Did you know your body has a trunk? No, not like an elephant's trunk. Your trunk encompasses the chest, back and abdomen. In this lesson you'll learn about some of the main muscles of the chest and abdomen.
Axial vs. Appendicular Muscles
Ever think about what's underneath your skin? What would your face look like without skin covering it up? Or your chest? Or your back? Probably pretty gruesome, or maybe pretty cool, depending on who you are. Well, underneath all that skin that you're used to looking at every day are the muscles that do all the work of moving your body.
If you've been following some of our other lessons on muscle anatomy, you may recall that, just like the skeleton, the muscles of the body can be divided into two groups based on location. The appendicular muscles are attached to the appendicular skeleton. Those would be the muscles making up your arms, shoulders, hips and legs. And, the axial muscles are the muscles attached to your axial skeleton. So, that would be the muscles of your head, neck, chest, abdomen and back.
The axial muscles may not have as much range of motion as your appendicular muscles do, but they are important in providing support and protection for your internal organs. They also control facial expression, neck movement, spinal rotation and even breathing. That's because your axial muscles are found on your face, your neck and the trunk of your body. Did you know you had a trunk? Not a trunk like an elephant's - this one is made up of your chest, abdomen and back.
Muscles of the Chest
Let's take a look at some of the muscles of the chest and abdomen in this lesson. Muscles of the chest, also called the thorax, include both smooth muscles and skeletal muscles. You may recall from other lessons that smooth muscles are found in many of your internal organs and blood vessels. They are involuntary muscles, meaning they aren't under conscious control. For example, you don't have to think 'digest' to make your stomach digest food or to make your intestines move food through your body. Smooth muscles:
- Have smaller individual muscle cells than skeletal muscles
- Lack the striations (or stripes) that are visible on skeletal muscles (that's why they're called smooth muscles)
- Contract slower than skeletal muscles do
The diaphragm is important for breathing.
One of the main smooth muscles inside the chest is the diaphragm. Located between the upper chest and abdomen, the diaphragm is a dome-shaped muscle responsible for enabling us to breathe. It works together with muscles of the ribs to alter the pressure in the lungs as you breathe in and out.
On the outside of the ribs we have the skeletal muscles. Skeletal muscles make up most of your external muscles and are attached to the bones of your skeleton. These muscles are voluntary muscles, meaning that they are consciously controlled by you; you can choose to move them. For example, when you want to throw a ball, you have to think about the action; your arm doesn't just pick up and throw a ball without you knowing it. Skeletal muscles:
- Have larger muscle cells (or fibers) than smooth muscles
- Contract faster and with more force
- Have striations on them that we don't see in smooth muscles
Location of the serratus muscles
Skeletal muscles of the chest are attached to the ribs and sternum. These muscles also attach to the scapula, clavicle, vertebrae and neck. Muscles located in between your ribs are called intercostal muscles ('inter' because they are 'in-between'). To the sides of your rib cage are the serratus muscles, which originate on the ribs and insert at the back, along the scapula. Together with the pectoral and teres muscles, they assist in shoulder movements and in raising the arm, making them appendicular muscles.
The pectoral and teres muscles assist with shoulder movements and arm raises.
Many muscles of the chest function to pull the body inward. What do I mean by inward? Well, imagine you're standing up straight, or you can just go ahead and stand up straight if you want. Now, hunch or pull your shoulders and arms toward your chest. This movement is pulling your upper body inward, toward the center of your chest, and is performed by the skeletal muscles of your chest.
Muscles of the Abdomen
Underneath the upper chest are axial muscles of the abdomen. These can be divided into the oblique and abdominus muscles. They surround the internal organs, providing a protective barrier between them and the external environment.
The oblique muscles wrap around the sides of your trunk in a circular pattern. There are two oblique layers: an external and an internal layer. The external layer is located directly under your skin; this is followed by the internal layer and then by a layer called the transverse abdominus. Together, these three layers help compress the abdomen and flex and extend the vertebral column. Basically, they're the muscles that allow you to bend sideways.
The oblique muscles have external and internal layers.
The front of the abdomen is made up of the rectus abdominus muscles, or your 'abs' for short - you know, the ones that make up the coveted 6-pack, even though really it's an 8-pack.
This is the most superficial (that means closest to the top) of all the abdominal muscles. You can see its location below, where it originates down at the front part of the pelvic girdle, known as the pubis symphysis, and inserts all the way up on your ribs and sternum. These muscles work together with the obliques and the transverse abdominus to help you maintain good posture. They also depress your ribs, pulling them down toward your stomach, and flex your vertebral column; you can see this function when you do sit-ups.
The rectus abdominus muscles extend from the pubis symphysis to the ribs and sternum.
Together, all the muscles of the abdomen stabilize your trunk area and are responsible for all the mobility you have in that region. That's why, as annoying as crunches are, it is important to maintain strength in these muscles.
So, let's go back and review. On the upper body we have muscles of the chest and muscles of the abdomen. The upper chest muscles include the intercostal, serratus, pectoral and teres muscles. These muscles attach to one or more of the following: the ribs, sternum, scapula, clavicle, vertebrae and neck. They pull the shoulders and chest inward, towards the center of your ribs. Some of them, like the pectoral, teres and serratus muscles, are also involved in shoulder movements. The chest is separated from the abdomen by the diaphragm, a large smooth muscle that enables us to breathe by changing the air pressure in the lungs.
Underneath the diaphragm are the abdominal muscles. The oblique muscles run horizontally around the sides of the trunk. Together with the transverse abdominus, they help compress the abdomen, flex and extend the vertebral column. Running vertically along the abdomen are the rectus abdominus muscles, which depress the ribs and flex the vertebral column. Together, all the muscles of the abdomen and chest stabilize the trunk and help you maintain good posture.
After this lesson, you'll be able to:
- Identify the upper chest muscles and describe their function
- Differentiate between smooth and skeletal muscle
- Explain the importance of the diaphragm
- List the abdominal muscles that work together to help maintain posture and identify where they are located