What is the core?

While most people seem to have reached a general consensus on what the leg is, you’ll find as many definitions of what the core is as there are articles…

Some people think of the core as the 6-pack abs, and the big muscles of the back. Others say it’s everything from the neck to the knees (excluding the arms).  But if we look at the core’s main functions, then it’s helpful to think about the core as all the muscles intended to support and move the spine and pelvis. 

These muscles are usually divided into two broad categories* depending on their structure and function; the stabilisers and the movers (or global mobilisers as they are often referred to in medical and scientific literature). In a well-balanced system, the two groups will work together in harmony to produce efficient movement.  

The core stabilisers  

The core stabilisers are the deep internal muscles closest to the spine which help stabilise the body and are also capable of producing force. This group of muscles have a large surface area and a higher predominance of Type 1 (slow twitch) muscle fibres making them more suitable for endurance tasks (such as supporting your body all day). They also have a significantly higher number of mechanoreceptors (sensory receptors that respond to movement and changes in force/pressure) than the movers, making them important for maintaining spinal equilibrium (the correct working of the spinal segments) [1], [2].

The core stabilisers include:

  • Transversus Abdominis (TrA) – arguably the most important, but often neglected muscle of the core, the TrA is the deepest of the abdominal muscles and is crucial for core stability. Often known as the ‘corset’ muscle, it has horizontal muscle fibres which wrap around the body from front to back, and extend from the ribs to the pelvis.
  • Multifidus – a series of small muscles deep in the back that span the whole length of the vertebral column.
  • Internal obliques – sandwiched between the TrA and external obliques, the internal obliques support the abdominal structures. They are sometimes classified as a mover for their role in lateral flexion (side bending) and rotation.
  • Pelvic floor muscles – help stabilise the core, hold the internal organs in place, and assist with bodily functions.
  • Diaphragm – most commonly known as the ‘breathing’ muscle, the diaphragm also plays a role in maintaining spinal stability.

The core movers (mobilisers) 

The core movers are the more superficial muscles closer to the skin which help move the body (and look good on Instagram…). They typically function concentrically to produce large torques for movement and power [3]. These muscles have a higher predominance of fast twitch (Type II) muscle fibres when compared with the stabilisers – meaning among other things, they can contract quickly to produce fast movements, but fatigue more quickly.

Easier to feel and consciously activate/squeeze, the core movers include: 

  • Rectus abdominis – the muscle most people refer to as the ‘abs’ or six-pack. The muscle fibres run vertically and their primary role is to flex the trunk. If you ask an athlete with weak (or switched off) core stabilisers to brace, squeeze, engage the core, it is likely they will activate this muscle first.
  • External obliques – flex and rotate the trunk.
  • Erector spinae – a collection of 3 long muscles running down the back near the spinal column.
  • Latissimus dorsi – Lats – the large muscle of the back.
  • Hip adductors – a group of 4 muscles that move the thigh and leg, also assisting with postural and pelvic control, and hip flexion and rotation.
  • Hamstrings – a group of 3 muscles at the back of the leg, that flex the hip and extend the knee.
The glutes 

Because they don’t stabilise the spine directly, the glutes are not always included with the core muscles. However, they are very important for control of the pelvis (which impacts how the spine is stacked) and should be active in the majority of core strengthening exercises. They play several roles so don’t fit well into the above categories: 

  • Gluteus medius & minimus – commonly known as the side glutes, help stabilise and control the pelvis (hips) and leg, but also acts as a hip abductor. 
  • Gluteus maximus (the big glutes) – also help stabilise the pelvis, but primarily work as a hip extensor.
Recap – muscles of the core

Any easy way to remember how the body would ideally like use the muscles of the core, is to think about them as lights: 

Stabilisers are like a strong light that is permanently switched on – great for when you want to sit inside reading something to de-stress for hours on end with weight plates balanced on your head..

