There has been a significant amount of work published in the past 15 years investigating the essence of spinal stability (Brown et al., 2003; Chiang and Potvin, 2001; Cholewicki et al., 1997 and 1999; Essendrop et al., 2002; Hodges et al., 2001; Quint et al., 1998; Solomonow et al, 1998 and Wilke et al., 1995). Much of this work is applicable to the investigation behind the concept of core stability. This chapter reviews the evidence behind the anatomical, biomechanical and physiological principles of core stability. Table 8 displays the evidence table for intervention studies that was used when considering guideline statements for this section.
It has been widely reported that spinal stability is primarily the result of interaction between different components of the neuromuscular system (Gibson and McCarron, 2004; Granata and Orishimo, 2001; Kaigle et al., 1995 and Panjabi, 1992a). This can provide both dynamic, controlled movement and variable degrees of stiffness dependent on the functional requirements of the task being performed by an individual or the external forces acting on the individual (McGill et al., 2003).
There is a considerable variation in the literature from both scientific sources down to the popular press that influences much of the fitness industry on what anatomical structures provide the foundation for core stability. The fitness industry is responsible to a large extent for the practical application of core stability training methods to the general population. Unfortunately, much of the material on which they base their programmes are derived from poorly researched publications that are highly biased towards anecdotal evidence and even influenced by companies that produce ‘fitness products’. The aim of the following section is to provide a comprehensive review of what structures have been shown to contribute to core stability and to highlight areas that warrant further investigation.
Simply stated core stability is a term that would appear to encompass structures that provide stability to the trunk. Soft tissue structures that connect the pelvis, spine, ribs and shoulders that have varying levels of responsibility in terms of movement, stability and combined function would make up some of the essential components needed to provide adequate core stability. Panjabi (1992a) described three subsystems that provided a conceptual model for spinal stability: passive, active and neural. The passive subsystem was described as being made up of the vertebrae, discs, joint capsules and ligaments (Panjabi, 1992a). Notably he did not specifically include the pelvis, ribs or scapulae in his model. The active system was described as consisting of the muscles and tendons surrounding the spinal column. Thirdly the neural and feedback subsystem constituted the various force and motion transducers, located in ligaments, tendons, and muscles, and in the neural control centres. Core stability could be considered as the interplay between these three components .
To provide structure to the following the above subsystems will be used as a format to describe the components of core stability. Whilst the concept behind these terms has been proposed by Panjabi (1992a) the following discussion is not necessarily an expression of his views rather a representation of the available evidence from data searches performed.
The components of the passive subsystem do not provide significant stability to the spine whilst in the neutral zone (Panjabi 1992a and 1992b). They provide soft tissue attachment points for muscles and fascia of the active subsystem and attempt to provide a safe anatomical passage for some of the structures that make up the neural and feedback subsystem. The osseous structures that fit within the core stability framework, starting inferiorly, include the pelvic girdle made up of the coccyx (4 fused vertebrae), sacrum (five fused vertebrae) and 2 ilia (Williams et al., 1989). The lower limb connects with the pelvic girdle directly through the connection with the hip joint (Calais-Germain, 1993). The spine interacts with the pelvic girdle through the lumbosacral junction at L5/S1(Bogduk, 1997). The sacroiliac joints within the pelvic girdle play an important role in controlling torsional movements through the lumbopelvic region (Vleeming et al., 1990a and 1990b; Lee, 1999). The lumbar spine is made up of 5 lumbar vertebrae (Bogduk, 1997). The thoracic cage is made up of 12 ribs on each side of the thoracic spine (Edmondston and Singer, 1997). The shoulder girdle articulates with the thoracic cage at the scapulothoracic joint (Motttram, 1997). This effectively provides the superior border for core stability structures and has a similar function as the pelvic girdle but for the upper limb that is, creating a stable platform for the upper limbs to move from and to control the initial transference of upper limb forces to the trunk (Mottram,1997).
Two adjacent vertebrae constitute a spinal segment (Bogduk, 1997; Norris, 2000). They are attached (or articulated) by joints, ligaments, capsules and overlying muscle attachments (Maitland, 1997). There are 3 joints at each spinal segment, the ‘articulating triad’(Norris, 2000). The intervertebral disc forms the joint between the vertebral bodies and the two zygapophyseal facet joints are made up of the inferior articular processes on either side of the upper vertebra and the superior articular processes on either side of the lower vertebra (Norris, 2000; Williams, 1989). The zygapophyseal facet joints have a function in controlling movement and transmitting loads (Goel et al, 1993). They are not designed to be weight bearing joints and only become so in extremes of spinal movement and pathological states such as degenerative disc disease that results in increased facet joint contact pressures and subsequently degenerative changes of the articular surface (Norris, 2000).
