The need for a consistent, accepted definition or common understanding for core stability has widespread benefits to health professionals, academics, researchers, athletes, patients and clients. With the increasing need for health professionals to produce treatment plans for third parties like insurance companies, coaches or trainers consistency and acceptability of a definition is critical for effective communication. If a term is to be used with such universal acclaim then it is essential that it is based on sound scientific reasoning and evidence.
To many the ‘pseudo’ new concept of core stability was borne out of the work by Bergmark (1989) who coined the term local and global muscles and a University of Queensland research team that pioneered much of the early work on stabilisation theories involving the transverse abdominus and multifidus muscles (Hides et al., 1994, 1996; Hodges et al., 1996, 1997, 2000; Richardson et al., 1990, 1995, 2002). These findings have been adapted to suit a range of lumbar stabilisation exercise programmes (Stott, 2002). The acknowledgement of the importance of this principle would appear to extend back much further in time. The yoga gurus of India have for centuries described physical states of being that limit them from practising yoga. Two obstacles they describe are vyādhi, (sickness which disturbs the physical equilibrium) and angamejayatva (unsteadiness of the body) (Iyengar, 1979). The body is considered to be the prime instrument of attainment for the yogi. The essence of all yoga movement sequences is to maintain postural control so the body is strong enough to maintain static postures during long periods of meditation (Bhaktivedanta, 1983). In the eastern philosophy of martial arts such as Wing Chun a similar important concept is acknowledged (Chu and Ritchie, 1998). In more recent times Joseph Pilates described the principle of the “power house” which was the fundamental principle behind his much revered ‘Contrology’ exercise system (Larsen, 2004; Gallagher and Kryzanowska, 1999; Muscolino and Cipriani, 2004a and 2004b; Pilates and Miller, 1945). There are many core stability exercise programmes that claim to facilitate the activity of these muscle groups and purport to improve health, strength, performance and body image (Berk, 2001; Blount and McKenzie, 2000; Craig, 2003; Frediani, 2003; Westlake, 2002 and 2003). From a scientific perspective there is relatively little evidence to support the fundamental principles of many of these programmes (Lycholat, 2004). The fact that there is no universal agreement on the parameters of what core stability is makes it extremely difficult to study the concept. Some authors argue that the concept is too “isolationist” and human movement should not be considered in such terms (Siff et al., 2000).
It would appear that a number of terms are used synonomously for core stability including core strengthening, lumbar stabilisation, trunk stabilisation, spinal stabilisation, dynamic stabilisation, motor control (neuromuscular) training, neutral spine control and muscular fusion just to list some of the more common terms (Akuthota and Nadler, 2004). How can it be that all these terms have a similar meaning? They certainly all create a lucid image for individuals familiar with the terms however on reviewing the literature subtle and often significant differences can be found in the application of these terms.
Unfortunately many of the definitions give rise to quite paradoxical arguments. The term functional stability is used freely to describe a level of progression from very basic core stability exercises to those that are more challenging and require greater degrees of control. It could be argued that all stability exercises are functional and task specific. Conversely it would seem absurd to use the term non-functional stability to describe any task. Does not every movement or task have a function? For example, teaching an individual to isolate a transverse abdominis muscle contraction in 4-point kneeling would not appear to be functionally specific for a tennis player however could be for a toddler, disabled person or a wrestler. Perhaps more appropriate terms would be sport specific or task specific stability. These terms could only be used if it was acknowledged that there was a basic foundation level of stability that behaved as a default level of stability that withstood the forces of normal everyday living.
Stability has been defined by Gibbons and Comerford (2001a) as, “central nervous system modulation of efficient low threshold recruitment and integration of local and global muscle systems.” This conceptual definition of stability would allow for all joints in the body to be considered within the above terms. To define core stability using the same parameters it would appear to be as simple as defining the boundaries of the ‘core’. This has consistently been poorly defined in the literature, thus resulting in a variety of definitions terms and concepts.
