When you think about core strength do you think about doing crunches and planks? Or taking a nice deep breath in through your abdomen? Well, in reality we should potentially be thinking about both (plus a number of other factors)!
Concepts around the core and ideal exercises to strengthen and stabilise it have developed a lot over the years. Originally a lot of emphasis was put on just the Transverse Abdominis muscle (TA) and the 'hollowing' of this muscle to prevent lower back pain. However, we now understand that many muscles play a role and hidden muscles like the diaphragm are much more important than we once thought!
Why is core stability important?
Core stability is defined as the ability to maintain equilibrium and control of your spine/pelvis, especially during movement.
Even tiny movements require proper stabilization to occur, not only in the local joints involved but in other areas along the functional chain. Often this means activating our core muscles before our limb muscles. Without proper core stabilization, we can't produce strength.
Stability is not just about generating maximum stiffness and rigidity though, it's about contracting all of the right muscles, in the right amount, at the right time. This allows us to produce optimum production, transfer and control of force and motion to the terminal segments in the kinetic chain e.g. arms or legs.
Stability depends on: Active structures e.g. muscles, Passive structures e.g. ligaments and control through Neurologic systems.
So if core stability isn't just about our six pack muscles, what else do we need to take into account? The core can be divided into two main areas...
The 'Inner Core'
Consists of the Diaphragm, Pelvic floor, Transversus Abdominis, Internal Obliques, Iliocostalis and Longissimus (parts of the Erector Spinae group), Multifidus, Lumbar spine and Pelvis including the Sacro-coccygeal joint.
The Diaphragm- has been shown in MRI studies to have postural function that can be voluntarily controlled and is independent of breathing. It can create abdominal pressure which has a significant spinal unloading effect, counter-acting spinal flexion due to it's position against the front of the thoracolumbar spine. Diaphragm control can be assessed by looking at breathing mechanics including lower rib flare. Often those with poor breathing mechanics show inward movement of the abdomen on breathing in, with rib flare and excessive chest movement.
The Pelvic Floor- consists of the Levator Ani muscles (Ilio-coccygeus, Pubo-Rectalis and Pubo-coccygeus), Coccygeus and surrounding ligaments. Many of these muscles attach into a slightly more superficial muscle called the Obturator Internus, which can be worked on with manual therapy.
The diaphragm and pelvic floor work together, so tightness or weakness in one can directly effect the other. Disorders of continence and respiration were strongly related to frequent back pain (Smith, Russel and Hodges 2006 study).
The Transversus Abdominis and Internal Obliques are the only two abdominal muscles that attach into the lumbar spine, via the thoracolumbar fascia. When not working well, other muscles such as the psoas (hip flexor) muscle overworks, creating more of a compressive force to mimic stability.
The 'Outer Core'
Consists of the Rectus Abdominis, External Obliques, Quadratus Lumborum, Gluteals, Hamstrings, Hip Adductors, Latissimus Dorsi, Erector Spinae and Mid to Lower Thoracic spine.
The outer unit is arranged in muscle slings, which work together to produce smooth, supported movement. For example, when walking (gait)...
Anterior Oblique Sling- obliques and hip adductor muscles help with pelvic rotation during the mid stance phase.
Posterior Oblique Sling- hamstrings, SIJ (sacro-iliac joint) ligaments, glute max and lats help SIJ stability and spring during the stance phase.
Lateral Sling- hip abductors, adductors, QL and SIJs help with a smooth swing phase and stabilizing the body during mid stance.
Deep Longitudinal Sling- calf and lower leg muscles such as peroneus longus and tibialis anterior, hamstrings, SIJ ligaments and erector spinae help preload the leg prior to heel strike, prevent too much foot pronation and and help with propulsion.
(For more information on the gait (walking) cycle and phases read my blog on 'Feet...'!)
Combining Inner and Outer Units
These inner and outer units support each other. The inner core needs to be strong to control the forces generated by the outer core muscles. The outer core muscles need good eccentric strength to prevent inner core damage.
We can assess muscle strength and control through muscle testing (see my blog post 'How resilient are you? Muscle activation, strength and the nervous system')
Exercises for the core
Diaphragmatic Breathing progressions... emphasizing abdominal movement as well movement through the sides and back of the lower ribs.
Pelvic Floor... Kegels or pelvic floor release techniques.
McGill and the 'Big Three' core exercises...
Dr. McGill's work has emphasized the need for core stabilization, without excessive spinal compressive/shear forces. He has come up with three of the 'safest' core exercises: The Curl-ups, Side Bridge and Bird-Dog, and classified others based on potential loads on the spine vs potential for a 'good workout' e.g. lots of muscle activity.
Paloff Presses... consisting of isometric anti-rotational control.
Planks/side planks... and lots more!
In Conclusion...
Problems with the core can not only lead to local issues within the inner and outer units, such as diastasis recti and urinary incontinence... because we need core stability before moving and getting optimum strength in the limbs, poor core control could lead to dysfunction or pain elsewhere too through faulty compensations.
Often, I tend to test core control by using muscle testing, breathing observations, spinal/pelvic palpation and orthopedic testing to work out what part of the core is dysfunctional and therefore what needs hands on treatment and/or home exercises. As always, it is then putting this into the broader picture of pain or dysfunction in each individual (i.e. biopsychosocial/holistic approach).
So... you may need to look after your diaphragm as well as doing those planks and curl ups!
Extra Reading:
For patients- https://www.coreexercisesolutions.com/ - core and pelvic floor exercises/advice.
Emma Goodman's classes including Holistic Core Restore https://www.holisticcorerestore.com/coach/emma-goodman/, https://www.emmafitness.co.uk/about/
For practitioners- Jesper Dahl's Core Seminars.
https://pubmed.ncbi.nlm.nih.gov/10618161/- Research by Paul Hodges
Research and Blogs from Dr Greg Lehman