Bogduk (1997) described 12 possible degrees of movement at each spinal segment. These possible movements included distraction, compression, translation (lateral, medial, posteroanterior and anteroposterior), side flexion (left and right), axial rotation (left and right), flexion and extension. A coordinated approach by all three subsystems is necessary to control these potential movements (Panjabi, 1992a, 1992b; McGill et al., 2003).
From a biomechanical perspective sacroiliac joint stability has been described as being derived from a combination of form and force closure (Liebenson, 2004, Vleeming et al., 1990 a and b). Form closure is related to the inherent anatomical properties of the sacroiliac joint (Vleeming et al, 1990a and 1990b). Force closure is related to how the muscles, ligaments and the thoracolumbar fascia aid in stabilising the pelvis (Pool-Goudzwaard et al., 1998). During activities such as walking a unilateral force is transmitted up the leg and introduces a shear force through the sacroiliac joint. The muscle-ligament-fascia system works to compress the sacroiliac joints together to ensure stability and hence force closure (Liebenson, 2004a, Vleeming et al., 1990a and 1990b). The study by Avery et al. (2000) demonstrated a possible role for the pelvic floor muscles in the force closure system. The combined evidence for this concept is moderate. No studies were identified that disproved or opposed the above principles.
The ligamentous structures of the spinal segments and the pelvic girdle have definite biomechanical roles in controlling end of range spinal movement and may also factor in the neural feedback system (Panjabi, 1992a). Solomonow et al. (1998) demonstrated how stimulation of the supraspinous ligament influenced activity in the multifidus muscle in both cats and humans, thus providing some evidence to this concept. In terms of the passive subsystem at the pelvis the sacrotuberous and long dorsal sacroiliac ligaments are responsible for limiting nutation and counter-nutation, respectively (Liebenson, 2004a). Nutation is the forward motion of the sacral promontory into the pelvis about a coronal axis within the interosseous ligament (Lee, 1999). Conversely, counter nutation is the backward motion of the sacral promontory about a coronal axis within the interosseous ligament (Lee, 1999). These movements can be bilateral or unilateral.
Ligaments of the spinal segment have been classified as neural arch, capsular and ventral (Willard, 1997). The neural arch ligaments constitute the ligamentum flavum, interspinous ligament, supraspinous ligament and intertransverse ligament (Williams et al., 1989). The capsular ligament surrounds the zygapophyseal joint capsule and is reinforced by the ligamentum flavum (Bogduk, 1997). The fundamental biomechanical difference between fibrous joint capsules and ligaments is the arrangement of collagen fibres (Lee, 1999). The functional role of ligaments are to resist tensile stresses between bones therefore the collagen fibres are aligned in a longitudinal arrangement (Bartlett, 1999; Lee, 1999). The collagen fibres within the fibrous joint capsule are arranged in an irregular, random pattern to allow a degree of extensibility to permit joint movement (Lee, 1999). The ventral ligaments are described as being the anterior longitudinal ligament and the posterior longitudinal ligament (Willard, 1997; Norris, 2000).
The ligaments of the neural arch have been considered to act as a single functional structure (Norris, 2000). Dissection studies have described how the deep abdominal muscles may assist spinal stability by influencing the transmission of forces through the ligaments and fascia (Willard, 1997). The following is an example of this proposed feature. The posterior border of the interspinous ligament is thickened into the supraspinous ligament. The supraspinous ligament combines with the thoracolumbar fascia which further merges with the deep abdominals (Norris, 2000). It has been proposed that this chain of interconnecting structures can prevent the ligamentum flavum from buckling towards the spinal cord due to the force generated by the deep abdominal muscles (Willard, 1997; Norris, 2000). This interspinous-supraspinous-thoracolumbar ligamentous complex not only attaches the fascia to the back of the lumbar spine but also creates a pathway for tension in the extremities to be transmitted to the vertebral column (Norris, 2000). This would become an important consideration during rehabilitation and performance training.
