Webtoon Advanced Structural Understanding of the Arms
GCC Academy
Hello, I am Webtoon artist Uksoo
Let’s talk about the arm this time
There’s more to discuss about the arm than you might think, so we’ll divide it into two parts
This time, we’ll focus on the anatomical explanation of the arm,
and in the next part, we’ll look at
how the contours and internal details form when the arm actually moves
Also, we’ll take a closer look at the hands,
which could be considered the second face
The arm plays an important role in expressing various emotions in the upper body
For example,
when they stretch their arm high into the air,
it gives a feeling of freedom and hope
On the other hand, when the arms are in pockets or crossed,
it can express a defensive or passive psychological state
Additionally, it can give us a sense of who the character is just by their posture
Take this pose, for example
You’d know immediately what kind of character this is, without them saying a word
With broad pectorals, a robust deltoid, and clear biceps,
And the biceps brachii,
along with the clearly defined muscles of the upper arm,
makes you think,
I shouldn't mess with him
The position and movement of the arms are important elements in revealing a character’s emotions and intentions
The Anatomical Structure of the Arms
Now, let’s start by learning
about the bones of the arm
First, here is the sternum
Now, we need to attach the arm bones to this
How should we attach them?
If evolution
were human, it might have been a tough decision
This is because, unlike quadrupeds,
the human arm has a very wide range of motion
and it needs to be strong
The arm doesn’t just bend and straighten
It also moves side to side, front to back, up and down
Furthermore, the shoulder joint, where the arm attaches,
also has to be able to move freely
Nothing can be fixed in place
It has to be firmly secured while also allowing movement
This created a tough problem for evolution
So, after much consideration, evolution created the scapula and the clavicle
Here, the scapula is attached here
And here’s the other scapula
The humerus attaches here,
and then this scapula is connected to the clavicle
It connects to the front here
The back part is hidden and not visible
And the clavicle connects to the sternum as we discussed last time
This forms a delicate mechanism
where the arm, scapula, clavicle,
and the entire sternum are all connected
It’s really amazing how evolution worked this out
The way it all fits together is remarkable
It's so beautiful that it almost makes me believe
there’s some higher being involved in creation
Anyway, now we’ve started talking about the arm bones,
let’s proceed to a detailed explanation
The arm bones can be divided into
the arm attachment bones and the free arm bones
The arm attachment bones,
which connect the arm to the upper body,
are composed of the scapula and clavicle
The clavicle
and the scapula
The scapula
The clavicle
On the other hand, the free arm bones are made up of several bones for the functional movement of the arm
First, let’s talk about the scapula,
also known as the shoulder blade
The scapula serves as the starting point of the arm and plays a crucial role in supporting the arm
The scapula has several functional areas
Here, we have the slanting part of the scapula
his is called the scapular spine,
The scapular ridge in Chinese characters
The scapular spine is the point where many muscles of the back and arm attach
It also serves as a prominent landmark that reveals clear lines
When you follow this ridge upwards, it connects to the front part
This is where the scapular ridge rises from the back
Now, let’s talk about the acromion,
which is this point right here
The acromion, as I explained earlier,
connects with the clavicle and fixes itself to the sternum
It’s an important point when drawing face shots as well
Next, we have the shallow dish, which is the area here
This is where the arm bone directly connects to the shoulder
Lastly, there’s the coracoid process, this small protrusion here
It contributes greatly to the movement and stabilization of the arm
We will revisit this point when we discuss the humerus later on
It’s a bit complicated, right?
