Core stability is the resultant variable of a multitude of factors. This creates a very challenging situation when attempting to create a meaningful measuring system. Considerable attempts have been made to measure core stability objectively however the challenge remains to provide a definitive, reliable and valid tool. A number of systems are currently used in diffferent settings (Ellenbecker and Roetert, 2004; Richardson et al., 1999). The general scenario is to choose a technique that can reliably measure one or two variables that make up core stability that are most relevant to a particular rehabilitation requirement or athletic endeavour . This chapter is not an exhaustive review of all testing methods for core stability however an attempt has been made to introduce and critically appraise some of the better known measures and introduce some of those that are less popular.
Simple clinical tests such as the Active Straight Leg Test described by Mens et al. (1997) provide rapid feedback concerning the stabilising properties of the muscles and identify any compensatory strategies. This test involves the individual lying in supine with their legs extended, then actively raising one leg to 45°. The clinician then observes the movement pattern to see if there has been any loss of control, for example loss of neutral lumbar curve or other compensatory strategies such as rotation of the pelvis, lateral flexion of the trunk or excessive rectus abdominis activity. The advantage of such tests are that they are easily reproducible, economical and can be performed independently by the individual to monitor their own progress and performance which encourages an active role in their rehabilitation. This style of test still has a considerable degree of subjectivity and may only be relevant for low velocity movement patterns. It is also limited by the amount of variables that it can provide feedback on, for example there is no consideration given to the endurance capacity of the stabilising muscles. Lehman et al. (2004) investigated a similar test, the prone leg extension, and found that muscle activation patterns were inconsistent in the group of assymptomatic subjects tested. A limitation of this study was the choice of muscles that electromyographic readings were taken from. No readings were taken from local stabilising muscles. The fact that there were inconsistent readings in the global muscle activation patterns suggested that this form of testing may be unreliable. Further studies are required to evaluate the reliability and validity of this form of testing.
It is possible that a battery of simple, graded clinical tests may be able to provide a more accurate indication of the functional status of an individual’s core stability function. Wisbey-Roth (2000) described a system of graded, clinical tests with numerical scores that were awarded depending on the level of successful performance. The cumulative scores then provided a reference point of the individual’s functional core stability capacity at a specific point in time. The test could then be repeated following theapeutic or sport specific intervention to determine the degree of progress. The negative feature of these sorts of cumulative score tests is their insensitivity to underlying pathology or deficits. It would not specifically highlight a component of core stability that was lacking. It would only provide an insight to the overall level of functioning. Sahrmann (2002) proposed a similar method of graded exercise testing for movement impairment. This review did not identify any studies that investigated the reliability or validity of graded clinical tests for core stability.
There are a number of areas where cumulative scores from graded clinical tests have been very successful including the Glasgow Coma Scale (Teasdale and Jennet, 1974), Functional Capacity Score (Oliveri et al., 2005) and the Cincinnati knee score (Risberg et al., 1999). A rating scale for core stability could be useful when performing a physical examination on a sportsperson preseason to objectively rate a component of their physical capacity. If a valid scale of measures was created a database of norms could soon follow. This would provide a useful source of reference. A practical application could be a footballer on a transfer list being given a core stability score as part of their medical report. Interested clubs could then consider the core stability value of the player in relation to the commercial value and potential injury risk or even performance level. As chapters VII and VIII will highlight research is lacking with respect to the impact of core stability on injury and performance. It would therefore be some time before a concept such as this could be implemented or accepted.
Richardson et al. (1990) used a stability equation based on anatomical features to measure the efficacy of a range of exercises. Their equation was quite simplistic and only considered the anatomical forces acting in a rather limited way. This may be appropriate when examining movement patterns in one plane however more complex tasks would not be well represented by the equation described in this paper. Consideration of physiological individual differences that can effect core stability such as muscle fibre type (Jorgensen et al., 1993), angle of lumbar lordotic curve (Steele and White, 1986), body mass and an individual’s ability to activate their nervous system (Liemohn et al., 2002) would ideally need to be accounted for in a definitive equation. More difficult components to consider are the influence that pain has on inhibiting muscle activation patterns and the subsequent effect this could potentially have on biomechanical efficiency, injury and performance (Arvidsson, 1986).
