Skeletal Muscle Series Part 3

Trunk Muscles

Your trunk or torso is the central part of your body where your head and limbs are attached. Minus your brain, your trunk is a protective home to all the vital organs of your body. In addition, the trunk muscles help in essential body processes like movement, breathing, bowel movements, and urination. Most of them originate from and attach to the skeletal system in the trunk and may be conveniently divided into the following groups:

Muscles of the Chest or Thorax.
Muscles of the Back
Muscles of the Abdominal Wall
Muscles in the Pelvic Area
Muscles of the Perineum

The paragraphs and tables below provide a generalised description and comprehensive* lists of muscles found in the trunk. Please note: the word muscle is removed from the actual muscle names to avoid repetition.

*Comprehensive but may not be inclusive of all the skeletal muscles found in the trunk region.

Muscles of the Chest

The muscles of the thorax or chest function to facilitate breathing by altering the volume of your thoracic cavity. When you breathe in, your chest rises, increasing the volume of your thoracic cavity. On the other end, when you exhale, your chest sinks or falls, decreasing the capacity of your thoracic cavity. Most of the muscles involved in respiration attach to your sternum and ribs and move these bones during respiration. The remaining muscles of your chest are involved in the movement of your upper limbs.

MUSCLES OF THE CHEST
Muscles involved in Respiration (Breathing)
Muscle	Action/Function
1. External intercostals	Inhalation (breathe in) and exhalation (breathe out) Expands the thoracic cavity
2. Internal intercostals 3. Innermost intercostals	Complete (forced) exhalation Bring the ribs closer together
4. Diaphragm	Inhalation and exhalation The primary muscle for inhalation Separates your thoracic cavity and abdominal cavity
5. Subcostalis 6. Transversus thoracis	Lower the ribs during complete exhalation
7. Levatores costarum 8. Serratus posterior superior and inferior	Help with inhalation and exhalation
Muscles involved in Upper Limb Movement
Muscle	Action/Function
9. Pectoralis minor	Throwing motion of the shoulder and arm by rotating the shoulder anteriorly Football players (contact sports) have strong pectoralis minor muscles
10. Pectoralis major	The pecs (colloquial term) are the most superficial and largest muscles in the chest area Its functions include flexion, adduction, and rotation of the arm. So when you think of pecs, think of Arnold Schwarzenegger during the prime of this bodybuilding career. They were like mountains.
11. Subclavius	Stabilises the clavicle during movement
12. Serratus anterior	Moves the arm from the side to the front Assists with inhalation
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Note: For simplicity and brevity, shoulder muscles are included in the article on skeletal muscles of the upper limbs.

Muscles of the Back

For simplicity, the muscles of the back are conveniently divided into extrinsic and intrinsic back muscles. Extrinsic back muscles are concerned with helping respiration and movement of the upper limbs. The intrinsic back muscles are involved in movements of the spine and maintenance of posture.

MUSCLES OF THE BACK
Extrinsic Back Muscles
Muscle	Action/Function
1. Trapezius	Rotates, retracts, and elevates the scapula
2. Latissimus dorsi	Adducts and rotates the arm
3. Rhomboid major and minor	Elevates the scapula
Intrinsic Back Muscles
Muscle	Action/Function
4. Erector spinae Iliocostalis Longissimus Spinalis	Extends and bends the vertebral column to the sides.
5. Semispinalis	Extends head, neck, and thorax
6. Multifidi	Stabilises the spine
7. Rotatores	Rotates, stabilises, and extends the spine
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Note: The muscles at the back of your neck, splenius capitis, splenius cervicis, and levator scapulae, are sometimes grouped with the back muscles. In this series of articles, they’re grouped with skeletal muscles of the neck. Also, serratus posterior superior and inferior are muscles that can also be grouped under back muscles. In this series of articles, they’re grouped with skeletal muscles of the chest.

Muscles of the Abdominal Wall

The skeletal muscles in the abdominal wall assist in breathing and facilitate bowel movement, urination, and childbirth (females).

MUSCLES OF THE ABDOMINAL WALL
Muscle	Action/Function
1. External obliques 2. Internal obliques	Twisting at the waist Bending to the side (lateral flexion of the vertebral column)
3. Transversus abdominis	Squeezes the abdomen during bowel movement, urination, childbirth, and forceful exhalation
4. Rectus abdominis	More commonly called ‘abs’ or ‘six-pack', this muscle is responsible for flexion of the vertebral column (as in sitting up)
5. Quadratus lumborum	Bending to the side (lateral flexion of the vertebral column)
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Muscles of the Pelvic Area

The muscles of the pelvic area make up what is called the pelvic floor or pelvic diaphragm. They separate your pelvic cavity from your perineum, where the anus and genitalia are located. The levator ani is deemed the most essential muscle of this group because it diverts pressure from your abdominal cavity to your large intestine to help you move your bowels. It’s also responsible for the formation of sphincters around the urethra (urination) and anus.

MUSCLES OF THE PELVIC AREA
Muscle	Action/Function
1. Levator ani is made up of the following muscles: Iliococcygeus Puborectalis Pubococcygeus 2. Coccygeus	Supports the organs in the pelvic cavity Resist increases in abdominal and pelvic pressure when you cough, sneeze, exercise, etc These muscles give you the incredible ability to control the release of urine, stool, and flatus (pass gas)
3. Obturator internus 4. Piriformis	Rotation and abduction of the thigh Stabilises the hip joint
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Muscles of the Perineum

The perineum is a term used to describe the area where the genitals and anus are. The point midway between the two structures is called the perineal body. It’s a fibromuscular tissue that acts as a point of attachment for the muscles of the perineum. It’s located just beneath the skin, between the genitals and anus in both males and females.