The facet joint capsule has been described as a ‘bridge’ of connective tissue connecting the ligaments of the neural arch with the vertebral body (Wilard, 1997). The posterior longitudinal ligament is not the strongest ligament limiting flexion of the spine (Bogduk, 1997). Bogduk (1997) reported that 52% of the resistance to flexion in the lumbar spine is provided by the combined ligamentum flavum and the facet joint capsule. The ligamentum flavum has the highest amount of elastic fibres of any ligament in the body and is held under tension when the lumbar spine is in a neutral position (Butler et al., 1978). The anterior longitudinal ligament is very strong and extends from the occiput to the sacroiliac joint capsule (Norris, 2000). Rapid loading of the longitudinal ligaments results in a ‘stiffening’ response due to their viscoelastic properties. Due to the hysteresis effect these ligaments become stiffer with repeated loading and therefore become prone to fatigue failure (Hukins, 1987). If the local stabilising muscles were working optimally then it would be assumed that their functional role would be to limit the passive structures from being pushed to their failure point. In this evidence based review no studies were found to prove this point.
In reference to anatomy and biomechanics the thoracic spine has been largely neglected compared with the the cervical and lumbar spines (Edmondston and Singer, 1997). The rib cage and its articulations enhance the stability of the thoracic spine (Shea et al., 1996). Berg (1993) hypothesised that the rib cage-sternum complex may comprise an additional ‘column’ for load transfer and stability. No studies were identified that provided evidence for this. A number of investigative studies demonstrated the importance of stability provided by the costovertebral joints (Oda et al., 1996), thoracic ligament complex (Panjabi et al., 1981; Shea et al., 1996) and intercostal fascia (Jiang et al., 1994). The reduced disc height to vertebral body height ratio compared with the lumbar spine reduces motion of the thoracic functional spinal unit (Kapandji, 1978). Zygapophysal joint alignment in the thoracic spine also limits flexion and anterior translation (White, 1969) however enhances torsional stiffness (Singer, 1989).
While there has been considerable research and opinion on the benefits of a neutral pelvic position and the importance of a natural lumbar curve (Chek, 2004; Cholewicki et al., 1997; Norris, 2000; Panjabi, 1992a) the natural thoracic kyphotic curve appears to be primarily related to the osseous asymmetry of the vertebral bodies and tension in the ligament complex (Singer, 11989). There appears to be little supporting evidence to suggest that increasing muscle activity in the thoracic spine will alter the degree of thoracic spine kyphosis. In a load-carrying task, Klausen (1965) reportedly found greater electromyographic activity in the thoracic spine extensors compared with the lumbar extensors however the thoracic curvature remained unchanged while the lumbar curvature was altered. This finding suggested that osseous and non-contractile tissue have a greater influence over thoracic spine curvature than muscular control (Edmondston and Singer, 1997).
Edmondston and Singer (1997) concluded from their review that, “posture correction is achieved predominately through changes in the cervical and lumbar regions while the less mobile thoracic spine remains relatively unaltered.” They acknowledged that the thoracic curvature would influence patterns of load-bearing movement and the degree of stiffness may produce compensatory changes in the cervical and lumbar regions that naturally have more movement. It would appear the more anatomically static thoracic region provides stable attachment points for muscles and fascia that have the potential to influence intra-abdominal pressure and dynamic control of the lumbar spine. There is moderately good evidence of synergistic muscle activity in the trunk producing a stabilising effect (Brown et al., 2003; Cholewicki et al., 1997 and 1999; Essendrop et al., 2002 and 2004; Hodges et al., 2001; Sapsford and Hodges, 2001; Stokes et al., 2000; Radebold et al., 2000; Wilke et al. (1995). In contrast to these studies which were generally from experiments measuring spine stiffness from rapid or informed loading, Kumar (1997) found that with prolonged load the IAP readings were inconsistent when compared with the magnitude of the load applied. The Kumar (1997) study did not record EMG activity of transverse abdominus or multifidus which would have provided a more indepth analysis of this finding.
