Apr 2014 19

The muscles of the abdomen

By: Dave Wheeler

If you say “abs”, most people think of six-packs, the diet Coke man and sit-ups.

 

 

Image courtesy of imagerymajestic / FreeDigitalPhotos.net

 

In fact your abs are made up of a group of 4 muscles which wrap around your abdominal area like duct tape around a parcel: front to back, diagonally in a cross, and vertically.

Together they perform a whole range of functions:

  • bending forward & sideways, and twisting your body;
  • helping keep your internal organs like your stomach, liver &  pancreas in place;
  • assisting in breathing;
  • acts of expulsion like coughing, sneezing, vomiting, defecating and childbirth.

Let's take a look at each of these 4 muscles.

 

Rectus Abdominis

 

Your rectus abdominis are a strap-like muscle running vertically from your sternum down to your pubic region.
 
They’re divided lengthways by a strip of tendon-like tissue called an aponeurosis. The rectus abdominis are divided horizontally by two more strips of aponeurosis. This is what gives the muscle it’s characteristic six-pack shape on superheroes.
 
The rectus abdominis have a whole load of functions:
 
  1. The fibres of the muscle run vertically, so as they contract, they pull your sternum (chest) towards your pubic region - in other words, cause you to bend forward. This action is called flexion of the trunk.
  2. They maintain pressure inside the abdomen when lifting heavy weights or during those acts of expulsion mentioned above. 
  3. When your breathing is forced, e.g. after exercise or if you have a medical condition like emphysema then your rectus abdominis will force up your diaphragm forcefully expelling air from your lungs.
  4. It’s an important postural muscle, balanced by the muscles running up your lower spine. If the muscles at the front are stronger or weaker than the muscles at the back, then you’ll be pulled off-centre which can lead to all sorts of aches and pains.

External Obliques

 

The outermost layer of muscle, closest to the skin, are the external obliques.

Your external obliques run diagonally downwards and inwards (imagine putting your hands in the pockets of a pair of jeans - the direction of the muscle fibres runs in the same direction as your hands).

They start from the bottom of your lower 8 ribs and run diagonally down towards your pubis. Most of the muscle fibres of the external obliques attach at the aponeurosis running down the length of the strap-like rectus abdominis, though the lower fibres attach to the pubis itself.

In fact they run from along the entire length of those ribs - so it’s like a large sheet starting from near your spine going all the way to your mid-line then running downwards and inwards.

The external obliques have a number of functions:

  1. They assist the rectus abdominis in flexion of the torso - Imagine the muscles on both sides  contracting (bilateral contraction), pulling in on one another. 
  2. When they contract one side only (unilateral contraction), then they pull that side towards the midline of the body, in other words, they rotate the body towards the other side (this is called contralateral rotation). 
  3. Unilateral contraction also causes side-bending. If the muscles of the external obliques contract on one side then they pull you into a sideways bend to the same side (ipsalateral flexion).

Internal Obliques

 

 

Directly underneath (that is, deep to) the external obliques are the internal obliques.

Like the external obliques which lie on top of (or superficially to) the internal obliques, these are a sheet of muscles which wrap around the body from front to back, running diagonally. Unlike the external obliques, the internal obliques run downwards and outwards, so that they take the shape of a roof on a house when looked at from the front.

They start along the ridge of your hip bone (called the iliac crest) and run upwards and inwards towards the aponeurosis of the rectus abdominis, with the outer fibres (technically, the lateral fibres) attaching to the front of the last 4 ribs).

The actions of the internal obliques are similar to, but subtly different from the external obliques:

  1. They assist the rectus abdominis to flex the torso when they contract bilaterally.
  2. When they contract unilaterally they pull the torso into rotating towards that same side (this is because they run downwards and OUTWARDS, unlike the external obliques.
  3. Unilateral contraction also causes ipsilateral side-bending.

​Transverse Abdominis

 

If you’re familiar with Pilates then you’ll know about the transverse abdominis.

These are the deepest layer of abdominal muscles laying deep to the internal intercostals.

They start from the lumbar area of the back and wrap all the way around the body horizontally to join the aponeurosis of the rectus abdominis. Effectively, they’re a natural corset.

Because they wrap around the abdomen, the transverse abdominis are important postural muscles. They’re responsible for a number of different functions:

  1. They stabilise the spine & ribs, and for maintain core stability.
  2. They compress the abdominal contents (the viscera - that is, the internal organs) and help keep them in place.
  3. They also contribute to acts of expulsion, particularly childbirth (parturition). People with a chronic cough may often complain of an ache in their sides - caused by minor muscle strain in the transverse abdominis.

Remedial massage and the muscles of the abdomen

The most common abdominal muscle to get damaged - either through overtraining or straining (such as sneezing or coughing excessively) is the rectus abdominis, but it’s not rare for others like the transverse abdominis to be affected.

Some people with certain posture types are more prone to muscle strains there than others. Your remedial massage therapist needs to understand this, and also that your abs go all the way round to your back!

In a later blog post, I’ll get more geeky and talk briefly about how all of these muscles connect together at the aponeurosis.

As always, if you think I can help, give me a call.