perception of stiffness in manipulative physiotherapy

fiysiolherapy %og andndachcc (19%) 11, 35 44 0 Lawrence Erlbaum Associates Limited 1995

Perception of stiffness in manipulative physiotherapy

Christopher Maher

While physiotherapists commonly use manual tests to assess the stiffness of the spine, these tests have been shown to have poor to fair reliability. This paper reviews the literature on stiffness perception and suggests two possible approaches to enhance the objectivity and accuracy of stiffness assessment. T h e first approach is to abandon manual assessment of stiffness and instead rely upon instruments to measure stiffness. T h e second approach involves continuing with manual assessment but adopting strategies that have been shown in other disciplines to enhance manual stiffness assessment. These strategies include clearly defining what physiotherapists mean by stiffness, identifying and controlling factors in the clinical environment that affect stifhess perception, and instituting training programmes that provide quantitative feedback on performance. Changing the method of rating stiffness magnitude, so that stiffness is rated relative to reference stiffness stimuli rather than the physiotherapist’s expectation of normal, may also improve the value of manual assessment of stiffness.

INTRODUCTION

In all areas of physiotherapy, the use of touch is important; however, its use as a therapeutic device is probably most obvious in the field of manipulative physiotherapy. Within this field the physiotherapist uses manual examination tests to make judgements about the presence of tissue thickening, muscle spasm, bony prominences or abnormalities of movement. While a number of terms are used to describe the movement abnormalities, probably the most common is ‘stiffness’. It is this use of the hand as a sensory organ to collect information on stiffness that is the subject of this paper.

C. Maher, School of Physiotherapy, Faculty of Health Sciences, University of Sydney, East St (PO Box 170). Lidcombe, New South Wales 2141, Australia Accepted for publication September 1994

The perception of stiffness has been mainly investigated outside the field of physiotherapy with much of the research undertaken within the food industry, where it is necessary to control food texture in order to enhance consumer ac- ceptability. While this research may not initially seem of relevance to physiotherapy, this is not the case. A significant problem that manipulative physiotherapists face today, the poor reliability of manual assessment of stiffness, was recognised by the food industry 50 years ago and has been the subject of much research since that time. It is quite likely that the results of this research have relevance to physiotherapy. The aim of this paper is to review the literature on stiffness perception in order to provide some insight into the use of manual tests of stiffness in manipulative physiotherapy and so provide options for developing more objective and accurate clinical methods of assessing stifhess.

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36 PHYSIOTHERAPY THEORY AND PRACTICE

BACKGROUND

Re1 i a bi I ity studies

One of the more common forms of research into the perception of stiffness in manipulative physiotherapy is reliability studies. The studies to date that have examined the reliability of judgements of postero-anterior (PA) stiffness have produced conflicting results. While some studies have concluded that physiotherapists produce reliable estimates of PA stiffness (Jorgensson, 1993; Jull and Bullock, 1987; Minucci, 1987), they are typified by poor design or inappropriate statistical analysis. Common problems are the use of agreement measures that do not correct for chance agreement when analysing categorical data, and the use of Pearson’s r or the t-test when analysing continuous data. Those studies that have used a strong design and appropriate analysis (Matyas and Bach, 1985; Binkley, Stratford and Gill, 1992; Maher and Adams, 1994) have concluded that stiffness judgements have fair to poor reliability.

One of the unfortunate limitations of reliability studies is that while they alert physiotherapists to the potential inaccuracy of judgements of stiffness, they do not identify where in the judgement process errors occur. This is unfortunate, because if it were known where the errors oc- curred, it may be possible to develop more reliable methods of stiffness assessment. This option of developing more reliable methods of assessing PA stiffness was originally suggested by Matyas and Bach (1985) but has received little attention to date. One way to begin this development would be to consider the various steps involved in judging PA stiffness and to estimate, based upon the available research, how much error each step is likely to contribute to the total process.

Nature of the task

While many authors advocate the manual assessment of PA s t f iess in patients with spinal pain (Mennel, 1960; Stoddard, 1983; Grieve,

1984), the assessment procedures are most com- prehensively described by Maitland (1 986), who suggests two methods for assessing PA stiffness. The first makes use of the thumb tips and the second the ulnar border of the hand. With the thumb tips method, Maitland advises that the pressure should be applied to the contact vertebra by the arms and the trunk rather than through the use of the intrinsic muscles of the hand, with the fingers straddling the region of contact to provide balance and steadiness for the thumbs. The second technique requires the area of the ulnar border between the pisiform and hook of the hamate of one hand to be placed over the spinous process of the contact vertebra. The other hand then reinforces the contact hand and motion is produced by a rocking movement of the upper trunk with the force transmitted through the arms and shoulders which act as springs.