Core stability rehabilitation in the thoracic region could potentially require manual therapy intervention as part of the programme to correct biomechanical anomalies. An example would be during active movement assessment of unilateral arm elevation that produced an asymmetrical spinal movement response. This response could be due to an upper thoracic movement dysfunction and may be best managed with manual therapy techniques. Joint mobilisation techniques such as those described by Maitland (1997), Mulligan (1999) and Kaltenborn (1993) cannot be excluded from a core stability programme as abnormal mechanics will affect the neural and active subsystems. Normalisation of physiological and accessory joint movements would appear to be critical in maintaining ideal timing of muscle activation patterns for muscles acting on the trunk. Such statements are currently largely based on observation and anecdote, direct evidence has not been forthcoming to date however some evidence for this can be found in the literature for manipulative therapy. Keller and Colloca (2000) reported on a clinical trial where they investigated the effect of mechanical force, manually-assisted spinal manipulative therapy on trunk muscle strength pre and post manpulation. They found that muscles produced stronger muscle activation patterns immediately following manipulation of the underlying joints. This study could be improved by extending the length of follow up time to determine the lasting effects of manipulation. Research is necessary to develop a greater understanding for the role of the thoracic spine in the core stability concept. The influence that manual therapy techniques can have on improving muscle activation patterns and thereby core stability also warrants further research.
A number of models have been proposed to describe the functional role of the trunk muscles as stabilisers and mobilisers (Bergmark, 1989; Comerford and Mottram, 2000; Goff, 1972; Sahrmann, 2002) The classification schema by Bergmark (1989) divided muscles into two functional categories, the local stabilising and global stabilising systems. The local stabilising muscles were considered to have a primary role in maintaining segmental stability. Many of these muscles have their origin or insertion on the lumbar vertebrae. Panjabi’s (1992a and 1992b) hypothesis of clinical instability highlighted the role of these muscles in maintaining the ‘neutral zone’. The neutral zone has been described as the area of physiological intervertebral motion within which the spinal motion is produced with minimal internal resistance (Gibbons and Comerford, 2001a).
Muscles that make up the local stabilising system are generally considered to be continuously active throughout movement, their activation is not dependent on direction of movement and due to their anatomical close proximity to the centre of rotation of the spinal segments they may also have a proprioceptive role (Gibbons and Comerford, 2001a; Richardson et al., 1999). Panjabi (1992a) suggested they have a role in maintaining the lumbar lordotic curvature.
The muscles that constitute the global stabilising system are multi-segmental, more superficial, generate force to control movement, contractions tend to be eccentric to control motion segments throughout range of movement, activity is direction dependent and muscle activation patterns are phasic (Gibbons and Comerford, 2001b; Richardson et al., 1999).
A third functional classification has been proposed by Comerford and Mottram (2000) which they have labelled the ‘global mobilisers’. The function and characteristics of these muscles is to generate torque to produce range of movement, shock absorption of load, activity is direction dependent and phasic in nature. Muscle contractions are generally concentric in nature therefore produce movement through concentric activity rather than the eccentric control displayed by the global stabiliser muscles. Gibbons and Comerford (2001a) suggested that the global mobility muscles work eccentrically to decelerate high loads and would adopt a stabilising function when under load or when subjected to high-speed movements. Table 7 displays the classification of the local and global muscles of the trunk according to their proposed stability function.
Table 7. Muscle Functional Classification Table. Based on work of Bergmark, 1989; Richardson et al, 1999; Gibbons and Comerford, 2001b and Norris, 2000).
|Local Stabiliser||Global Stabiliser||Global Mobiliser|
|Transversus Abdominus||Oblique Externus||Rectus Abdominus|
|Deep Lumbar Multifidus||Spinalis||Iliocostalis|
|Psoas Major (Posterior Fascicles)||Gluteus Medius||Piriformis|
|Intertransversarii||Quadratus lumborum lateral fibres||Hamstrings|
|Interspinales||Oliquus internus abdominis|
|Iliocostalis Lumborum pars lumborum|
|Quadratus lumborum, medial fibres|
|Obliquus internus abdominis (thoracolumbar fascia insertion)|
Further terms that are used to describe the muscular function of the trunk include the ‘inner’ and ‘outer unit’ (Lawrence, 2003; Richardson et al., 1999). The corset or girdle effect created by the deep stabilising muscles that provides a stiffening effect on the spine is also known as the inner unit (Richardson,et al., 1999 and Chek, 2004). The inner unit of musculofascial support is independent of the outer unit both anatomically and functionally (Richardson et al., 1999). Muscles that are part of the inner unit include the transverse abdominus, multifidus, pelvic floor, diaphragm and posterior fibres of the internal oblique (Richardson et al., 1999; Chek, 2004).
