, so that little hook on the scapula.
These points of attachments are the origins, not the insertions. The long head originates on the supraglenoid tubercle and the short head originates on the coracoid process.
The biceps then inserts onto the radial tuberosity, so the tuberosity of the radial bone. This muscle acts to flex the forearm and it also can supinate the forearm.
Another point to mention about the biceps is that as the tendon comes to insert in the radial tuberosity, you’ve got a flat tendon sheath, an aponeurosis, which comes off medially as it enters the anterior compartment of the forearm. It’s not shown here, but you’ve got this bicipital aponeurosis. That’s the name that is given to this flat tendon, this flat connective tissue, which fans out on the medial side of the tendon. It blends with the deep fascia of the anterior compartment of the forearm. That’s something to be aware of, this bicipital aponeurosis, which comes off the medial side of the biceps tendon.
The next muscle is the coracobrachialis. As the name suggests, the coracobrachialis originates on the coracoid process and it inserts underneath the biceps on the mid-shaft of the humerus. It lies deep to the biceps muscle originating on the tip of the coracoid process and inserting on the mid-shaft of the humerus on the medial side. It inserts here. What this muscle does is it flexes the arm at the glenohumeral joint.
We’ve just got a diagram of it here. This is the coracoid process. You can see it running just underneath the biceps and it inserts onto the humerus medially on the shaft. That’s the coracobrachialis muscle.
At the distal end of the humerus, you’ve got the brachialis muscle, which also lies deep to the biceps brachii. This muscle, it’s actually shown incorrectly on this model, but it inserts on the ulnar tuberosity, so on this side of the arm. It lies deep to the biceps.
I’ll just flick over again to the old. You can see it lying beneath the biceps brachii. It lies anteriorly on the humerus and it inserts on the ulnar tuberosity.
Those are the three muscles of the anterior compartment – the biceps brachii, the coracobrachialis and the brachialis.
The brachialis muscle again is innervated by the musculocutaneous nerve and it’s a flexor of the forearm at the elbow joint, so the same action as the biceps brachii at the forearm.
Finally, the posterior compartment of the arm only has one muscle. You’ve got the triceps brachii. The tri- referring to the three heads of this muscle. You’ve got the long head and the medial and the lateral heads.
I’ll just dissect away the latissimus dorsi and the rotator cuff muscles and we can see the origin of this muscle on the infraglenoid tubercle of the scapula. This head here, which you can see is the lateral head. This lies superficial. And underneath it, you’ve got the medial head, which originates a little bit below.
The triceps muscle, all the heads of the muscles, the long head and the medial and the lateral head converge to form this tendon which inserts onto the olecranon, which is a process on the ulnar bone. What this muscle does is it extends the elbow. It brings it backward like this.
The muscles of the posterior compartment are innervated by the radial nerve, which comes off the brachial plexus and winds around to innervate the posterior compartment.
You can see the origin point of this long head means that this part of the triceps can actually adduct the arm slightly at the shoulder joint. That’s another thing worth bearing in mind. Those are the muscles of the upper arm.