Hello everyone! It's Megan from Kenhub here, and welcome to our tutorial on muscles of the trunk wall. In today's tutorial, we'll be discussing the muscles of the trunk wall as well as their function and we'll be looking at muscles at both the thorax and the abdominal wall.
So let's begin with the muscles of the thoracic cage. We'll start off by looking at the intercostal muscles. The intercostal muscles can be divided into the internal and external intercostal muscles. Together, these muscles are a group of intrinsic muscles of the chest wall and they occupy the intercostal spaces. So in this illustration on the right, we can see the internal intercostal muscles. These muscles function to lower the ribs during expiration or when we exhale as well as to support the intercostal spaces and stabilize the chest wall.
Before we move on to the external intercostal muscles, I just want to draw attention to the direction of the muscle fibers. So as you can see in this illustration, the internal intercostal muscles run downwards and backwards whereas if we look in the next image of the external intercostal muscles, we can see that they run downwards and forwards. The external intercostal muscles are superficial to the internal intercostal muscles and they function to raise the ribs during inspiration or when we inhale. These muscles also support the intercostal spaces and stabilize the chest wall.
Medially, we have the transversus thoracis muscle which we can see here highlighted in green. This is a thin muscle found on the anterior surface of the inner thoracic wall. During expiration, this muscle acts to lower the ribs by pulling the rib cartilage caudally when it contracts. So in this way, this muscle supports expiration. However, it's not considered to be among the primary respiratory muscles.
The next muscles we're going to look at are the subcostal muscles. These muscles are unique in that they can span across one or multiple ribs becoming more numerous within the inferior regions of the posterior thoracic wall. These muscles extend from the inner surface of one rib to either the inner surface of the next rib or to the inner surface of the one below that. The subcostal muscles are located in the same plane as the innermost intercostal muscles and they also assist these muscles during respiration.
The following group of muscles we're going to look at are the scalene muscles. These group of muscles consist of the posterior scalene muscles, the middle scalene muscles and the anterior scalene muscles. Although these three muscles are cervical muscles, they run to attach to the first and second ribs. If we look at the scalene muscles side by side, we can see that the posterior scalene muscle attaches to the second rib whereas the middle scalene muscle and the anterior scalene muscle attach to the first rib. The contraction of these muscles results in the elevation of the ribs, therefore, they are also considered as accessory muscles of inspiration.
Another muscle of the thorax is this small muscle seen here highlighted in green which is known as the subclavius muscle. This small muscle, though technically a muscle of the shoulder girdle, depresses the clavicle when it contracts while at the same time elevating the first rib. Located on the lateral aspect of the chest wall, we can see serratus anterior which is a fan-shaped muscle. The superior, intermediate and inferior part of this muscle attaches to ribs one through nine. This muscle acts as an accessory muscle of inspiration among its other functions. When the scapula is in a fixed position, the contraction of this muscle lifts the ribs.
The next muscle we're going to look at is the pectoralis minor muscle which is also a fan-shaped muscle. When this muscle contracts, it pulls the scapula forward and downward towards the ribs. When the scapula is in a fixed position, the pectoralis minor also elevates the third, fourth and fifth ribs and expands the rib cage, therefore, serving as an accessory muscle during inspiration.
So we just saw the pectoralis minor which is the smaller of the two pectoralis muscles. Now here we have the pectoralis major muscle which is also fan-shaped. This muscle which is larger than its counterpart is comprised of a clavicular portion, a sternocostal portion, and an abdominal portion. The pectoralis major is an important muscle of the shoulder joint which is vital for the adduction and anteversion of the shoulder joint. But in addition to that, this muscle plays a significant role during inspiration and so it can be considered an inspiratory muscle. Also, when the arms are in a fixed position, the pectoralis major lifts the trunk which could be helpful in climbing.
Now, let's move on to look at the muscles of the lateral and anterior abdominal wall. So here highlighted in green is the external oblique muscle. This muscle is one of three muscles of the lateral abdominal wall. Unilateral contraction of the external oblique bends the trunk to the same side and rotation of the trunk to the opposite side. Bilateral contraction of this muscle bends the trunk forwards. The external oblique muscle is also active during expiration and it helps maintain the abdominal tone or abdominal press. This basically means that it increases intra-abdominal pressure which supports the process of defecation, urination, and expiration.
As I just said, there's three muscles of the lateral abdominal wall and each of them has a different orientation of muscle fibers. For example, in this illustration, we can see that the fibers of the external oblique run downwards and forwards whereas in the next illustration of the internal oblique, we can see that they pass upwards and forwards. The internal oblique muscle is the middle layer of the three muscles of the lateral abdominal wall and carries out much of the same functions as the external oblique. Unilateral contraction bends the trunk to the same side, however, it rotates the trunk to the same side also whereas with the external oblique this action causes rotation to the opposite side. Bilateral contraction of the internal oblique also bends the trunk forward and it also helps maintain abdominal press. Both the external and internal oblique muscles get their names as the result of the alignment of their muscle fibers.