Richardson et al. (1999) proposed a three-tier model of assessment to assess motor control deficits in the lumbopelvic region. Their model is graduated in terms of sophistication from screening tests using such devices as pressure biofeedback up to diagnostic tests that include invasive measures. Their first tier of testing included simple observational tests of the abdominal drawing-in action whilst controlling lumbopelvic posture during progressive leg loading. This test can be augmented with a pressure biofeedback unit which can provide some quantitative feedback on whether the deep stability muscles are working. Although there is a degree of objectivity this test cannot provide information on specific details of motor control deficits.
The second tier of testing described by Richardson et al (1999) is suitable for clinical assessments and involves non-invasive graded tests augmented with pressure biofeedback and surface electromyography. The tests are designed to be performed by physiotherapists who are familiar with compensatory strategies and can also incorporate the tests into treatment strategies or use results to provide direction for treatment.
The third tier of testing included invasive measures such as electromyography with fine-wire electrodes inserted into the deep stabilising muscles. Richardson et al. (1999) are also developing a protocol that involves the combined use of ultrasound imaging, measures from a pressure sensor and surface electromyography. This protocol is currently under investigation at The University of Queeensland, Australia with the aim that it will be able to accurately diagnose motor control deficits and also direct and evaluate re-education strategies.
A different perspective on the assessment of stability function was proposed by Comerford (2000a and b) who compared strength and stability assessments. He stated that in a strength assessment the individual was assessed by either passing or failing a high load test (or assessed with load) while in a stability assessment, the individual passes or fails a low load test (the load being limb load or movement). Comerford (2000) defined stability dysfunction as being, “... identified by the failure of the movement system under low load testing.” Strength dysfunction was defined as being, “identified by the failure of the movement system under high load” (Comerford, 2000).
The conceptual difference between strength and stability proposed by Comerford (2000) provides support to core stability assessments that are based on movement dysfunction tesing protocols as described by Richardson et al. (1999), Sahrmann (2002) and Wisbey-Roth (2000). Emerging new research in the fields of anatomy, physiology and biomechanics on muscle function with respect to spinal stability provides the opportunity for such protocols to be vindicated by evidence. To give further credit to such concepts that are effectively based on expert opinion it is necessary for appropriate investigative studies to be performed on such assessment protocols.
The lack of objective measures for stability has been one of the major weaknesses of the core stability concept. The use of musculoskeletal diagnostic ultrasound (MDU) as a form of feedback and for measuring cross-sectional areas of deep stabilising muscles such as iliopsoas and multifidus provides direct objective data however it is not necessarily a functional measure that could be related directly to performance or specific functional tasks. Another disadvantage of real time ultrasound and electromyographic techniques is the direct financial cost of both in terms of purchasing the equipment and clinical time required to perform such procedures. If time and money weren’t limitations then the performance of these tests would be common place as they do provide very useful information as described by a number of authors (Kermode, 2004; Richardson et al., 1999). The most appropriate use of MDU would appear to be the early phase of core stability rehabilitation where emphasis is on teaching very specific isolated muscle actions of transverse abdominus and multifidus (Richardson et al., 1999).
Electromyographic (EMG) techniques are frequently used in core stability programmes and research for assessing muscle activation patterns and rates of fatigue (De Luca, 1984; Roy et al., 1989 and 1995). The advantage of surface EMG is that it is non-invasive (Oddson and De Luca, 2003). Koumantakis et al. (2001) found moderate intra-rater reliability for testing the fatigability of multifidus however this study needs to be expanded to consider inter-rater reliability. The use of EMG results when considering muscle activation patterns must be done with caution as a number of researchers have identified inconsistent muscle firing patterns between asymptomatic subjects when performing identical movement patterns. (Lehman et al., 2004; Basmajian and De Luca, 1985). The reliability of such testing has been challenged by a number of researchers (De Luca, 1993; Lariviere et al., 2003; Roland, 1986; Thompson and Biedermann, 1993). Problems such as cross-talk between electrodes, electrode placement, skin preparation for surface electrodes and variations in muscle activation patterns within normal populations undermine the reliability of this form of objective measure.
