To highlight the problem with the popular use of the term core stability one just has to look at its most popular application, training. Core stability training is often used to describe strengthening (overload or high threshold training) of the proximal trunk muscles (Gibbons and Comerford, 2001). The net effect here is potentially a cocontraction of all regional muscles (local and global muscles). Gibbons and Comerford (2001a) suggested that it may not be appropriate to extrapolate the research on low threshold movement dysfunction and training of the local stability muscle system with strength training of other muscle groups (global muscles). The indiscriminate and haphazard use of the terms strength and stability is a consistent problem with the core stability concept Gibbons and Comerford, 2001a and 2001b).
On review of the evidence concerning specific exercise programmes focused on trunk musculature the majority followed guidelines of segmental stabilisation as described by Richardson et al. (1990 and 1999) (Davidson and Hubley-Kozey, 2005; Hubley-Kozey and Vezina, 2002; Marshall et al., 2005; Souza et al., 2001). Some programmes were more general in approach to exercising the trunk musculature and applied only some of the principles such as a neutral pelvic position (Arokoski et al, 2004, Haynes, 2004; Koumantakis et al., 2005). The majority of the related research is derived from studies in exercise and back pain (Arokoski et al., 2004; Nelson et al., 1999; O’Sullivan et al., 1997; Richardson et al., 2002). The literature consistently described some desirable traits that reportedly constituted the foundations of good core stability such as low threshold spinal stabilisation exercises performed with the pelvis in a neutral position (Bardin, 2003; Hagins et al, 1999).
The established elements of what constitutes a physical training programme include training intensity, duration, frequency and exercise type (Astrand and Rodahl, 1986). One could assume that core stability training could fit into these parameters however on review of the available evidence it would appear that the traditional strength training guidelines do not apply directly to core stability training. Arguably the physiological adaptations required to enhance core stability muscle function are as specific as their functional roles (Boyle, 2004; Liebenson, 2003). This evidence based review has highlighted that training protocols described for strength and power training should not be directly correlated with core stability training. Overload training programmes tend to recommend a training intensity of between 60-80% of 1 repetition maximum (RM) (Astrand and Rodahl, 1986; Campos et al., 2002) A number of studies demonstrated improved function of the core stabilising muscles at training intensities less than 40% of a maximal voluntary contraction (Davidson and Hubley-Kozey, 2005; Hubley-Kozey and Vezina, 2002; Sung, 2003). This would indicate that factors other than muscle fibre hypertrophy have contributed to enhanced muscle control such as neuromuscular adaptations (Sung, 2003). This lends support to the programmes that encourage low intensity exercises of the deep stabilising muscles (Richardson et al., 1999 and Stott, 2002).
Campos et al. (2002) evaluated the effect of three resistance-training programmes (low repetition 3-5 RM, intermediate repetition 9-11 RM and high repetition 20-28 RM) on the physiological responses in men. The high repetition group (2 sets of 20-28 RM) were the only group to show improvements in endurance capacity, aerobic power and time to exhaustion. The low repetition group (4 sets of 3-5RM) demonstrated maximal strength gains. The target muscle group in this investigation was the quadriceps muscle. The methodology of this study should be repeated specifically on muscles that make up the core stabilisers. In its current form the results of this study provide an indication that endurance training requires high repetition rates.
While the benefits of low intensity stabilising exercises have been heralded in core stability exercise programmes for the past decade there appears to have been a lack of investigation into the effects of higher intensity exercise after the initial muscle activation patterns have been enhanced through low grade exercise. The natural progression following achieving enhanced muscle activation patterns during low intensity stabilising exercises has tended to be functional training. Functional training has been described as, “a continuum of exercises that teach athletes to handle their own body weight in all planes of movement” (Boyle, 2004). Essentially functional training involves maintaning the principles of core stability training and reproducing these concepts into task specific exercises. The specificity of training principle is well established and this progression makes perfect empirical sense however this review failed to find intervention studies on this topic. Interestingly no studies were found that considered the effects of progressing to higher intensity core stability training following achieving adequate standards of basic, low intensity exercise. Perhaps this progression is unnecessary for everyday life and normal activities of daily living though this level of conditioning could potentially influence performance at elite exercise levels. There may also be a role for this progression in high velocity , high impact sports such as rugby where perturbations to spinal alignment are caused by large forces. This is an area of potential future research.