On considering the anatomical, physiological and biomechanical features of structures that could be considered as part of the core this paper proposes that from review of the current evidence base that the core is made up of the pelvic girdle commencing with the pelvic floor muscles inferiorly (Avery, 2000; Sapsford and Hodges, 2001; Sapsford et al., 2001) , the vertebrae of the lumbar spine and thoracic spine (Berg, 1993; Bogduk, 1997; Edmondston and Singer, 1997), muscles of the trunk including the diaphragm (Cholewicki et al, 1997; Gardner-Morse and Stokes, 1998; Hodges and Gandevia, 2000a), ribs (Berg, 1993), fascia (Vleeming et al., 1995), ligaments (Solomonow, et al., 1998) and discs (Panjabi, 1992a and b) finishing superiorly with the scapulothoracic joint (Mottram, 1997). In simple terms all musculoskeletal structures between the gleno-humeral joints and the hip joints can influence an individuals core stability function. Within this anatomical range the area that has been given most attention in the literature has been the ‘central cylinder’ that is responsible for the control of intra-abdominal pressure (Hodges et al, 2001). The majority of the previously mentioned structures either make up the walls, floor and ceiling of the cylinder and thereby influence trunkal movement and control. The scapulothoracic joint is not directly part of the proposed cylinder model however the scapula has very important soft tissue attachments that influence trunk stability (Vleeming et al., 1995). There are obvious anatomical similarities between the ilia and the scapulae, they provide the stable base for peripheral movement to originate from. It is this authors opinion that the functional anatomical characteristics of the scapulae justify their inclusion in the core stability model as the most superior anatomical border.
The previous example of inconsistencies and confusion with definitions demonstrates how the concept of core stability has become fragmented and generic in meaning (Siff, 2000). It would appear that a small body of evidence has been extrapolated diversely to allow many existing concepts and philosophies on exercise and rehabilitation to become credible and perhaps fashionable. It has also allowed opportunists to benefit financially by capturing some of the health and fitness market by marketing “core stability” products e.g. stability balls and balance boards as being inherently related and necessary to improving core stability function (Lycholat, 2004).
Some expert opinion suggests that the concept of core stability has become too narrow to be clinically useful as most definitions don’t acknowledge peripheral stability or peripheral core stability (Siff, 2000). Most human movement requires a hand or foot to be in contact with a surface so without good peripheral stability the motor task will be deficient even with perfect core stability (Siff, 2000). Such arguments appear to predispose the coining of new terminology even before an existing concept has completed being developed. Comerford and Mottram (2000) try to embrace the above flaws with the all encompassing term movement dysfunction. The risk here is that this concept is too general to apply meaningfully.
On reviewing the literature core stability appears to be primarily focused on central (spinal) joints (Lawrence, 2003; Norris, 2001, Westlake, 2002). Some authors acknowledge that the concept of “core stability” relates to the deeper inner “core” of muscles around any joint which displays anatomical and physiological characteristics that make them primarily suitable for promoting stability of a joint rather than movement (Siff, 2000; Comerford, 2000). This broader definition of the concept could allow the extension of the theory to peripheral joints. Is it reasonable to consider core stability exercises at the wrist, knee or ankle? Whilst the exact definition of what core stability is and where the boundaries lie is debatable, the bigger question is do muscles behave in such a specific way? The work by Basmajian and De Luca (1985) suggested that there was significant variation in EMG activity between individuals performing the same movement pattern. Many authors suggested that some muscles have an inherent function to be stabilisers around joints and others as mobilisers (Comerford, 2000; Cholewicki et al., 1997; Panjabi, 1992a and b). If this is true then why would there be individual variation in muscle activation patterns between individuals performing the same movement task? Does variation in muscle activation pattern highlight dysfunctional movement patterns in some individuals and potentially precipitate injury? If so, when does a movement pattern become dysfunctional? Is dysfunction measured by efficiency of movement or symptom led? If efficiency of movement was a parameter then this could be quantified by measures of time, energy consumption, speed, accuracy and successful completion of task. The problem here in relation to sports science is that many successful athletes have unorthodox movement patterns that would possibly be considered dysfunctional when compared with the general population. It is possible that core stability is the common thread between all individuals and individual variation is as a result of movement patterns of the global mobilisers. While this hypothesis remains unstudied it does open a Pandora’s Box of possible new research in the field of core stability.