Movers are like strobe lighting (the fast-flashing lights they warn TV viewers about). Great when you’re out at a techno party and want to dance, (but not practical to support sitting reading; yes 6-pack, we are talking to you).

What does the core do? 

In daily life a strong, a well-functioning core is important for: 

  • Support & Posture – the core should provide the foundational support for the body at all times  
  • Stability & Balance –the core should stabilise the spine and pelvis, responding to movement so you don’t lose your balance 
  • Transfer of forces – a well-functioning core connects the legs to the trunk and transfers the forces between the lower and upper body 
  • Distribution of forces – the core (if used optimally) can help distribute heavy forces more evenly throughout the whole body. This helps prevent excessive overload of one specific part of the body and reduce the risk of potential injuries. 
  • Movement – as the core connects the whole body, it is facilitates all movement. However it’s primary movement roles are rotation, flexion of the trunk.  

The stress formula’s relationship with the core

If you ask an athlete what the formula for stress is, you might get answer like: “Accidentally leaving my paddle at home on the day of a big race…”

But in physics the stress formula is: σ = F / A or in plain English: 

Stress = Force/Area  

If we relate this to athletic performance (which generally adheres to the laws of physics), this means that the smaller the area of the body where force is been directed to, the higher the stress in that particular area (and higher the injury risk), and conversely the greater the area which the force is being distributed across, the smaller the stress on any one specific area.  

The stress formula is particularly relevant to the core as its’ very large surface area (and connections along the whole length of the spine), make it ideally suited to reducing stress on specific areas of the body. However for the core to act as a stress reducer like this, it is not enough for it to be strong, it needs to be well-balanced, and integrated into all movements. 

To remember how the stress formula works, you could think about icecream – if you’ve had a very hard day (Force), then you need a very large tub of icecream (Area) to make the Stress small… (btw I don’t recommend using this analogy when coaching kids, unless you want to spend the rest of the session explaining why eating huge quantities of icecream isn’t a good idea..).    

Why is the core especially important for kayakers? 

A strong, well-balanced, integrated core is important in kayaking for supporting powerful, yet efficient technique. It can: 

  • enable the optimal transfer of power from the footrest through the legs to the trunk and onto the blade 
  • support a strong frame that facilitates rotation, helping to prevent the arms taking over the ‘pull’ phase 
  • generate torque (important for rotational power) 
  • help support good posture in the boat which allows for optimal breathing and efficient movement (including being able to dissociate the arms from the core)
  • balance the forces that result from putting pressure on the blade by distributing them throughout the body, (helping to avoid excessive boat roll, sideways leaning, collapsing of the spine in one or more planes, and slip) 
  • help to prevent unnecessary leaking of energy in all phases of the stroke 
  • assist with balancing the boat  

Note that a strong, well-balanced, integrated core alone doesn’t guarantee the things on the list above, it just provides some of the building blocks needed for them.  

Continue Reading
In Part 2 of this series, we’ll look more closely how core use relates to paddling.

Footnotes
*A two category model of classification for muscles of the core is widely used in fitness and scientific literature, however some researchers and practitioners favour a more diverse classification system such as Gibbons and Comerford’s three category model which divides the muscles of the core into: stabilisers, mobilisers, and load transferers [4].

The two category system is easier for most athletes to understand, and for the same reason ‘movers’ is used throughout this series of posts in place of ‘global mobilisers’. 

References
1. Mense S., Innervation of the thoracolumbar fascia. European Journal of Translational Myology, 2019; 29(3)
2. Niewiadomy et al., The influence of rotational movement exercise on the abdominal muscle thickness and trunk mobility – Randomized control trial. Journal of Bodywork and Movement Therapies, 2021; Volume 27.  
3. Bergmark A., Stability of the lumbar spine: a study in mechanical engineering. Acta Orthop Scand Suppl. 1989; 230:1-54
4. Gibbons SGT, Comerford MJ., Strength versus stability: part 1. Concepts and terms. Orthop Division Rev. 2001; 2:21-27