The effect of traditional strength training on spinal stability has been poorly reported (Linton and van Tulder, 2001; van Tulder et al., 2000). Studies that have considered the effects of strength training about the spine have tended to use pain as the outcome measure rather than stability. The holistic benefit of an integrated strength and stability exercise programme on an individual’s pathology and performance could prove to be a very beneficial combination that warrants further scientific investigation (Boyle, 2004).
Maintaining the neutral lumbar spine position with low intensity exercises appears to be important from a specificity of training point of view (McGill et al., 2003; Richardson et al., 1990 and 1999; Stott, 2002). Classic abdominal curl exercises were originally performed with the pelvis placed in a posteriorly tilted position (Stott, 2002). This not only resulted in increased intradiscal pressure but also recruited the global muscles such as rectus abdominus greater than the deeper stabilising muscles like transversus abdominus (Norris, 2000). Of note many of the early Contrology exercises described by Joseph Pilates (1945) were originally performed with the pelvis tilted in a posterior direction. The influence of recent researchers like Essendrop et al. (2002), Hides et al. (2001), Hodges and Gandevia (2000), Panjabi (1992a) and Richardson et al. (1999), has resulted in a change in the method of Pilates teaching to incorporate the neutral spine. This has bourne a whole new genre of exercise known as Clinical Pilates. The proponents of this branch of exercise have been proactive in implementing new research and findings in the area of segmental spinal stabilisation exercise into a formal training programme (Stott, 2002). By having accesible exercise programmes like Clinical Pilates it allows easy access for the public to exercise programmes based on scientific evidence. What the public is not aware of is that many studios teach the traditional Pilates techniques that do not apply the findings from recent research. It is very important to consider the fundamental principles of an exercise programme that is being considered primarily for its core stability benefits. As there is no universal registration body that controls Pilates standards there is an unfortunate inconsistency in how this exercise format is taught.
There is much variation in the types of core stability training programmes described in the literature however there appears to be some consistent parameters that exist between the different programmes. These common attributes may provide the essence to what parameters make up the essential components of core stability training. The following discussion will attempt to define the essential components of a core stability training programme based on a review of the available evidence. Recommendation for further research will be made as appropriate.
Core Stability Training (CST) Protocol
Stage 1 of CST: General Assessment and Biomechanical Analysis
A comprehensive subjective examination is important as information about variables that can influence core stability can be identified. Of prime significance are mechanisms of injury, presence of pain, previous operations, congenital disorders and physical training techniques (McRrae, 1997; Sahrmann, 2002).
A biomechanical analysis commencing with a static observation then a dynamic overview of the lumbopelvic, thoracolumbar, scapulothoracic, upper and lower limbs provides an initial reference point for assessment of core stability function (Norris, 2000; Richardson et al., 1999; Horsley, 2002; Sahrmann, 2002) Consideration must be given to all factors acting on this region that may cause a perturbation from the position of ideal equilibrium (neutral pelvis) in both a static and dynamic environment such as muscle length (Toppenberg and Bullock, 1986), joint stiffness (Cyriax, 1996; Kaltenborn, 1993; Maitland, 1997; Mulligan, 1999), neural mobility (Butler, 1991) and muscle function (Richardson et al., 1999; Wisbey-Roth, 2000; Sahrmann, 2002)
A valid objective measure for core stability should be taken to assist with assessment and diagnosis of components that need to be optimised and to allow for comparative monitoring in the future (see Chapter VI). The measures taken should depend on the presenting condition and the ultimate goals of the individual. Components that have objective measures include spinal alignment (Liemohn et al, 2002; Toppenberg and Bullock, 1986), muscle control (Richardson et al., 1999; Sahrmann, 2002; Wisbey-Roth, 2000), muscle activation patterns (Hodges, 2005, Oddson and De Luca, 2003) and endurance capacity (De Luca, 1984; Koumantakis et al., 2001; Roy et al. 1989 and 1994).