I remember when I first heard a lecture on the shoulder blade,
I had no idea what they were talking about
So, let me simplify it for you
If we simplify the shape of the scapula, it’s basically triangular
It could look like this
And there’s a protruding ridge coming out like this
This is the basic shape of the scapula
But this is too simple, so let’s go a bit further
Looking at this material,
you can see how the scapula emerges from the body
You can see where the scapular spine is located
As mentioned earlier, the shoulder blade moves just like the arm does
In order to raise our arms outward, not only do the arm bones move,
but the shoulder blades must also shift like this
Like this
At first, the shoulder blades
are simply positioned in place
But as we lift our arms, the shoulder blades also move accordingly
However, due to their connection with the clavicle
the shoulder blades rotate in a circular motion around the clavicle’s connection point
This movement affects the silhouette
of the latissimus dorsi, or the broad back muscle
In individuals who are lean or have less muscle mass,
the shoulder blade silhouette is more visible
Next, let’s discuss the clavicle
The clavicle, also known as the collarbone,
was previously covered when discussing the torso, so we will go over it briefly
As you can see, the clavicle connects with both the pectoralis major and the deltoid muscles
The structure formed by the humerus, scapula, clavicle, and sternum
functions similarly to the latch of a door
The clavicle connects firmly to the sternum at this point
The clavicle enhances the stability of the shoulder
while allowing for a wide range of motion
by responding flexibly to various shoulder movements
However, it can be difficult to grasp the exact shape
of the shoulder girdle when viewed from the front
It often appears as just a straight or flat structure
This is how it actually looks
The clavicle and scapula connect here,
here,
and here, like so
In the past, criminals were made to wear a cangue around their neck
The way the scapula and clavicle connect forms a structure similar to that
When a person raises their arms,
the movement of the scapula
follows a circular trajectory,
all because of these connecting points
Understanding the structure of the shoulder girdle
will give you a much clearer grasp
of many upper-body close-up shots
Now, let’s move on to the free arm bones
The free arm bones primarily consist of the humerus,
ulna, and radius
Those are the prats
First, let’s talk about the humerus,
or the upper arm bone
It makes up about half of the total arm length, including the hand
The key point of interest here
is the shape of the humeral head, which fits into the glenoid cavity
It's right here
This is an enlarged view of that area
Rather than fitting straight into the cavity,
the humeral head extends outward at an angle
Like so
Just like an AirPod
The bean sprout look, right?
In a similar way, the humeral head is inserted at an angle into the glenoid cavity
This anatomical structure creates a limitation in the arm’s range of motion
Try raising your arm to the side as if you’re reaching upward
How was it?
You’ll notice that beyond a certain point, the arm no longer moves freely
But if you raise your arm forward and continue lifting it above your head,
then it moves smoothly
This happens because the AirPod-like humeral head gets obstructed by the acromion
when the arm is raised sideways
If you continue forcing the movement,
the shoulder blade will have to move along with it
This is where the mechanics of the shoulder girdle come into play again
In contrast, lifting the arm forward and above the head does not cause this obstruction
The humeral head moves freely without getting caught under the acromion
This interplay between the humerus and the shoulder girdle mechanics
is always quite fascinating
The major muscles that attach to the humerus
include the pectoralis major and the latissimus dorsi from the torso
The deltoid muscle, which occupies the upper arm,
and the biceps brachii and triceps brachii,
and the brachioradialis, connected to the lower arm,
the wrist extensor muscles, and the wrist flexor muscles
As you can see from the pictures,
the muscles attached to the humerus
are essentially a hub for the upper body muscles,
including the chest, back, and arms
Almost all major muscles pass through this humerus
You know that pose where people flex their arms
to show off their biceps?
There’s a reason for that
Next, let’s look at the ulna
The ulna connects from the elbow to the wrist
In Chinese characters,
the term refers to
a unit of measurement
Measurement
Historically, the length of the ulna has remained relatively consistent,
making it a useful reference for measurement units
The ulna is a primary bone forming the joints for flexion and extension
The concave-shaped head of the elbow
interlocks with the trochlea of the humerus,
allowing unidirectional movement
The ulna also creates key anatomical landmarks,
such as the protrusion at the elbow
and the visible bump at the wrist
Now, let’s move on to the radius
Let's take a look
The radius is the counterpart to the ulna, forming the other half of the forearm bones
If the ulna is responsible for the bending and extending movements of the arm,
what’s the purpose of the radius?