Ideally core stability tests need to be quick and simple to perform in a clinical environment without being a drain on financial resources. Static pelvic alignment could provide some indirect information on how the core stability muscles are functioning. Pelvic alignment can also be influenced by non-contractile structures so an objective measurement would need to be considered with both static and dynamic variables in mind. This measurement would add very useful data to an initial examination and could be considered in association with dynamic tests such as muscle length and control as well as static investigations such as xrays. By taking a considered overview of the available information from a physical examination there is potential to deduce significant findings from pelvic alignment measures.
Pelvic tilt has been assessed using a pelvic inclinometer. Toppenberg and Bullock (1986) described a technique for measuring pelvic tilt in the saggital plane. They found that their technique was accurate ± 0.25 °. It would be useful to further this study to determine if there were any consistent findings in terms of muscle function with specific deviations of pelvic alignment. Electromyographic or musculoskeletal diagnostic ultrasound could be used to determine if there was any consistent muscle activity that corresponded with pelvic tilt angle. This could provide the clinician with fundamental information that could guide the direction of rehabilitation or training. Lam et al. (1999) investigated the kinesthetic awareness of subjects with back pain compared with normals using an eletromagnetic tracking device. This study did not report any significant differences however the methodology used was fairly insensitive to subtle changes as they considered a large spinal segment from T10-S2. Electromagnetic tracking devices may have a place in measuring spinal position sense however methodological changes are required to provide more information of segmental movement.
Pelvic tilt and lumbar lordosis are intimately linked, with the depth of the lumbar lordosis being influenced by degree of pelvic tilt (Day et al., 1984; Crowell et al., 1994; Bullock-Saxton, 1993). Norris (2000) concluded from a review of the literature that inclinometers were reliable and valid in comparison with lateral radiographs for measuring pelvic tilt (Day et al., 1984; Crowell et al., 1994; Bullock-Saxton, 1993). Levine et al. (1997) reported on correlations of a trigonometric formula for measuring the degree of lordosis with a supine leg lowering task. The formula used by Levine et al. (1997) was: θ = 4 arctan(2H/L), where θ represents the lordotic index, L is the length of the curve from L1 to S2 and H is the deepest part of the lordotic curve. They found that the mean lordosis measured using the above formula was 50.9° in normals and 40.4° in subjects with weak lower abdominals. This type of test would be easy to perform clinically however more research is needed to correlate this with other dynamic test results to determine if there is a useful degree of sensitivity in different populations.
Isokinetic testing has been used as a measure of core stability function over the years especially during the 1980’s however its popularity has waned somewhat recently. While it does have the benefits of good intertester reliability (Dvir, 1996) it generally produces results of very isolated muscle function that are not specific for the tasks the muscle is being tested for (Headley, 2000). Ellenbecker and Roetert (2004) found that results from an isokinetic trunk muscle assessment correlated with results from a functional medicine ball toss. While not being able to provide all the data of an isokinetic test this simple test had the benefits of providing the same results for the overall pattern of muscle dysfunction and was inexpensive, quick and easy to perform.
Brumagne et al. (1999) found that a piezoresistive accelerometer was a reliable method of measuring lumbosacral angle and may have useful applications in more dynamic settings. This could possibly help bridge the gap between static and dynamic tests. This technique warrants further research.
If greater clarity could be achieved in terms of defining core stability then more consistent testing methods could be employed. Currently the clinician has a choice of core stability components to measure such as muscle activity, alignment, fatigability and postural control. Some methods such as EMG techniques with fine needle electrodes measure the function of distinct variables very accurately however others only provide general trends of function such as graded clinical tests. As objective measures develop following new innovations and more tests of reliability and validity are performed, greater opportunity exists to more accurately determine the impact of core stability function on other variables such as human performance and injury.
Many of the testing protocols described in this chapter have been used in studies that have been considered in the evidence tables for Key Questions 2-6. Studies considered as direct evidence for Key Question 2 can be seen in Table 9. Expert opinion and observational studies are not included in these tables however have been incorporated into the previous discussion and Chapter XI where appropriate.
|Table 9. Intervention studies for Question 2: Is there any valid objective measure for core stability? Can any components of core stability be measured to provide a useful clinical tool?