Stage 2 of CST: Treat pathology and implement biomechanical corrections
It may be appropriate to provide treatment to specific musculoskeletal injuries that may directly cause movement dysfunction or pain. (Commerford and Mottram, 2001). Pain can cause muscle inhibition which results in compensatory strategies and dysfunctional muscle activation patterns (Arvidsson et al., 1986; Hides et al, 1994 and 1996; McConnell, 2002). This can be very important with spinal rehabilitation as pain can limit exercise capacity which prevents efficient treatment of musculoskeletal dysfunction (Richardson and Jull, 1995). It may be appropriate in these cases to create a ‘pain free window’ to allow the performance of appropriate exercises (Hackley and Wiesel, 1993). The pain free window can be provided by oral medications or interventions such as nerve root, facet joint or epidural injections. Physiotherapy treatment has been shown to be effective in decreasing pain and improving function and could be used in conjunction or independently of other therapeutic interventions. Efficacy of physiotherapy treatment can be found in the Physiotherapy Evidence Database (PEDro) (Moseley et al., 2002).
This phase involves optimising the previously identified biomechanical deficiencies to achieve a more desirable spinal alignment and to achieve a state of relatively low tonic activity of the local stabilizing muscles while the spine is in the neutral zone (McGill et al., 2003; Panjabi, 1992b). This includes both active and passive structures. Implementation of strategies to decrease excessive compressive forces through the spine that are caused by tight or over active soft tissue structures (Norris, 2000). For example the iliopsoas muscle can cause a significant increase in extensor loading of the spine as well as decrease flexion range if shortened or overactive (Gibbons et al., 2002).
It is desirable to improve spinal alignment as a precursor to improving muscle activation patterns, strength and endurance components (Richardson et al., 1999). Acknowledgement of the mobility of neural structures is important as the restriction of neural mobility may influence muscle activation patterns (Butler, 1991). It is reported that neural mobility can be restricted by adhesions around nerve roots, vertebral joint dysfunction and muscle spasm (Butler, 1991). The area of neuro-dynamics is one that warrants further research.
There is no clear evidence in the literature concerning optimum muscle stretching techniques. The role of stretching is contested in the literature with its position being challenged in rehabilitation, prevention and performance enhancement (Shrier, 2002). Stretching soft tissue structures and mobilising neural structures anecdotally appear to be very important during rehabilitation (Butler, 1991). Scientific evidence has not delivered a concise answer with regard to stretching and CST. More specific investigations are necessary based on the above techniques. Sustained stretching has been reported to recruit slow motor units (Burke, 1968). This may benefit the local stabilising muscles and possibly the global stabilising muscles as they are purported to respond to low loads by encouraging slow motor unit recruitment. This physiological response to sustained stretch may be problematical with the stretching of global mobilising muscles. Implementing contract-relax stretching techniques may be more appropriate for global mobilizing muscles (Norris, 2000). This has not been vindicated by evidence to date. Gibbons and Comerford (2000) reported that the global mobility system is dysfunctional when it responds to low loads e.g. like postural sway where gluteal muscle insufficiency results in overactive hamstrings in single leg stance. Clark (1999) reported that stretching a mobiliser may encourage them to respond to low loads. This is contrary to their proposed function as global mobilisers that have a dominate role in tasks requiring speed and loading with fast motor unit recruitment (Comerford and Mottram, 2000). This is an important component that requires further investigation. It may be possible to create a biomechanical alteration by stretching a muscle group and then immediately performing specific exercises that ensure the optimal muscle recruitment pattern is maintained. This hypothesis warrants investigation.
Stage 3 of CST: Implementation of neuromuscular phase
Commence low intensity exercise focusing on the ‘local muscle’ stabilising system performed in a neutral spine posture (Davidson and Hubley-Kozey, 2005; Hubley-Kozey and Vezina, 2002; Marshall et al., 2005; Souza et al., 2001). These muscles include: multifidus, transversus abdominis and iliopsoas (Erb, 2004; Gibson et al., 2002; Richardson et al., 1999).
Intensity levels up to 30-40% of maximal voluntary contraction (Davidson and Hubley-Kozey, 2005; Hubley-Kozey and Vezina, 2002; Sung, 2003). Tonic, continuous contractions. No phasic or ballistic movements (Richardson et al., 1999).