The ulna is responsible for the bending and extending,
while the radius is mainly responsible for wrist rotation
As you can see,
the ends of both the ulna and radius are significantly larger at their respective functional joints
For example,
the radius here
plays a major role in wrist rotation,
and you can see that the bone near the thumb side of the wrist is much broader
Meanwhile, the ulna
interlocks with the humerus at the elbow,
making its upper portion much larger
The radius is wider at the lower end where it connects with the wrist bones,
enabling pronation and supination movements of the hand
Before continuing, let’s briefly explain wrist rotation
The default position of the hand is with the palm facing forward,
known as supination
When you rotate the hand so that the back of the hand faces forward,
that movement is called pronation
The opposite is called supination
Let's go back
An interesting fact is
that the head of the radius
always moves in sync with the direction of the thumb during both pronation and supination
These bones not only facilitate pronation
and supination but also play a role in absorbing impact
When the arm experiences a fall or shock,
the ulna and radius help distribute the force, preventing injuries to the wrist and elbow
At the same time, they generate counterforce,
aiding in movement propulsion
That covers the bones of the arm
Now, let’s discuss an interesting feature of the forearm,
the carrying angle
If you observe the arm in a supinated position,
you’ll notice that
the forearm angles slightly outward
This angle is called the carrying angle,
also referred to as the valgus angle
Usually we say the carrying angle
It is an evolutionary adaptation that developed from quadrupedal to bipedal locomotion
When the arms are relaxed in a supinated position,
they are naturally affected by the protruding pelvis
As a result, the arms angle outward
Since pelvic structure varies significantly
between genders,
this also affects the carrying angle,
which is very interesting
As you can see, women generally have wider pelvises
than men, resulting in a greater carrying angle
Typically, men have a carrying angle of 0 to 15 degrees,
while women have fom at least 10 to 20 degrees
This angle also contributes to differences in upper body strength between men and women
Because flexion and extension movements
follow a more linear path in men,
their muscle force is more efficiently transmitted
Therefore, men’s arms tend to stay closer to their bodies,
and when they exert force,
they can extend their arms
straight more effectively
On the other hand, due to the larger carrying angle in women,
their elbows tend to angle outward,
resulting in a more rounded and flexible motion
This difference is often reflected in posing styles
Men frequently take poses where their arms extend
outward in a Superman-like fashion
Meanwhile, women’s poses often emphasize outward hand gestures,
naturally showcasing
their carrying angle in a more fluid and soft manner
Overview of Arm and Shoulder Muscles
I mentioned that arm muscles can be broadly classified into flexor and extensor muscles
Flexors, also known as flexor muscles,
engage when bending the arm
A key example is the biceps,
or biceps brachii,
along with the brachialis
These muscles generate force at the elbow to facilitate arm flexion
In contrast, extensors are muscles that straighten or extend the arm,
with the triceps brachii being the primary extensor
One key takeaway here is
that flexors are generally
more developed than extensors
his is why when the arm is at rest in a natural position,
it tends to stay slightly bent
The flexor muscles exert more influence
When you build muscle, what happens?
That's right
It gets bigger
This is why flexor muscles, being the stronger force generators,
tend to be more pronounced compared to extensors
Likewise, this is also why the arms naturally
turn slightly forward when relaxed
Since the anterior muscles of the arm,
the flexors, are stronger than the extensors,
the arm naturally adopts a slightly forward-facing posture when at rest
This explains why the balance between flexors and extensors isn’t perfectly symmetrical
This natural flexed posture can also be observed in the hands
Just like the arms,
the hands receive much more force from the flexor muscles than from the extensor muscles
In contrast, for the legs,
the extensor muscles of the thighs are more developed than the flexors,
resulting in a more pronounced extensor muscle shape
Now, let’s take a look at the arm muscles
Starting with the shoulder, the origin of the arm bones,
the muscles of the shoulder girdle
These muscles are layered in complex formations,
so instead of analyzing them one by one, we will group them for easier understanding
Even knowing all of them in detail may not be particularly useful,
as much of them are actually covered by the trapezius muscle
However, understanding this part
allows for a deeper mastery of back muscle details
So let’s approach this without feeling overwhelmed
Here, you can see the shoulder blade
First, let’s look at the teres major,
which is located at the lowest layer
Above it, the teres minor is attached slightly forward,
connecting to the head of the humerus
Additionally, there is the infraspinatus muscle,
which extends above and below the spine of the scapula,
attaching to the humeral head
The infraspinatus is technically divided into the supraspinatus and infraspinatus,
but distinguishing them in this context doesn’t add much value
So let’s treat them as a single unit for now
These muscles primarily function to rotate the arm
Having examined these multiple layers,
you’ll notice that they are again covered by the trapezius and deltoid muscle
As a result, when drawing the back,
only a small portion of the shoulder muscles is visible
It’s somewhat amusing
yet also feels like we’re studying something we barely get to see
Now, let’s move on to the actual shoulder muscles
A broad, powerful shoulder
What man wouldn’t want to have one, whether thin or muscular?