MUSCLES OF THE PERINEUM
Muscle	Action/Function
1. Sphincter ani externus and internus	Keeps the anus closed Helps in the expulsion of stool
Superficial Perineal Pouch
Muscle	Action/Function
2. Superficial transverse perineal muscle	Stabilises the perineal body
3. Bulbospongiosus 4. Ischiocavernosus	Expel the remaining urine or semen from the urethra and assists in the erection of the penis (men) Clenches vagina and assists in the erection of clitoris (women)
Deep Perineal Pouch
Muscle	Action/Function
5. Deep transverse perineal muscle	Supports the perineal body Expel the remaining urine in both sexes
6. External sphincter muscle of the urethra	Contracts to voluntarily control the flow of urine The internal sphincter muscle of the urethra is a smooth muscle (not skeletal muscle) and isn’t under voluntary control
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Clinical Significance of Some Selected Muscles of the Trunk

The skeletal muscles found in your trunk have some essential functions: they help protect the vital organs in your thoracic, abdominal, and pelvic cavities. They also contribute to important body activities such as movement, breathing, urination, and moving your bowels. However, they also get sick, injured, or become dysfunctional. Below are some examples.

Poland syndrome is a medical condition where a baby is born with underdeveloped or missing chest muscles (pectoralis major and minor). Only one side of the body is usually affected. The cause of this rare disease is unknown and treatment is available in the form of injections or implants to fill in the deficient muscle areas. It is more common among males than females, occurs sporadically and isn’t inherited from a parent.
A tear, rupture, or laceration of your diaphragm muscle can lead to an acquired diaphragmatic hernia. It’s a condition where one or more abdominal organs move into the thoracic cavity displacing a lung because of a defect (hole) in the diaphragm. Road traffic accidents, falls, stab wounds, and gunshot injuries are major causes of acquired diaphragmatic hernias. Abnormal development of the diaphragm in an unborn baby may also lead to a diaphragmatic hernia (congenital one). The treatment for both types of hernias is surgery.
The main problem with back muscles is pain. The lower back muscles are the most common cause of low back pain. The muscles here can generate spasms that can be excruciating and debilitating. Learn more about back pain by clicking here.
An abdominal strain can mean a rupture, tear, or overstretching of the abdominal muscles. It can be caused by:
- Excessive laughing, sneezing, coughing
- Too much exercise of the abdominal muscles or not allowing the muscles enough recovery time.
- Improper lifting of heavy objects (yes, it’s not just your lower back that’s in trouble).
- Abrupt and abnormal twisting, bending, or fast movements of the abdominal muscles.
Treatment includes cold and hot therapy, pain relievers, adequate rest, and the need to wear abdominal binders.
A person with pelvic floor dysfunction can’t correctly coordinate and relax the pelvic floor muscles to have a successful bowel movement or urination. Causes of pelvic floor dysfunction include:
- Being obese
- Advancing age
- Straining the pelvic muscles too hard when you lift something heavy or move your bowels
- Pregnancy and pelvic surgery
- Traumatic injuries to the pelvic floor muscles (road traffic accidents)
Both males and females can suffer from perineal muscle injuries. They’re usually secondary to a fall which could lead to muscle tears, inflammation, and swelling.

Maintenance and Recovery of Trunk Muscles

Chest muscles. The muscles involved in respiration benefit best from breathing exercises. There are different types of breathing exercises ranging from diaphragmatic breathing, pursed-lip breathing, and deep breathing, just to name a few. You can also consult with your doctor or a physiotherapist to determine which breathing exercises are suitable for you.
The muscles recruited in upper limb movement require enough time to rest and recuperate. Hot packs and pain relievers can help alleviate the pain. A pulled or strained chest muscle will give you sharp pain. If the pain becomes chronic, physical therapy can offer some benefit. If the pain is constant (not on and off), feels like pressure on your chest, with pain shooting down your left arm, jaw or back, consult your doctor or the nearest ER immediately. The pain may be originating from your heart muscle and not from your chest muscles.

Back muscles. A pulled back muscle may sound like an insignificant injury, but the resulting pain and spasms can be shockingly severe. The time it takes to heal a pulled muscle may vary, but they usually resolve between four to six weeks. Your posture, the mattress you sleep on, and the activities you engage in will also affect your road to recovery. A wide variety of specialists can help you with a pulled back muscle, like chiropractors, physiotherapists, orthopaedic surgeons, etc. Click here for more information on back problems.
Abdominal wall muscles. Strained or pulled abdominal wall muscles are commonly seen among active people and athletes. It’s usually secondary to sudden twisting (like swinging a bat) or abrupt hyperextension of the spine (as observed among gymnastics movements). The force taken to perform these movements are stronger than the muscles leading them to stretch, tear, or even rupture. Core (midsection) strength training exercises work best for abdominal wall muscles. It’s usually incorporated into the training program of athletes. Certified athletic trainers, physical therapists, and even some personal trainers can help you strengthen your core.
Muscles of the pelvic area. Hmm, how do you maintain and exercise muscles involved in moving your bowels? These muscles are often underrated and abused daily. Show them that you care by adjusting your lifestyle, adequately managing your stress levels, and doing some Kegel exercises. Yes, Kegel exercises aren’t just for pregnant women.
Muscles of the perineum. Among pregnant women, recovery after childbirth includes taking care of the perineal muscles. Exercises that strengthen the pelvic area also contribute to the healing and recovery of perineal muscles. Trauma to the perineal muscles may respond to cold compresses, exercise, and possibly electrotherapy.

The muscles of the trunk are complex and serve several vital functions, as previously mentioned. Therefore, each group of muscles deserves special attention when sick or injured. In addition, the maintenance and recovery of each muscle group may also require a different approach, therapy or treatment to heal or sustain function.