The outer unit has a combined role of stability and movement initiation (Richardson et al., 1999). Movement is the primary function and this takes place in unison with the stabilising role of the inner unit (Richardson et al., 1999 and Lawrence, 2003). The outer unit has been further described as being made up of four subsystems (Lee, 1999). These systems have been labelled the the posterior oblique, the deep longitudinal, the anterior oblique and the lateral system (Vleeming, et al., 1990a and 1990b). The four subsystems are effectively another term to describe the specific roles of the global muscles.
The posterior oblique system of the outer unit is made up of the latissimus dorsi, gluteus maximus and the thoracolumbar fascia. The anterior oblique system includes the external and internal oblique, the contralateral adductors of the thigh and the anterior abdominal fascia.The lateral system includes the gluteus medius and minimis and the contralateral adductors of the thigh. The deep longitudinal system includes the erector spinae, deep lamina of the thoracolumbar fascia, sacrotuberous ligament and the biceps femoris muscle (Vleeming, et al., 1990a and b and 1995; Lee, 1999). Alteration of normal muscle activation patterns or weakness of the inner or outer units would reduce the force closure mechanism of the sacroiliac joint (Vleeming et al., 1990b; Lee, 1999). Lee (1999) proposed that this could result in compensatory movement strategies to accommodate the dysfunction. She suggested that this could culminate in pathology in the lower back, hip and knee. This pathology would most likely be in the form of tissue trauma as a result of the cumulative effect of undesirable forces. This indicates that by maintaining adequate control of these muscle groups and their associated synergies and agonist versus antagonistic relationship that core stability training programmes could either prevent or rehabilitate injuries. The amount that each system contributes to spinal stability is currently still under investigation (Richardson et al., 1999).
The functional unit of local stabilisation (Richardson et al., 1999) or cylinder of stability (Chek, 2004; Hodges et al., 2001) has been used to describe the synergistic actions of the pelvic floor, transversus abdominis, multifidus, and the diaphragm. This conceptual cylinder is made up of the transverse abdominis and multifidus as forming the walls, pelvic floor as the base and diaphragm as the lid. Evidence suggests that with activation of the transverse abdominis muscle there is coactivation of the multifidus, diaphragm and pelvic floor muscles (Avery et al., 2000; Hodges and Richardson, 1996; Hodges et al., 1997; Hodges and Gandevia, 2000; Richardson et al., 1999).
The coactivation of the pelvic floor and diaphragm with the transversus abdominus would act to increase the intra-abdominal pressure and potentially stabilise the spine (Hodges and Gandevia, 2000a; Hodges et al. 2001). The hypothesis here is that this synergistic activity would provide an additional extensor torque to the spine. The pressure in the ‘cylinder’ is optimised if as the abdominal wall is engaged and drawn in causing the pelvic floor to be drawn up a deep breath is taken to force the diaphragm down (Hodges et al., 2001). If the breath is held the intra-abdominal pressure is sustained (Valsalva maneuver). This may be appropriate for heavy loads to prevent bending stress through the lumbar spine (cylinder) (Hodges et al., 1997; Hodges, 1999; Hodges et al., 2001; Norris, 2000). Hodges et al. (2001) found a causal relationship between intra-abdominal pressure and lumbar extensor torque in an in vivo study. Notably this study was performed on a small sample size of 5 males.
The role of intrathoracic pressure in trunkal stability has been much less lauded. Intrathoracic pressure is created during inspiration by expanding the lungs against the rib cage. The role of intrathoracic pressure within the core stability concept appears to have been given very little attention. Norris (2000) considered that the coordination of combined inspiration and abdominal hollowing to increase intra-abdominal and intrathoracic pressure was unsuitable for most rehabilitation programmes. The effect of timing inspiration with effort could result in the use of the Valsalva maneuver which could potentially raise blood pressure to dangerous levels (Linsenbardt et al., 1992).