For many men, it is the most desirable muscle group
and a top priority in bodybuilding
The key muscles here include the coracobrachialis and the deltoid
The coracobrachialis connects to the shoulder and functions to bring the arm forward
It also affects the armpit area when the arm is raised
Since it is a relatively small muscle, it doesn’t stand out much
Next, we have the shoulder muscle itself
The deltoid,
yes, the famous deltoid muscle
The deltoid is the main muscle used to lift the arm laterally
That’s why lateral raises with dumbbells
are a common exercise for shoulder development
The deltoid extends from the spine of the scapula
and the acromion, continuing to the middle of the clavicle
On the other end, it extends down
to about the midpoint of the humerus
In a flat diagram, it may be difficult to grasp the overall shape,
but structurally, it is attached to the bone like this
It forms a C shape,
extending from the scapular spine to the clavicle
Does that make sense?
At the lower end, the deltoid connects to the humerus
Rather than being a simple triangular shape, its structure resembles the canopy of a fighter jet
This silhouette is quite common in plastic model kits
Now, moving from the deltoid, let’s look at the muscles that define the presence of the upper arm
Back in the day, we used to associate bodybuilders with their biceps
That’s exactly where this muscle is located
The main muscles in this area are the brachialis,
the biceps brachii, and the brachioradialis
The brachialis and biceps brachii are primarily involved in elbow flexion
These muscles are heavily used when lifting objects,
so they are built
with two distinct layers
The lower brachialis originates
from the midpoint of the humerus and attaches to the ulna
It is positioned closer to the joints than to the center of the humerus,
and it has a slightly diagonal orientation
The biceps brachii has two heads that originate from the upper part of the scapula and extend downward,
attaching to both the radius and ulna
Thus, in the front of the arm, the biceps brachii takes center stage,
while the brachialis defines the contour from behind
The brachioradialis originates from the anterior portion of the humerus
Specifically, it starts from the lower half of the humerus at the anterior side
It then extends downward to attach near the elbow joint
As you can see, the brachioradialis plays a crucial role in flexion movements
Even though it belongs to the upper arm muscles,
it primarily shapes the forearm’s contour
When the hand is in a supinated position, this muscle forms a noticeable protrusion on the forearm
It is one of the key muscles responsible for changes in forearm shape depending on pronation and supination
On the posterior side of the upper arm, we have the triceps brachii,
commonly referred to as the triceps
That's the muscle
This muscle functions as an extensor,
primarily used for extending the arm
It is activated when extending the elbow or lowering the arm backward
The triceps brachii originates from three different points on the scapula
and humerus before converging at the elbow
This muscle has three distinct heads,
which merge at the olecranon process, where it forms a broad tendon
That's why it looks like this
Overview of the Forearm Muscles
Now, things are about to get even more complicated
You might be thinking, It was already complicated, what do you mean?
No,
up until now, it was relatively simple
The forearm,
especially the muscles responsible for wrist and hand movements,
is where things truly get intricate
Let’s start with the muscles related to flexing the hand
These are located on the anterior side of the forearm
The flexor digitorum profundus,
flexor pollicis longus,
and flexor digitorum superficialis
control finger movements
Meanwhile, the flexor carpi ulnaris,
palmaris longus,
and flexor carpi radialis
contribute to wrist flexion
Pretty amazing, right? Next!
We also need to look at the extensor muscles
The extensor digitorum,
extensor pollicis longus,
extensor pollicis brevis,
and abductor pollicis longus support the movement of the fingers
Additionally, the extensor carpi radialis longus, extensor carpi radialis brevis,
and extensor carpi ulnaris regulate wrist extension
Next!
