What is Core Stability?
The body relies on deep stabilising muscles for resting postural tone and for movement. These stabilisers are commonly referred to as ‘the core’.
This name has been accepted in physiotherapy and fitness circles, but recent research is dramatically changing the way therapists teach traditional ‘core’ exercises.
The original core theory is based on three main muscle groups providing stability for the spine and trunk. These include:
- transverse abdominus (TA) which attaches to the pelvic or illiac crests, lower 6 ribs and posteriorly to the spine via the thoracolumbar fascia or connective tissue. This forms the middle ‘cylinder’ of the core
- the diaphragm underneath the lungs, responsible for control of breathing via the autonomic nervous system
- the pelvic floor which lies at the base of the cylinder and protects the abdominal contents
Traditionally these three major muscle groups have been the focus of specific strengthening exercises for the core. Common exercises have included the classic abdominal ‘brace’ or hold, pelvic floor exercises, and diaphragmatic breathing techniques.
It is now increasingly understood that the old ways of bracing the core are not necessarily the best way to reduce pain and improve function.
A Modern Understanding of the Core
Deep stabilisers, also known as local muscles lie within close proximity to joints of the ribs, spine, pelvis and hips. They are predominantly made of slow twitch or type 1 muscle fibres, which are fatigue resistant and responsible for tonic postural control. In contrast, global or outer muscles are type 2 fibres and composed of fast twitch fibres. These muscles are required for short bursts of speed, and contribute primarily to movement and joint orientation.
There is now strong evidence of a feed-forward mechanism for the deep stabilisers (type 1 slow twitch muscles). What this means is that stabilising muscles contract before movement occurs to provide compressive or stabilising forces for the joints of the hip and spine. Electromyography (EMG) has been used to detect this phenomenon, and any movement away from midline (lifting your arm, kicking a ball) will trigger this anticipatory contraction in a bid to prevent injury. In individual’s with previous history of injury of the spine, this feed-forward mechanism can be delayed due to ongoing back muscle dysfunction.(1)
Anatomy of the Hip, Abdominals and Lumbar Spine
In addition to the well-known core muscles, other muscles are needed for fluid movement and stability at the hips, spine and lower limb. At the hip, these include quadratus femoris, obturator internus and externus and the gemelli. These deep external rotators are responsible for stabilising the head of the femur with the help of the internally rotating gluteus minimis. Together these have been called the ‘rotator cuff’ of the hip region.(2)
The iliopsoas complex consisting of illiacus, psoas major and psoas minor is also important for both stability of the lumbar spine and flexion at the hip. Illiacus is predominantly slow twitch, while psoas major has been found to be predominantly fast twitch in cadaveric studies.(2)
Multifidus is a sometimes forgotten muscle of the core. It consists of small muscles also known as fasciculi which span from the sacrum at the base to the axis at the top of the spine. It play a very important role in core function with deep fibres thought to be responsible for stabilisation of spinal segments (type 1 slow twitch) while more superficial fibres are responsible for joint movement and orientation (type 2 fast twitch).(2) In low back injury, mutifidi have been shown to atrophy rapidly due to pain inhibition, causing changes in long term spinal stability.(1)
The outer abdominals including obliques and rectus abdominus are important for gross movements of the trunk but are not considered to be components of the inner core.
So, How Does the Core Work?
The core as a symphony orchestra……
A wonderful analogy is to imagine the core as a symphony orchestra. Certain instruments need to be played at the right time and volume, whereas others need to remain quiet. The core is very dynamic and thus the muscles need to work together for optimum movement to occur.
The core is often misunderstood and as a result overworked or over-tightened, particularly the outer abdominal muscles. Regardless of whether a muscle is type 1 or 2, it is essential for all muscles to be able to relax (lengthen) and contract (shorten) when required. Just as the orchestra relies on different instruments working in synergy, the human body requires the correct pattern of activation for efficiency of movement and gait.
Eccentric and concentric muscle contraction…..
It is useful to understand the eccentric and concentric action of all muscles (including core muscles). If you consider the muscle biceps, as you lift a shopping bag biceps contracts and shortens (concentric muscle action), but when you lower the same shopping bag biceps is still working (in addition to the rotator cuff and triceps) but biceps is now lengthening (eccentric muscle contraction).
This also applies to muscles of the core such as iliopsoas and transverse abdominus which need to be strong and long, so they are able to move through range. This debunks the myth of bracing the core muscles (which can lead to tight, short muscles). A good rehabilitation program includes re-training for both concentric and eccentric muscle function.
Good core muscles are required for good posture. Weak postural muscles lead to muscle imbalance and compensatory habits. You may suspect a person has poor stabilising muscles as they are often at the office or party leaning on the bench, or sitting slumped or cross-legged. The body loves symmetry and stability and better facilitation of core muscles can go a long way towards improving posture.
Physiotherapy Assessment of the Core Using Real-Time UltraSound (RTUS)
Seeking an appropriate diagnosis is an important part of re-training core muscles. Pathology such as a hip labral tear or a disc injury can cause pain inhibition and atrophy of deep stabilising muscles. This can lead to overactivity of global muscles in a compensatory mechanism, and supervision may be required to assess key areas of muscle dysfunction.
Real-Time UltraSound is a tool used by clinicians to assess deep stabiliser recruitment patterns. It can be used to visualise activation patterns and endurance of upper and lower transverse abdominus (left versus right side), pelvic floor and multifidus. It provides excellent visual feedback which helps with specific re-retraining of muscles.
A Modern Approach to Exercise Therapy
modern approach to core stability takes into account the whole body. This means strengthening where muscles are weak, stretching where muscles are tight, and down-training when overactive globals are causing poor patterns of movement. It also includes thorough assessment of upper limb alignment (dynamic scapular control), lower limb alignment (ankle, knee, hip) and cervico-thoracic, thoraco-lumbar and lumbo-pelvic stability. Pain also needs to be addressed in order to reduce pain-inhibition mechanisms which can impact on proper muscle recruitment.(3)
A Pilates therapeutic exercise program is a great way to initiate relaxation of global muscles, facilitate activation of stabilisers and improve their endurance. Research indicates that supervised exercise therapy targeted specifically at local stabilisers can improve function, reduce pain and restore the normal feedforward mechanism, thereby reducing risk of recurrent injury. Once activation and endurance of deep stabilisers has been restored, functional exercise that involves both local and global muscle activation can be included to progress rehabilitation.(4)
1. MacDonald, D , Moseley, G L, Hodges, P W (2009). ‘Why do some patients keep hurting their back? Evidence of ongoing back muscle dysfunction during remission from recurrent back pain’, Pain.
2. Retchford, T H, Crossley, K M, Grimaldi, A, Kemp J L, Cowan, S M (2013) ’Can local muscles augment stability in the hip? A narrative literature review’, J Musculoskelet Neuronal Interact, 13(1):1-12
3. Cowan SM, Bennell KL, Hodges PW, Crossley KM, McConnell J. (2003) ‘Simultaneous feedforward recruitment of the vasti in untrained postural tasks can be restored by physical therapy’, J Orthop Res, 21 pp.553-8.
4. Tsao H, Hodges P (2007) ‘Immediate changes in feedforward postural adjustments following voluntary motor training’, Exp Brain Res, 181, pp.537-46.