The innermost layer of the lateral abdominal muscles is the transversus abdominis muscle and it lies deep to the internal abdominal oblique muscle. The fibers of this muscle run transversely across the abdomen which is why it's given its name. Unilateral contraction of the transversus abdominis results in ipsilateral rotation of the trunk or to put it more simply, rotates the trunk to the same side. Contraction of this muscle also increases intra-abdominal pressure through abdominal press which supports the processes of urination, defecation and expiration.
The next muscle we're going to look at is the rectus abdominis muscle which is a muscle of the anterior abdominal wall. This muscles is more commonly known as abs or what people refer to as a six-pack when this muscle is well-defined. The rectus abdominis muscle appears to have multiple muscle bellies and this is due to the fact that it has three to four tendinous intersections that adhere to the anterior layer of the rectus sheath. It acts with the lateral muscles of the abdominal wall to increase intra-abdominal pressure through abdominal press. This muscle also acts to stabilize the vertebral column, tense the abdominal wall, straighten the pelvis, flex the lumbar spine, and support exhalation by pulling down the thorax during contraction.
The final muscle of the anterior abdominal wall is the pyramidalis muscle which we can see here highlighted in green. This small muscle is a rudimentary muscle that relates to the pouch muscles of monotremes and marsupials. Originally, this muscle is supposed to play a role in tensing the linea alba but it no longer plays a role in higher mammals.
Now let's look at the muscles of the back and posterior abdominal wall starting with the trapezius which is a large trapezoid-shaped muscle. This flat triangular-shaped muscle stabilizes and secures the shoulder blade at the thorax among its other functions. Unilateral contraction of the trapezius muscle rotates the head to the same side also known as ipsilateral rotation whereas bilateral contraction raises the head in the cervical vertebral column which is known as dorsiflexion.
The next muscles of the back we will look at are the two rhomboid muscles which are two diamond-shaped muscles of the shoulder girdle. These muscles are the rhomboid major muscle and the rhomboid minor muscle. As you can tell from these two images, the rhomboid major muscle is the larger of these two muscles but they both basically perform the same function. Contraction of these two muscles results in the adduction and elevation of the scapula and the movement of the inferior angle of the scapula towards the vertebral spine or rotation. These two muscles which are located underneath the trapezius muscle also act to stabilize the scapula during both rest and movement of the arm.
Now let's have a look at the levator scapulae muscle which is also a muscle of the shoulder girdle. As its name suggests, this muscle acts to elevate the scapula and when the scapula is in a fixed position, the contraction of this muscle results in the lateral flexion of the cervical vertebral column.
Next we'll have a look at the serratus posterior muscles. There are two serratus posterior muscles – the serratus posterior superior and the serratus posterior inferior. These two superficial muscles lie above the intrinsic muscles of the back and they contribute to the stabilization and movement of the vertebral column and the thorax. As we can see here, the serratus posterior superior muscle is located superiorly and acts to raise the ribs thereby supporting inspiration. Bilateral contraction of the serratus posterior superior muscle also extends the vertebral column whereas the unilateral contraction of this muscle rotates the vertebral column to the opposite side. The more inferiorly-situated serratus posterior inferior muscle helps during expiration by luring or depressing the ribs. When this muscle contracts bilaterally, it extends the spine while unilateral contraction rotates it.
The next muscle we'll look at is the latissimus dorsi muscle which is a large wide muscle that covers almost all of the muscles of the posterior trunk. It supports various movements of the shoulder including the inward rotation, adduction and retroversion of the humerus. The latissimus dorsi also supports breathing function. For instance, the activation on both sides of this muscle causes a compression of the rib cage which facilitates the process of expiration. This muscle is particularly strained during continuous coughing attacks. In addition when the humerus is in a fixed position, the latissimus dorsi also elevates and moves the trunk anteriorly. These two functions of this muscle are particularly useful during certain activities like climbing, pull-ups or cross-country skiing.
In the next illustration, we can see a muscle that contributes to the stabilization and movement of the spine and pelvis and it's called the quadratus lumborum muscle. The bilateral contraction of this muscle extends the lumbar vertebral column while unilateral contraction bends the trunk to the same side also known as lateral flexion of the trunk. The quadratus lumborum is also considered an accessory muscle of expiration because it fixes the twelfth rib during movements of the thoracic cage thereby supporting expiration.