The thoracolumbar fascia gain is an anatomical principle that has been compared with erecting a tent where the guy ropes (latissimus dorsi, gluteals, thoracolumbar fascia and internal obliques) act together as a team to support the main structure (lumbar spine) (Lawrence, 2003; Norris, 2002). When the transverse abdominus muscle contracts and draws in the abdominal wall a synergistic relationship with the internal obliques is invoked that causes them to contract and increase tension through the thoraco-lumbar fascia (Hodges, 1999). Evidence for this concept has been provided in studies by Gibson and McCarron (2004) and Vleeming et al. (1995). The spinal extensors are encased in a layer of fascia therefore when they contract they act to stretch the fascia and tension it (Norris, 2002). The combined actions of the latissimus dorsi (pulling fascia upwards), gluteals (pulling fascia downwards) and the transversus abdominus and internal oblique (pulling fascia sideways) is to resist bending forces through the spine especially when lifting (Liebenson, 2004c; Norris, 2002; Vleeming et al., 1995). This coordinated approach to spinal stability requires all muscles to be activated in an appropriate sequence and to have adequate endurance capacity (Elfving et al., 2003; Thomson, 2002). This highlights the need for core stability training programmes to consider coordinated muscle function and not just isolated activity.
The transversus abdominis is the deepest abdominal muscle and has been the subject of intense interest in spinal stability models and research (Hodges, 1999; Richardson et al., 1999; Norris, 2000). The transversus abdominis muscle has its origin on the iliac crest, lower six ribs, and the lateral raphe of the thoracolumbar fascia and inserts medially at the linea alba (Williams et al., 1989). The horizontal fibre orientation results in causing a reduction of the abdominal circumference, increased intra-abdominal pressure and increased tension in the thoracolumbar fascia (Hodges, 1999).
The contributing role of transversus abdominis to spinal stability has been investigated from two view points. Firstly, a mechanical evaluation of the transversus abdominis and its ability to contribute to spinal control, and secondly, the influence of the central nervous system on transversus abdominus during specific tasks (Hodges, 1999). Hodges (1999) critically reviewed the evidence on the recruitment pattern of transversus abdominis. Much of the earliest research on transversus abdominis as a potential contributor to spinal control was performed by Cresswell et al. (1992). In this study they considered the surface electromyographical activity of rectus abdominus, internal oblique and external oblique muscles during isometric trunk extension that caused an increase in intra-abdominal pressure. As there was minimal electromyographic activity in these muscle groups it was proposed that the increase in intra-abdominal pressure was due to transversus abdominis involvement. Transversus abdominis was also found to be the only abdominal muscle continuously active during trunk flexion and extension in both dynamic and isometric situations (Cresswell et al., 1992).
The erector spinae muscle group has been found to behave in a phasic manner during trunk movements (Hodges, 1999; Cresswell et al., 1992). This provokes the question of what contributes to the extensor torque of the lumbar spine or provides the stablity to keep it in the neutral zone whilst the erector spinae is being latent. The increased intra-abdominal pressure as discussed above provides some of the extensor torque to the lumbar spine however studies have demonstrated the role of multifidus and iliopsoas (psoas major and iliacus) as also having major contributory roles in spinal stability (Gibbons et al., 2002; Wilke et al., 1995). Neuromuscular processes appear to link transversus abdominis and multifidus coactivation (Richardson et al., 1999). A number of studies have highlighted the combined action of transversus abdominis and multifidus in maintaining spinal stability (Essendrop, 2002; Hodges et al., 1999).
Physiological evidence of the lumbar multifidus and the lumbar longissimus that is suggestive of their capacity as integral spinal stabilisers has come from studies of their histochemical composition, capillarisation and muscle enzyme activities (Richardson et al., 1999). Research indicates that these muscles have a greater capillary network, concentration of oxidative enzymes and type I fibres which would allow them to perform with a much greater endurance capacity compared with normal skeletal muscle which have a relatively even supply of type I and type II fibres (Jorgensen et al., 1993; Astrand and Rodahl, 1986). This supports the proposed role of these muscles in contributing to a tonic holding and therefore stabilising function of the lumbar spine.
The role of the internal oblique in the local stability mechanism hasn’t been fully explored (Richardson et al., 1999). There appears to be a stabilising role due to its posterior attachment to the lower lumbar section of the thoracolumbar fascia. It is therefore possible that it may contribute to stability when the transversus abdominis causes tensioning of the thoracolumbar fascia by assisting stabilising forces through the lower lumbar levels (Richardson et al., 1999).
In this review of the evidence behind the concept of core stability the scapulothoracic joint has been considered within the physical boundaries of the core stability model due primarily to the role that muscle and fascia that attach onto the scapula have in stabilising the trunk. Stability at the scapulothoracic joint is hypothesised to be due to a co-contraction between serratus anterior and the trapezius muscles (Mottram, 1997). It is thought that they have a similar stabilising role as transverse abdominus as muscles that are preactivated before upper limb movement (Mottram, 1997). This review found no direct evidence to prove this point however new research is currently investigating this feature.