Well, that's it
That's the end of the list
I figured that if we went through all of these one by one, you’d probably just turn this off
Honestly, I was a bit nervous about it myself
The forearm is packed with muscles responsible for wrist and hand movements
If the fingers had thick, bulky muscles attached directly to them,
they wouldn’t be able to move delicately, or do even basic movements
That’s why the power-generating muscles
are located in the forearm, and they control the fingers via tendons
However, finger movements are inherently complex
Fingers have to flex from the palm,
and each finger has two or three individual joints that must move independently
On top of that, the wrist must also move
And let’s not forget, the arm needs to pronate and supinate as well
Unlike broad muscles simply layered over the chest or back,
the forearm is filled with multiple fine muscle strands
The layering of these muscles creates the forearm’s silhouette
Given its complexity, understanding the order of these muscle layers
is just as important as knowing what they are
At the deepest layer,
you have small muscles like the coracobrachialis and brachialis,
which are attached to the scapula and humerus
Above that, the various flexor muscles of the wrist and hand run parallel to each other
On the posterior side, we have the shoulder blade muscles, the triceps brachii,
and several extensor muscles for the wrist and hand
Now, let’s examine the outermost layer
On the anterior side, the brachioradialis runs from the wrist up toward the humerus
It is positioned toward the back of the forearm
Meanwhile, the biceps brachii overlays the brachialis,
covering it snugly
From here, the trapezius and pectoralis major attach
to the scapula and clavicle
Lastly, the pectoralis major spans across the scapula and clavicle,
forming a cohesive structure
And with that, the anterior view is complete
On the posterior side, the trapezius and pectoralis major
follow the spine and integrate into the structure
Lower down, the brachioradialis originate
from the upper humerus and wraps around to the front
And just like that, the arm muscles are complete
Now, let’s go over the layering once more
From the front view, we group
the humerus and biceps brachii together
Below that, the brachialis and overlapping
wrist flexors form another group
To the side, the brachioradialis is grouped separately
Above, the pectoralis major overlaps the upper part
of the biceps brachii, while the deltoid caps it all off
There, that’s done
What about the posterior view?
It follows a similar pattern
Same thing
The triceps brachii is grouped as a whole
Below that, the wrist and hand extensors extend downward
The brachioradialis,
wrapping around toward the front, forms another distinct group
Above, the deltoid overlaps the triceps brachii
and shoulder blade muscles, forming the final layer
When we outline the silhouette from the skin’s surface, this is how it comes together
That feels pretty satisfying
That concludes today’s lecture
We only covered the anatomical aspects of the arm, and yet, it was a massive amount of information
It’s a bit overwhelming
And honestly, I’m getting hungry
But we’re not done yet
So far, we’ve only talked about the arm in a neutral, resting position
In other words, we’ve only covered the default state
We haven’t even begun to explore how muscles change
when the arm bends, twists, or rotates
And most importantly, we still have the hand to discuss
Even if you could get away with drawing arms in a simple way,
hands are a whole different challenge
The journey through arm anatomy is far from over
It's tough
But I hope you’ve enjoyed it
Great job everyone for going through this with me
Thank you
Arm Anatomy Pectoral Girdle: Connects the arm to the torso, has the scapula and the clavicle Free Arm Bones: Functional movement of the arm
Scapular Spine: For back and arm muscles, a landmark Acromion: Cavicle meets the shoulder blade
Glenoid Cavity: The humerus to the shoulder Coracoid Process: Arm movement Clavicle: Dynamic houlder movements
Humerus: The longest, around half of the arm Ulna: The inner forearm bone Radius: The outer forearm bone
Arm and Shoulder Muscles Arm Muscles Flexor Muscles: For bending the arm Biceps Brachii and brachialis
Extensor Muscles: To straighten the arm Triceps Brachii
Muscle Mass Difference: Flexor muscles are more developed than extensors, which is why the arm naturally curves forward when at rest
Shoulder Muscles Rotator Cuff Muscles: Attaching to the humerus, rotating the arms Coracobrachialis: Smaller, not really visible
Deltoid Muscle: The dominant shoulder muscle, from the scapular spine and acromion to the clavicle
Muscles for Forearm Rotation Pronator Teres & Pronator Quadratus: Control forearm pronation Supinator: Responsible for forearm supination
Muscles for Hand Flexion Deep, Long, Short Finger Flexor for fingers Flexor Carpi Ulnaris, Palmaris Longus, and Flexor Carpi Radialis for wrist
Muscles for Hand Extension Extensor Digitorum and more for fingers Extensor Carpi Radiali and more for writst
Muscles are located in the forearm Te hand is connected via tendons, allowing the fingers to flex or extend