Another muscle of the posterior abdominal wall is the psoas major muscle. This muscle which we can see here highlighted in green joins up with the iliacus muscle in the lower pelvis to form the iliopsoas muscle which is the strongest flexor of the hip joint. When the femur is fixed, the unilateral contraction of this muscle bends the spine laterally while bilateral contraction of the muscle raises the trunk from a supine position, for example, when doing sit-ups. Please keep in mind that the psoas major muscle is topographically part of the posterior abdominal wall but it's functionally classified as a hip muscle.
To end off this tutorial, we'll look at the intrinsic muscles of the back which are a group of deep back muscles at the vertebral column that range from the pelvis up to the head. These muscles can be divided into lateral tract, medial tract, and deep muscles. We are going to start with the muscles of the lateral tract and the first muscle that we'll look at is the iliocostalis muscle. This muscle has various points of attachment at the ribs and the transverse processes of the vertebral column. It can be subdivided into three parts based on muscle attachment. These parts include iliocostalis cervicis, iliocostalis thoracis, and iliocostalis lumborum. The bilateral contraction of the entire muscle extends the spine while unilateral contraction bends the spine laterally to the same side.
The next illustration we can see shows the longissimus muscle highlighted in green. This muscle also has various points of attachment at the ribs and transverse processes of the vertebral column and even attaches to the mastoid process of the temporal bone. This muscle can also be subdivided into three parts based on muscle attachment. These parts include longissimus capitis, longissimus cervicis, and longissimus thoracis. The function of this muscle differs depending on which part of the muscle is contracting. In a similar fashion to the iliocostalis muscle, the bilateral contraction of the entire longissimus muscle extends the spine while unilateral contraction bends the spine laterally to the same side. On the other hand, the bilateral contraction of the longissimus capitis extends the head while unilateral contraction flexes and rotates the head to the same side.
In the same way as the two muscles we have just discussed, the splenius muscle can also be divided into two parts. These are the splenius capitis muscle and the splenius cervicis muscle. Although this muscle can be divided into two parts based on attachment, it functions as one muscle. So the bilateral contraction of the entire muscle results in the extension of the cervical spine and the head. Unilateral contraction of this muscle flexes and rotates the head to the same side. The splenius muscle contributes towards the spinal transverse system along with the erector spinae muscles.
The next muscles we'll look at are the intertransversarii muscles which contribute towards the intertransverse system of muscles. The term intertransversarii refers to a series of small muscles that connect the transverse processes of neighboring vertebrae. Here we see the anterior and posterior cervical intertransverse muscles which are located in the cervical region. The bilateral contraction of these muscles stabilizes and extends the cervical spine. Unilateral contraction, however, bends the cervical spine laterally to the same side. And in the lumbar region, we find the medial and lateral lumbar intertransverse muscles. The bilateral contraction of these muscles stabilizes and extends the lumbar spine while unilateral contraction bends the lumbar spine to the same side.
Also part of the intertransverse system of muscles are these muscles seen here highlighted in green which are the levatores costarum muscles. The bilateral contraction of levatores costarum extends the thoracic spine, however, unilateral contraction of this muscle bends the thoracic spine to the same side while at the same time rotating it to the opposite side.
To start off the intrinsic back muscles of the medial tract, we'll look at the interspinales muscles which connect spinal processes of neighboring vertebrae. Contraction of the interspinales muscles extends the cervical and lumbar spines. The next muscles we'll look at are the spinalis muscles which originates slightly laterally from the interspinales muscles that we just saw. The spinalis cervicis as you can see from this illustration is found in the cervical region and the spinalis thoracis is found in the thoracic region. The bilateral contraction of these muscles extends the cervical and thoracic spines whereas unilateral contraction bends the cervical and thoracic spines to the same side.
In the next illustration, we can see the short and long rotatores muscles. Here we can see an example of the short rotatores muscle and here we can see an example of the long rotatores muscle. The bilateral contraction of these muscles extends the thoracic spine whereas unilateral contraction rotates it to the opposite side. Next we can see the multifidus muscle which courses between the transverse spinous process along the length of the vertebral column. The bilateral contraction of the multifidus extends the spine while unilateral contraction flexes it and rotates it to the opposite side.
Finally, the last muscles we're going to look at are the semispinalis muscles. These muscles include semispinalis capitis, semispinalis cervicis and semispinalis thoracis. If we look at these muscles side by side, we can see how they differ in location. So here we can see that semispinalis capitis is located in the head, neck and thoracic region whereas semispinalis cervicis is located in the neck and thoracic region. Finally, we have semispinalis thoracis which is mainly located in the thoracic region. Bilateral contraction of all three muscles extends the thoracic spine, the cervical spine and stabilizes the craniovertebral joints extending the head. Unilateral contraction on the other hand bends the head, the cervical spine and the thoracic spine while rotating them to the opposite side.
Now that we've talked about the semispinalis muscles, that brings us to the end of this tutorial on the muscles of the trunk. I hope you enjoyed it and thank you for listening.
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