In the last 10 years ‘core training’ or more specifically ‘core stability training’ has become extremely popular, both in the mainstream gym and sports training environments. The development of a strong and stable core is championed by many as the key to improved/pain free function and sporting excellence. However, there are a number of areas that must be considered in gaining a complete and well balanced understanding of this controversial and often misunderstood area:
- the structures that make up the core
- the function of the core
- how to activate the core
- what equipment is commonly used in core training
- what exercises could be prescribed in a core training programme
Quite simply, if the arms and legs were torn off, the core is what remains. Often the core is considered to include only the abdominal and lower back muscles. This is too narrow a view since when discussing the core muscles the powerful hip and upper back muscles should not be overlooked.
To truly, appreciate the structures involved it is useful to think of the body as being made up of a series of muscle layers – deep, middle and outer.
Deep muscle layer unit
(position sense muscles):
Movements of the spine and extremities can be divided into two categories, physiological and accessory movements. Gross physiological movements are responsible for large motions of the body and allow many functional tasks, such as bending and lifting, to be performed. In contrast, accessory muscles are responsible for controlling movements that occur within a joint, an example would be when bending to pick up an object from the floor the spine moves into a flexed position, but there is also accessory movement at each vertebral segment. Each segment depending on the task will bend, slide (shear) or rotate on top of each other. To control all accessory motions, there are small position sense muscles that cross from one vertebral segment to another.
Position Sense Muscles:
It is of vital importance to have good position sense muscle function if injury is to be avoided. McGill (2002), showed throughmonitoring the electrical activity of muscles that even small-uncontrolled movements of the spinal segments could lead to significant Impairment.
Middle muscle layer (inner unit):
Middle layer muscles include the transversus abdominis (TVA), internal obliques, lumbar multifidus, diaphragm and pelvic floor. When these muscles contract they create a non-compressible cylinder where the spine is stabilised and forms the working foundation from which the arms and legs can function optimally.
Richardson et al (1999), showed that inner unit activation occurs prior to involvement of the extremities and that faulty inner unit recruitment increased the likelihood of low back dysfunction.
Outer muscle layer (outer unit):
Outer layer muscles include rectus abdominis, external obliques, erector spinae, latissimus dorsi, the gluteals and the adductors and form muscle slings. These muscle slings contribute to the ability to maintain an optimal working relationship between joints and to integrate the various body segments for successful motion.
An example might be lifting shopping bags from the boot of a car. First, the hips and trunk need to stabilise, then the shopping bags need to be brought closer to the body to reduce the load. Activation of muscles such as latissimus dorsi and biceps brachii perform the action of drawing the shopping closer to the body. However, all this can only be done successfully if all muscle layers contribute.
Note: It should be seen that the core is made up of muscles from all three muscle layers.
Core Function – Fundamental Principles
Defining core function:
Elphinstone and Pook (1998), define the functional role of the core as:
“The ability of your trunk to support the effort and forces from your arms and legs, so that muscles and joints can perform in their safest, strongest and most effective positions.”
The core (trunk) can be thought of as the ‘crossroads’ of the body, providing a link between the lower and the upper body.
With the above in mind, the main focus of core training is to address any functional deficit in our trunk stabilisers in order to provide the necessary spinal support and a strong and adaptable platform for the actions of our extremities.
The risks of instability:
Panjabi (1992) defines clinical spinal instability as:
“A significant decrease in the capacity of the stabilising system of the spine to maintain the intervertebral neutral zones within physiological limits which results in pain and disability”.
Failure to stabilise/control ones core increases the risk of acute (short term) and chronic (long term) injury to the vertebral column. The key role of the trunk muscles in providing stability to the lumbar spine is well established (Granata and Morris, 2000). Many studies (Richardson et al, 1999; Hodges 2001; Hodges et al 2003) have identified changes in trunk muscle recruitment in clinical low back pain either as a contributing factor to the development of pain or as a result of pain.
It should be noted, that our increasingly sedentary lifestyles do little to promote the optimal function of the core. For example, habitual seated positions do little to promote neutral spines (see note below on the importance of neutral spine position) but rather promote flexed postures which actually place the core at a biomechanical disadvantage. Similarly, the use of back rests reduces the need for core activation therefore increasing the risk of acute and chronic injury to the spine and its associated structures.
Just as a sedentary lifestyle can have a negative impact on core function so too can some of our exercise choices. Within the fitness industry, for example, there is often an over reliance on fixed path resistance machines. Machines are popular choices with both trainers and clients for many reasons; since they offer a supported environment they place few if any demands on the core musculature. These machines also train the body in terms of individual muscle groups and so do little to promote the integrated function of our various body parts. So in effect, machines train us to be strong in isolated muscle groups whilst placing limited demands on the core – if not supplemented and balanced with exercises that progressively challenge the core, this is a recipe for dysfunction and injury.