Maitland (1 986) considers PA stiffness testing part of the general process of ‘passive accessory intervertebral movement’ testing. Maitland suggests that when an abnormality is found, it is necessary to compare the movement to that of the joints above and below the one being tested, the joints on the opposite side, and also to compare this with what would be considered normal movement for that joint. Test results are then recorded using either verbal descriptors (e.g. ‘normal’, ‘hypermobile’, ‘hypomobile’), movement diagrams (Maitland, 1986), categorical scales (Gonella, Paris and Kutner, 1982; Jull and Bul- lock, 1987) or visual analogue scales with criteria provided for each point on the scale (Watson and Andrews, 199 1).

A consideration of the task of rating PA stiflness as advocated by Maitland (1 986) reveals a number of possible reasons why the current clinical method of judging PA stiffness is unreliable. The first possibility is that humans may not possess a very good ability to perceive stiffness; that is, they may not be able to discriminate well between two different stiffness stimuli or to judge accurately the s t f iess intensity. The second possibility is that it is the process of determining whether the perceived stiffness intensity is in fact ‘normal’ that contributes to the unreliability. Third, it is possible that situational factors in the

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PHYSIOTHERAPY THEORY AM) PRACTICE 37

clinical environment may affect the physiotherapist’s ability to judge stiffness. Finally, there may not be complete agreement among physiotherapists as to what they mean by ‘stiffness’. Each of these factors will now be considered through a review of the literature.

PERCEPTION OF STIFFNESS

Stiffness perception studies can conveniently be classified as discrimination studies or scaling studies (Coren, Porac and Ward, 1966). Dis- crimination studies investigate how different two stimuli must be before they can be reliably dis- tinguished. In manual therapy, a discrimination threshold describes what difference in PA stiffness is required for a physiotherapist to reliably detect that one spinal level is stiffer than another. Scaling studies investigate the relationship between the subjective magnitude of a sensation and the intensity of the stimulus. An example of a scaling study in manual therapy would be the investigation of the relationship between the physiotherapist’s rating of PA stiffness on the Jull and Bullock scale (1987), and the objective measurement of PA stiffness.

Discrimination threshold: ’Physiotherapy studies‘

The only physiotherapy stiffness discrimination study to date (Trott, Evans and Frick, 1989) examined the ability of manipulative physiotherapists to perceive changes in the force- displacement curve of a simulated spine. In an increment detection task, the authors concluded that the subjects could reliably detect an increase in stiffness when the stiffness increased by 100%. In this study, the authors adopted the criterion that kappa values significantly greater than zero were evidence of reliable detection. It is more common, however, in psychophysical studies in- volving a ‘yes-no’ response task such as this, to adopt a criterion of 75% correct as evidence of reliable detection. Observing this criterion, when the initial s t f i e s s value was 0.4 N/mm, the subjects required increases in s t f iess greater than

or equal to 3ooo/0 for reliable detection. When the initial stiffness was 0.5 N/mm, the discrimination threshold was 200%.

The estimates of the discrimination threshold for stiffness provided by Trott et a1 (1989) are a little surprising given that the discrimination thresholds for the fundamental quantities used to derive stiffness - force and displacement - are of the order of 10% (Jones, 1986). While it has been shown that more complex physical quantities such as moment of inertia and viscosity have higher discrimination thresholds than the fundamental quantities they are derived from, the threshold is still only of the order of 30% (Kreifeldt and Chuang, 1979; Scott-Blair and Coppen, 1939a). Given this background, it is hard to explain the estimates of Trott et a1 (1 989), which are 20-30 times the value for force and displacement.

If the discrimination threshold for stiffness perception is 200-300%, then this could be one cause for the poor reliability of manual stiffness assessment. One judgement commonly made by physiotherapists in clinical practice is that one spinal level is stiffer than another. One study that measured the PA stiffness of the thoracic spine with a reliable and accurate measurement tool (Lee et al, 1993b) reported that the largest difference in PA stiffness of adjacent levels within the one subject was only 125%. Such a difference would be imperceptible with manual testing if Trott and co-workers’ estimate of the discrimination threshold is correct.

