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Origins, insertions, innervation and functions of the muscles of the arm.
Bodybuilding involves bulking to maximize muscle gains, cutting to minimize nonessential fats and, of course, weightlifting to grow those muscles. Competitors go to extreme lengths to make their muscles pop on show day from insane amounts of fake tan and gold shimmer dust to intentional dehydration. You might actually recognize this guy, Arnold Schwarzenegger, who gave bodybuilding the spotlight in the seventies.
Despite its popularity, bodybuilding causes controversy both relating to steroid use and whether or not it is actually a sport. We often associate bodybuilding and strength with big arms, probably because when someone wants to show off their muscles, they usually flex their arms. One muscle that we can see very clearly when the arm is flexed is the biceps brachii which we’ll discuss in today’s tutorial about the muscles of the arm.
Before we begin though, I want to clear up an issue that often arises when transitioning from everyday terms to anatomical terms.
So as a kid, your arm is your arm and your leg is your leg, but in anatomy instead, your arm or upper limb is divided into the arm and forearm and your leg or lower limb is divided into the thigh and leg. So when I say we’re going to talk about the muscles of the arm, I mean this region here, between the shoulder and the elbow joints.
Okay, now that we’ve got the terminology down, I’d like to give you a quick overview of what we’re going to be talking about in this tutorial. First, we’re going to be looking 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 arm which can be divided into two compartments – the anterior compartment and the posterior compartment.
As we talk about each muscle, we’ll learn about their origin, insertion, function, and innervation. And, 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 perspective 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 or fossa which is part of the scapulae 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 now to have a look at the muscles of the arm starting with the anterior compartment.
Now the anterior compartment of the arm consists of three muscles, and the first one we’ll talk about is the biceps brachii. As the name suggests, this muscle has two heads both with different points of origin, and 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 a single large muscle belly which inserts at the radial tuberosity. It also has a soft tissue attachment to the deep fascia of the forearm through a wide expansion of the biceps brachii tendon known as the bicipital aponeurosis.
So what does the biceps brachii do? Well, this muscle has several functions. It’s a powerful flexor of the forearm at the elbow joint which involves decreasing the angle between the arm and the forearm at the elbow joint. It’s important to note, however, that when the forearm is in the pronated position with the palms facing down, the biceps brachii loses its mechanical advantage. This means its involvement in flexion of the forearm at the elbow joint is more limited than the supinated forearm.
This muscle also assists in flexion of the arm at the shoulder joint and supination of the forearm. Supination is a movement that involves the rotation of the hand and forearm so that the palm faces upwards or forwards as demonstrated by our image here.
Before we move on, let me mention some minor functions of the biceps brachii. So the long head is said to assist in abduction of the arm at the shoulder joint which involves the movement of the arm away from the midline of the body whereas the short head is said to facilitate in adduction of the arm at the shoulder joint which involves the movement of the arm towards the midline of the body. Lastly, this muscle is also said to assist in internal rotation of the arm at the shoulder joint, and as you can see, this simply refers to the rotation of the arm towards the center of the body.
In order for the biceps brachii muscle to perform its functions, it requires innervation, and it’s innervated by the musculocutaneous nerve which is a branch of the brachial plexus.
So if we remove the biceps brachii, we can see this muscle highlighted in green which is the coracobrachialis muscle. And the coracobrachialis originates from the coracoid process of the scapula and extends distally to insert along the anteromedial surface of the humerus. This muscle assists in flexion and adduction of the arm at the shoulder joint and this muscle is also said to assist in internal rotation of the arm at the shoulder joint.
Like the biceps brachii, the coracobrachialis is innervated by the musculocutaneous nerve which we can see here, and the third and the last muscle of the anterior compartment of the arm is the brachialis. And even though the brachialis is located deep within the arm, its large belly makes the biceps brachii look much larger on the surface that it actually is. So for that reason, we have to say that behind every great biceps, there’s a great brachialis.
This muscle originates from the anterior aspect of the humerus and inserts onto the ulnar tuberosity. When the brachialis contracts, it causes flexion of the forearm at the elbow joint – in particular, when the forearm is in a pronated position. In terms of innervation, the brachialis is innervated by the musculocutaneous nerve and the lateral part of this muscle also receives some innervation from the radial nerve.
Okay, now that we’ve had a look at the anterior compartment of the arm, let’s flip it over so we can see the posterior compartment.
The posterior compartment of the arm consists of two muscles, and the first one we’re going to talk about is the triceps brachii. And as its name suggests, the triceps brachii has three heads – the long head seen here originates from the infraglenoid tubercle of the scapula, the medial head which is the smallest of the three arises from the posterior surface of the humerus inferior to the radial groove, and lastly the lateral head originates from the posterior surface of the humerus superior to the radial groove.
The three heads of the triceps brachii fuse into a common tendon which crosses the elbow joint before inserting onto the olecranon of the ulna. This muscle is responsible for extension of the forearm at the elbow joint and the long head also performs extension and adduction of the arm at the shoulder joint.
So when it comes to innervation of the triceps brachii, this muscle is innervated by the radial nerve, and although the triceps brachii is the main muscle of the posterior arm, there is another muscle found in this region which is often overlooked, and this muscle is the anconeus muscle, which you can see now highlighted in green.
The anconeus is a small triangular-shaped muscle found around the elbow region and it lies superficially and can be easily palpated at the posterolateral side of the forearm near your elbow. And this muscle originates from the lateral epicondyle of the humerus and inserts at the olecranon of the ulna. When the anconeus contracts, it assists in extension of the forearm at the elbow joint, and extension of the forearm at the elbow joint involves increasing the angle between the arm and the forearm at the elbow joint.
The innervation of the anconeus is supplied by the radial nerve. It’s worth noting that the triceps brachii and the anconeus are not only innervated by the same nerve, but they’re also often found either partially or completely blended together. Therefore, they sometimes may appear as one muscle in dissection.
Okay, so now that we’re familiar with the muscles of the arm, let’s get clinical.
So, linking back to our introduction, today’s clinical notes will discuss the issue of anabolic steroids. So, during your medical training, you might have come across treatment plans involving steroids. Corticosteroids are often prescribed by doctors to reduce inflammation, but they are not anabolic steroids. Anabolic steroids are synthetic hormones that resemble the male sex hormone, testosterone.
And like corticosteroids, they have some medicinal purposes, however, they can also be used illegally by bodybuilders and other athletes to help promote muscle growth and enhance performance, taking you from this to this. Sound appealing? Well, think again, because anabolic steroids have many serious side effects. Let me list some of them for you now: Severe acne, hair loss, liver disease, kidney disease, heart disease, altered mood, breast development or gynecomastia in males, facial hair in females, infertility, and even death.
So, although steroid users may be thrilled with their outer appearance, they may also be causing serious damage to their insides. Therefore, if you want to become swell, be smart and stick with the diet and exercise regime. Leave the steroids for the doctors to prescribe.
Okay, so before we begin our tutorial to a close, let’s quickly summarize what we’ve learned today.
So, we started off by talking about the bones of the shoulder joint which are the humerus and the scapula. Next, we moved on to talk about the muscles of the arm which we divided into two compartments, the anterior compartment and the posterior compartment, and finally, we concluded our tutorial with some clinical notes about anabolic steroids.
So that brings us to the end of our tutorial on the muscles of the arm. I hope you enjoyed it. Thanks for watching and see you next time.