Spinal discs sit between each pair of vertebrae, providing both shock absorption and an element of support for the spine. Ligaments run the entire length of the vertebral bodies (i.e. the anterior and posterior longitudinal ligaments) and between spinous processes and also help guide and support spinal movement. However, despite, these passive structures, without its supporting musculature the human spine is inherently unstable and can only withstand a load of 4-5 lb. before it buckles into flexion (Panjabi et al, 1989).
It is therefore a basic principle of core stabilisation that during movement, a failure to activate local stabiliser muscles will result in excessive forces being placed on these passive structures.
Intra-abdominal pressure (IAP):
To maintain stability and reduce pressure on the intervertebral discs in the lumbar spine some identifiable core muscles contract simultaneously causing an increase in pressure within the abdomen. As Norris (2000) states
“Intra-abdominal pressure is created by synchronous contraction of the abdominal muscles, the diaphragm, and the muscles of the pelvic floor.”The trunk should be thought of as a cylinder. The diaphragm forms the lid of the cylinder and the pelvic floor the base. The walls of the cylinder are created by the deep abdominals (TVA and the internal obliques).
During contraction of the abdominals the walls are pulled in and up while if a deep breath is taken, the diaphragm is lowered, compressing the cylinder and the abdominal contents from the top. Provided that the pelvic floor (the base of the cylinder) has sufficient integrity, it will resist the action of the diaphragm and the downward displacement of the internal organs (viscera). In this way, a non-compressible cylinder is created. This gives the torso stiffness and a more rigid structure. Such a structure is better able to resist the stresses placed on the lumbar spine, particularly during lifting movements. The spine is stabilised and forms the working foundation from which the arms and legs can function optimally. As Twomey and Taylor (1987) state, making the trunk into a more rigid cylinder reduces axial compression and shear loads and transmits loads over a wider area.
Note: a good example of the natural functioning of IAP would be when muscles contract reflexively to defend the abdomen from a direct blow.
The Thoracolumbar fascia (TLF):
The TLF is a broad, flat fascial sheath that stretches across the thorax and lumbar region and is involved in passive and active stabilisation of the spine. It serves as an anchor for many muscle attachments, especially that of the TVA, and aids stability for the second to the fifth lumbar vertebrae. The function of the TLF can be likened to the tightening of the strings on a girdle around the waist. Stability is created by lateral tension or a pulling action from the TVA and internal obliques that is transferred to the fibres of TLF creating a hoop-like tension through the TLF. This tension produces an extension force on the lumbar spine, which resists the natural pull of lifting movements into spinal flexion. This phenomenon has been referred to as TLF gain (Gracovetsky, 1985.)
The TLF can be seen as adding to the tension and the ability to resist stress of the walls of the non-compressible cylinder created by IAP and therefore, adding to our core stability.
A neutral spine position:
A neutral position for the lumbar spine is midway between full flexion and full extension as determined by the position of the pelvis. The greater the anterior tilt of the pelvis the greater the spinal extension while the greater the posterior tilt of the pelvis the greater will be the degree of spinal flexion.
A neutral spine position is maintained exclusively through muscular activity, thereby placing minimal stress on the passive structures of the spine (ligaments and discs). Furthermore, since in the neutral position the postural alignment of the spine is optimal this is also the best position from which the trunk muscles can work. Consequently, teaching clients to identify and maintain a neutral spine is a key part of any back stability programme.
Integrated core function:
While the above description of the IAP mechanism reveals a significant role for the inner unit musculature in providing core stability we should not overlook the contribution of the more superficial outer unit musculature in this stabilising role. As an example of this, it should be seen that the contraction of gluteus maximus muscles via their attachment to the TLF will have the effect of tightening this fascia. Consequently, efficient gluteal function is fundamental to back stability.
Activating the Core
The act of tightening or stiffening ones abdominal muscles (as if bracing for a punch in the stomach) is believed (McGill, 2002) to be the most effective method of stabilising the core. This bracing technique activates a simultaneous or co-contraction of the abdominal and lumbar extensor muscles. McGill recommends the performance of an abdominal brace in exercise/rehabilitative and functional situations (i.e. picking up a child or getting in and out of a car). To teach abdominal bracing McGill recommends stiffening a joint, like the elbow, to demonstrate. Actively stiffen the biceps and triceps and palpate the muscles on each side of the joint to get the idea. This can be practiced at different percentages of maximum contractions i.e. 10, 20, 50%. Once the basic idea has been grasped replicate this co-contraction on the torso. With abdominal bracing the abdominal wall is neither pushed out nor pulled in.
To progressively train the core muscles, select exercises based on increasing amounts of core contribution. This may be done using a variety of different training modalities, body positions and movements. A possible exercise progression is set out below. This progression first utilises floor-based positions and unstable surface training to address any existing deficiencies in core function. Clients would then be given more functional exercises in standing positions which seek to place demands on the core in all three planes of motion (sagittal, frontal and transverse).
Reasons for Participant Exclusion
All trainers should be aware that there are some conditions which, may be aggravated by physical activity and are beyond the scope of their practice. These would include a prolapsed/bulging intervertebral disc or facet joint syndrome. Such conditions should be identified during a thorough consultation/screening process.
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