Disc r i m i nation t h res h o Id: ’ No n- physiotherapy studies‘

A consideration of non-physiotherapy studies may explain the very poor discrimination reported by Trott et al (1 989), who used very low stiffness stimuli in the range 0.4-2.0 N/mm. Non- physiotherapy studies show that at low stifFness magnitudes (el N/mm in Jones and Hunter, 1990) the discrimination threshold rises sharply, a situation which is consistent with the study of other sensations. Also, the use of an increment detection task to estimate the discrimination threshold by Trott et al may also have contributed

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to their finding of poor discrimination. This psychophysical method has been noted to be more likely to demonstrate an elevated discrimination threshold at low stimulus intensities (Laming, 1986).

It is important to recognise that the stifFness stimuli used by Trott et a1 (1 989) are much lower than those observed by Lee and colleagues in the lumbar (1993a) or thoracic spine (1993b). In these studies, PA stiffiess ranged from 5.3 to 22.5 N/mm in the thoracic spine and from 9.7 to 27.0N/mm in the lumbar spine. It is likely, then, that the estimate of the discrimination threshold determined by Trott et a1 may not apply to the manual assessment of the lumbar or thoracic spine.

The estimates of the discrimination threshold obtained outside the physiotherapy literature (summarised in Table 1) suggest a much greater ability to discriminate stiffness than the study of Trott et a1 (1989), with the average figure being 12.5%. If this superior level of discrimination can be extrapolated to the clinic, where spinal PA stiffness values may range from 5.3 to 27.0 N/mm (Lee et al, 1993a, 1993b), then it would seem that humans should be able to distinguish between their patients based upon their PA stiffness. That research to date has not shown this to be the case probably means that factors other

Table 1 Stiffness discrimination threshold estimates from outside the physiotherapy literature

Author(s) Discrimination threshold

Harper (1952) Mioche et al (1991) Coppen (1942) Scott-Blair (1954) Scott-Blair and Coppen (1 939a) Scott-Blair and Coppen (1939b) Sheppard (1953) Sheppard and Scott-Blair (1952) Roland and Ladergaard- Peterson (1977) Jones and Hunter (1990) Guinot (1991)

Average

24% 4-11% 5 1 5 % 7-10%

9%

9% %lo%

9-24%

17%

32%

12.5%

1 %Z8%

than stifiess discrimination are likely to be re- sponsible for the poor reliability of clinical judgements of PA stiffness.

Sca I i ng studies: ’Physiotherapy studies’

Three studies have investigated the relationship between the physiotherapist’s estimate of stiffness and its physical magnitude. Chesworth and Koval (1 99 1) compared physiotherapists’ judgements of passive ankle stiffness as recorded on a movement diagram and the torque-angular displacement curves provided by an objective laboratory measurement tool. They found that the physiotherapists produced widely divergent curves and so concluded that the movement diagram drawn by a therapist may not accurately reflect the true passive stifhess. However, it may be that the physiotherapists’ performances were adversely affected by an incorrect expectation of the nature of the torque-displacement relationship. Many of the subjects’ movement diagrams showed a large amount of displacement associated with negligible torque followed by a region in which the torque substantially increased in a curvilinear fashion. While this is the case for most physiological movements of peripheral joints, it is not the case for dorsiflexion of the ankle where the torque providing resistance to movement is considerable by the time the plantargrade position is reached.

Watson and Andrews (1 99 1) compared physiotherapists’ estimation of the compliance of passive patella movement as recorded on a visual analogue scale and a categorical scale, with its objective measurement. The study noted a mean correlation of r= 0.59 between the subjective assessment of compliance and its objective measurement. There was however considerable individual variation, with some raters achieving correlations as high as 0.82, suggesting that at least some physiotherapists can achieve reas- onable estimates of the magnitude of stifkess. Less positive results were found in a study that required 15 physiotherapists to rank the L3 PA stifhess of three pain-free subjects (Viner, Mackey, Porter and Lee, 199 1). Viner et a1 (1 99 1 )

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PHYSIOTHERAPY THEORY AND PRACTICE 39

reported that while the raters could rank the stimuli significantly better than by chance alone, the accuracy of rankings was not high, with only 47% of raters correctly ranking all three subjects.