The latissimus dorsi originates from the sacral and iliac crests, thoracolumbar fascia, spinous processes of T7-T12 and the posterior surface of the lower ribs (Calais-Germain, 1993; Williams et al., 1999) . The muscle inserts on the bicipital groove of the humerus however also has a slip to the inferior scapula angle so therefore may influence scapula position (Calais-Germain, 1993; Mottram, 1997). Conversely, it can therefore be seen from the above attachment points that scapula position could influence lines of tension along the thoracolumbar fascia and therefore trunkal stability. Other muscles that should also be considered when assessing scapula alignment and function in relation to core stability are the rhomboids, subclavius, pectoralis minor and levator scapulae (Calais-Germain, 1993; Kibler, 1995). While not having attachments to the spinal column or rib cage the rotator cuff muscle group could also have some influence on scapula alignment and therefore secondarily on core stability (Horsley, 2005). The kinetic chain between the shoulder girdle and thoracic spine has been considered by a number of authors who acknowledged the influence of muscle imbalance and thoracic spine mobility on scapula control (Kebaetse et al., 1999; Lee, 1995; Janda, 1991). No direct evidence was found to support the influence of scapula position or control on core stability function.
Another possible consideration would be to consider the scapulothoracic joint as part of a second cell of peripheral core stability units. Muscle stabilising principles similar to the lumbar spine could be applied to the shoulder, elbow, hip, knee and ankle joints so that potentially these areas could be viewed as having their own unique core stability features. This potentially controversial view could provide geater continuity in describing the cause and effect of movement dysfunction throughout the body.
Control and Feedback Subsystem
McGill et al. (2003) acknowledged the importance of the sensory organs when considering the motor control response to sudden spine loading. They reported that the proprioceptive feedback gained from a variety of proprioceptors including muscle spindles, Golgi tendon organs, joint receptors and cutaneous receptors was an important drive to directing motor responses to attempted perturbations of the spine. Conversely, it has been well established from back pain research that subjects with lumbopelvic dysfunction have impaired muscle activation patterns, poor postural contol and a diminished concept of a neutal pelvic position (Radebold et al., 2000).
Panjabi (1992a) expressed his opinion that while there was no published evidence that spinal stability could be enhanced by improving neural control alone he felt the possibility existed. This current review found a number of studies that intimated that neural adaptations had occurred through low level training of the deep stabilising muscles (Davidson and Hubley-Kozey, 2005; Richardson et al., 1990; Hagins et al., 1999; O’Sullivan et al., 1997a). Muscles were trained at levels below that required to cause hypertrophy and strength gains however were able to be activated at higher intensities (measured by % maximum voluntary contraction) following training programmes. It has been suggested that this is the direct effect of enhancing neural pathways (Davidson and Hubley-Kozey, 2005; McGill et al., 2003).
A decade on from Panjabi’s (1992a) hypothesis came the work by Holm et al. (2002) who investigated the role of viscoelastic structures with special focus on their sensory motor functions. The intervertebral disc, fibrous joint capsule and spinal ligaments are included in the passive subsystem for their physical properties however their ability to monitor sensory information warrants their inclusion in the neural and feedback system. The background hypothesis for Holm et al. (2002) was that lesions in avascular supporting structures could cause perturbations in the proprioceptive function of various receptors and result in increased and prolonged muscle activation that may cause pain. They further suggested that subsequent irritation of low threshold nerve endings in the sacroiliac joint, intervertebral disc and zygapophyseal joint tissue may trigger a reflex activation of the gluteal and paraspinal muscles over time that may provoke further pain.
Another consideration here would be the impact of a ‘double crush’ like syndrome where a peripheral nerve pathway could be compressed twice e.g. centrally due to derangement of an intervertebral disc and peripherally due to tonic muscle spasm. In this scenario the nerve root irritation due to the disc compression would inflame the neural tissue and provoke muscle spasm along the course of the nerve root and therefore further compress the peripheral nerves potentially irritating them locally. This could then setup a chronic pain cycle. While there is a potential degree of overactivity due to the neural irritation there could also be associated inhibition of some muscle groups (Arvidsson et al., 1986). As the concept of core stability appears to rely on a coordinated pattern of muscle activation it would appear that aberations in the passive and neural subsystems would undermine core stability function.