Frequecy of exercise has been poorly considered. Davidson and Hubley-Kozey (2005) had subjects train 4 times per week at levels < 40% of a maximum voluntary isometric contraction. They found that muscle activation patterns improved significantly following this programme. Sung (2003) reported improved muscle activation patterns due to neural adaptations with a training frequency of 3 times per week. Hagins et al. (1999) demonstrated improvements in subjects who performed exercises 3-4 times per week for 4 weeks. They did not report methodology further to include repetition rate and number of sets. This was a shortcoming in all the above mentioned studies. DeMichele et al. (1997) found that two sessions per week produced the same strength gains as three. It would be incorrect to directly extrapolate this finding to exercises primarily working on stabilising muscles. The frequency of exercise required to produce neuromuscular adaptations is poorly reported in the literature and hence the optimal frequency for training is not established. Current studies such as DeMichele et al. (1997) can only act as indicators for optimum training frequencies. Repetitions of individual exercise and number of sets is poorly reported. Studies have ranged from 1 set of 15 repetitions (Stott, 2002) to neuromuscular fatigue (Nelson et al., 1999). More research is required in this area.
Richardson et al. (1999) recommended isometric type contractions as this would meet the functional characteristics of the local stabilizing muscles as they demonstrated minimal length changes in different spinal positions. Encourage co-contractions of the inner unit of muscles i.e. transverses abdominus (Hodges and Richardson, 1996, Hodges, 1999), multifidus (Hides et al., 1996 and 2001), diaphragm (Hodges and Gandevia, 2000) and pelvic floor (Sapsford et al., 2001; Sapsford and Hodges, 2001). Observe for patterns of breathing and be sure the individual is breathing normally whilst drawing the abdomen in. (Stott, 2002). Progress to movement patterns incorporating cocontractions with the outer unit (global muscles) ensuring they do not mask function of the inner unit (Comerford, 2000; Comerford and Mottram, 2001)
Stage 4 of CST: Advanced physiological adaptations
Improving the endurance capacity of stabilising muscles (McGill, 2001) and functional training (Boyle, 2004; Haynes, 2004; O’Sullivan, 2000). Good evidence available concerning benefits of endurance training (O’Sullivan, 2000; Roy et a., 1999) however functional training guidelines currently lack credible foundation studies.
This simplified 4 stage approach to CST has been created from evidence derived from intervention studies where possible. Further research is required to fill the many gaps to optimise the efficiency and efficacy of this protocol. While the stages have been set in a logical sequence of progression accounting for timing of desirable adaptations it is acknowledged that the maximum potential for optimisation of anatomical and physiological structures will not be achieved prior to each stage progression. Some elements of each stage would be performed concurrently. The challenge here for the clinician is that the current research base has very limited guidelines on determining firstly, the timing of exercise progression and secondly, what exercise type to progress to. Of the many established exercise systems ‘purported’ to enhance core stability such as Pilates, Swiss Ball training, yoga, martial arts and the Alexander technique they all describe progressions of exercise however the decision on when and how to progress an individual is often based on subjectivity or intuition on the part of the clinician or trainer. It would appear that limited work has been performed on exercise progression criteria. This creates inconsistencies in the management of individuals undertaking CST.
Most core stability exercise programmes are based on the hypothesis of lumbar stabilisation proposed by Bergmark (1989), in which he described the concept of local and global muscle function. McGill et al (2003) stated that any exercise can be classified as a stabilisation exercise depending on how it is performed. Essentially by ensuring that sufficient joint stiffness is achieved during the exercise, repetitive practice will develop the appropriate motor patterns for the lumbar spine (McGill et al., 2003). McGill et al. (2003) did not report on thoracic spine or scapulothoracic function as part of lumbopelvic control however the assumption would be that maintaining a neutral thoracic kyphosis and maintaining scapulothoracic control during stabilising exercises is advantageous to overall trunk stability. These theories warrant further investigation.
Intervention studies considered in the development of the guideline from Key Question 6 can be seen in Table 13. More detail for individual studies can be found in Table 19, Appendix H which contains the evidence table for Key Question 6.
|Table 13. Intervention studies for Question 6: Is there any evidence to support specific exercise programmes to enhance core stability?