Choosing the right Abdominal or ‘Core’ Exercise

Unless you’ve been living under a rock, you likely have heard of “core training”, “training the core”, “core stability” or some similar terminology. Often in the physio world this is in relation to back pain however people tend to use these terms interchangeably for abdominal and/or trunk exercises. Whilst I am not going to debate the validity of ‘core stability training’, I am going to explore the different uses of targeted abdominal exercises and their effect on the body. Not all abdominal exercises are created equal and what may appear to be an exercise to chisel your ‘6-pack’ may indeed be used for many other uses.

Anatomy

The musculature of the abdominals consists of 5 paired (right & left) muscles along with associated fascias. The abdominal muscles facilitate movement but also play important roles in protecting the abdominal viscera, increasing abdominal pressure to facilitate defecation, micturition, and parturition (childbirth).

Three of the muscles are aligned in a horizontal-type position (internal obliques, external obliques and transverse abdominis) while two run in a vertical alignment (rectus abdominis, pyramidalis). Fascia surrounds these muscles forming three layers to the abdominal muscles as well as three distinct fascial lines – the linea alba plus two semilunar lines.

Image: abdominal anatomy showing the layers of muscle

External Oblique

This is the largest, thickest of the abdominal muscles. It originates from the lower eight ribs, wraps around the trunk and forms the abdominal aponeurosis (fascia) which attaches into the linea alba. The external oblique creates trunk flexion when contracted in unisom and unilateral contraction causes same sides trunk flexion but rotation to the opposite direction (different to the internal oblique).

Internal Oblique

The internal oblique originates from the iliac crest (hip) and the transverse abdominis aponeurosis (fascia). It runs at a right angle to the external oblique attaching to the cartilage of the lower four ribs at the front of the chest. One sided contraction causes rotation and/or lateral flexion to the same side whereas bilateral contractions cause flexion of the trunk and abdominal compression for activities like breathing.

Transverse Abdominis

This muscle forms the deepest layer of the three layers. This is the most horizontally aligned abdominal muscle. At the back, this muscle originates from the lower six ribs, the iliac crest (hip) and the transverse processes of the spine. It wraps around the trunk to attach into aponeurosis at the anterior abdominal wall. Transverse abdominis acts to create compression of the abdomen when both sides contract or trunk rotation with a unilateral contraction.

Rectus Abdominis

Rectus is the 6-pack muscle, the one all the guys in the gym are crunching their life away to get chiselled. Rectus originated for the pubic symphysis and crest and attaches to the xiphoid process and cartilage of ribs 5-7. It creates the ab crunch movement (trunk flexion). The muscle runs vertically and left and right are separated by the linea alba.

Other non-abdominal muscles mentioned in this blog include Psoas Major/Minor, Quadratus Lumborum and Erector Spinae.

Image: anatomy of the psoas and quadratus lumborum muscles

What is EMG?

When a muscle contracts it emanates an electrical signal. Electromyography or EMG is the discipline of measuring these electrical signals from the muscles. As force develops in a muscle, more muscle fibres are recruited meaning an increase in the electrical activity of the contracting muscles. When this is measured we see an increase in the amplitude of the signal (see picture below). This electrical signal is detected by specialised surface electrodes places on the skin over the muscles (1). When placed on multiple muscles during exercise we are able to detect which exercises create more electrical activity in each muscle and therefore which muscle is more preferentially recruited or targeted.

Image: EMG tracing showing different muscle activity

Easy to perform abdominal exercises

Video: abdominal exercises discussed below

Which exercise to target which muscle?

Now let’s take this small select group of exercises we have highlighted above and match them with which muscle(s) that they are shown to activate most through EMG studies.

Rectus Abdominis: bent knee curl up, bent knee sit up

Internal Obliques: dynamic side bridge, isometric side bridge

External Obliques: sit ups, side bridges (both types)

Transverse Abdominis: side bridges (both types), sit ups

Psoas Major / Minor: straight leg raise

Rectus Femoris: straight leg raise, straight leg sit up

Erector Spinae: dynamic side bridge, isometric side bridge

Quadratus Lumborum: isometric side bridge

Please note that there is a limitless amount of ways to target your abdominal muscles and outlined above is a select group of exercises in which I was able to obtain scientific references. Point and case the below infographic from Adam Meakins (physiotherapist) showing other non-traditional abdominal exercises effects on the abs.

Image: activation of different abdominal muscles with certain exercises (image credit Adam Meakins)