Holm et al. (2002) used 80 domestic pigs during their investigation of the neuromuscular interaction between the spinal structures. They found that reflexive contraction of the multifidus and also longissimus muscles resulted following electrical stimulation of the lumbar afferents in the discs, capsules and ligaments. The muscular activity was greatest at the level of excitation and found to have weaker excitation at 1-2 levels above and below (Holm et al., 2002). This finding is suggestive that elements of the passsive subsystem that help to maintan the neutral zone as described by Panjabi (1992b) also have a role in monitoring sensory information and controlling spinal muscles especially the multifidus which has been reported to have a local stabilising role in a number of studies (Hides et al., 1996; Jemmett et al., 2004 and Moseley et al., 2002). They also found that the multifidus and longissimus muscles had increased levels of activation due to mechanical stimulation of the viscoelastic structures. Electromyographic recordings were greater if more than one viscoelastic structure was stimulated (Holm et al., 2002). Holm et al. (2002) also suggested that the viscoelastic structures provided kinesthetic perception to the sensory cortex.
Panjabi (1992b) hypothesised that if any one or more of the subsystems weren’t functioning optimally then overall spinal stability could be compromised. This could equate to influencing the core stabilising system to the same extent. Dysfunction of the passive subsystem could be the result of mechanical injury, development of microfractures in the endplates, disc protrusion or degeneration (Panjabi, 1992a). These disturbances in the passive subsystem could result in compensatory changes in the active subsystem due to the decreased load bearing and stability capacity of the passive subsystem. The alteration in muscle activation patterns would influence the core stabilising capacity. Any disruption to the neural components serving the active musculoskeletal subsystem could affect the muscle activation patterns and influence feedback of muscle tension to the neural system (Arvidsson et al., 1986; Panjabi, 1992a). It has been suggested by Panjabi (1992a) that perturbation of the active subsystem could influence the ability to provide compensatory muscle activation for stresses that challenge the passive subsystem and also erode the capacity to maintain trunk stability when subjected to unexpected multidirectional forces or heavy loading (Panjabi, 1992a). In this hypothesis it can be seen that trauma, disuse, degeneration and disease processes that affect the active subsystem would influence core stability directly. While there appears to be little direct evidence for this, there is some evidence to suggest that these noxious factors can influence muscle function and therefore potentiate instability of the trunk (Hides et al., 1994 and 1996; Silfies et al., 2005).
Dysfunction to the subsystems as described above would have far reaching effects on core stability rehabilitation programmes as these variables would need to be optimised or reduced below a threshold that could cause significant influence to the coordinated stabilising function of the subsystems. The majority of core stabilising rehabilitation programmes have an emphasis on physical training however the above discussion on reasons for dysfunction to the subsystems highlights the need to consider treatment of pathological states such as disc prolapse or ligament disruption with surgical and/or physiotherapy intervention. There is evidence to support both these modalities in the treatment of musculoskeletal disorders (Gibson, et al., 2005; Moseley et al., 2002).
In terms of the anatomical evidence for core stability much of the traditional anatomical work has been purely observational. Based on the observations and descriptions of structures researchers have hypothesised how they function in combination with each other. In this field there are very few randomised controlled trials so when developing guidelines based on anatomical evidence it would be very unlikely to ever be able to award the highest grading using the current criteria of the SIGN 50 (2004) protocol. There is a large proportion of observational and case series studies and from these expert opinions have been proposed (see Table 8). There has been some work on comparing biomechanical models with in vivo subjects and this has created a framework for predicting perturbations in the living model (Gardner-Morse and Stokes, 1998; Granata and Orishimo, 2001; Quint et al., 1998; Vleeming et al., 1995).
When comparing and contrasting observational studies in functional anatomy with the findings from biomechanical studies and biomechanical modelling, concepts and understandings begin to crystalise as findings from different sources assimilate. The continuity of findings allows concept development to gain momentum and credibility. There is a need for further studies in functional anatomy to allow the understanding of the core stability concept to progress.
The anatomical, physiological and biomechanical considerations behind the core stability concept are clearly complex. Future research will need to be well structured to provide further evidence especially in the areas of neural systems and synergistic muscle function and coactivation sequences. To see comprehensive evidence tables for the studies considered in this research question see Table 14 in Appendix C.
|Table 8. Intervention studies for Question 1: What anatomical, biomechanical and physiological evidence exists that supports the concept of core stability?