Muscles of the arm – Origin, Insertion & Innervation – Human Anatomy | Kenhub


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.