Scaling studies: ’Non- physiotherapy studies‘

The studies outside the physiotherapy literature suggest a better ability to scale stiffness. Studies have shown high correlations (r= 0.75-0.99) between judgements of stiffness and its physical magnitude (Finney and Abbott, 1972; Mioche, Auroy, Lepetit and Compagnon, 199 1 ; Munoz, Pangborn and Noble, 1986; Perez, Juliano, Bourne and Anzaldua-Morales, 1993; Sheppard, 1953). These high correlations are perhaps a function of the scaling methods used in many of the studies, where the subjects were provided with clear reference stiffness stimuli against which to anchor their judgements of stimulus magnitude. This scenario is unlike the clinical method advocated by Maitland and the protocols used in the physiotherapy reliability studies, where judgements of stiffness are made relative to what would be considered ‘normal’ for the level tested. This comparison is probably a difficult task and it may be that it is this aspect of the judgement process that contributes to the unreliability of PA stiffness assessment. This possibility is considered below.

ARE THE JUDGEMENT SCALES THE PROBLEM?

A wide range of factors has been investigated as potentially affecting stiffness perception (see Table 2 for a summary). This research has high- lighted a number of factors, apart from the stiffness of the stimulus, that will affect the perceived magnitude of a stiffness stimulus. For example, if the preceding stiffness stimulus is of high stiffness, it can produce a contrast effect so that the next stimulus is judged to be less stiff than if it were preceded by a low stiffness stimulus. Series effects also exist, where the perceived magnitude of a stimulus is affected by the series of stimuli within

which it is judged. The same stiffness stimulus will evoke a different sensation of stiffness if judged among a series of highly stiffstimuli than if judged among a series of low stiffness stimuli. Both these effects apply to other sensations and can be readily appreciated by the reader using stimuli such as weights.

The potential for the physiotherapist’s stiffness judgements to be affected by factors other than the PA stiffness of the stimulus to be judged could contribute to the poor inter-therapist reliability observed in previous research. This is particularly likely given that the scales used to record judgements are generally calibrated relative to ‘normal’, for example, Grade 1 on the Jull and Bullock (1987, p. 82) scale adopts the criteria: ‘Less resistance is perceived through normal displacement of the segment; displacement is through a slightly greater than normal range before the normal elastic limiting resistance is en- countered’ (emphasis added).

It may be that the physiotherapist’s ap- preciation of ‘normal stiffness’ can be affected by the factors mentioned previously and so contribute to the poor agreement between raters that has been noted in previous studies. Certainly this proposal is consistent with the stiffness perception literature and may explain why reliability studies have consistently shown that inter-therapist reliability is less than intra-therapist reliability.

It is important to recognise that novice users of the PA pressure will have particular difficulty making absolute judgements on scales such as those advocated by Jull and Bullock (1987). This is because the scales require the user to compare his or her perception of the magnitude of stiffness with normal PA stiffness for that spinal level. Unfortunately, the normal ranges of stiffness, or the potential effects of spinal level, gender and age, are not explicitly stated in manual therapy texts. Rather, it seems that clinicians must develop their own guidelines from experience. This is clearly a problem for novice users of the test, who will have little experience from which to construct their own scale of normal and additionally their judgements are more likely to be affected by series and contrast effects (Sheppard, 1953).

The food texture perception literature suggests

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a solution to this problem. It is possible to alter current practice so that physiotherapists make reference judgements rather than absolute judgements of PA stfiess. As mentioned previously, studies adopting this approach have shown high correlations between judgements of s t f iess and its physical magnitude. Finney and Abbott (1972) have shown that by training raters in the use of such procedures and by providing a series of seven reference stimuli against which to anchor

judgements, raters are able to provide estimates of peach firmness that correlate highly ( r= 0.957) with objective measurements. Similarly, it may be possible to enhance the objectivity and accuracy of clinical assessment of PA stiffness by getting physiotherapists to judge PA stiffness relative to reference stiffness stimuli. This method would address the contrast and series effects that can occur even with experts, and would also be useful when teaching novice manipulative

Table 2 Influence of factors on stiffness perreption

Author(s) Factor Effect

Coppen (1942)

Sheppard (1953)

Sheppard (1953) Harper and Baron (1950)

Scott-Blair and Coppen (1940)

Harper (1952)

Scott-Blair and Coppen (1939a) Coppen (1942) Scott-Blair and Coppen (1940)

Scott-Blair and Coppen (1940)

Scott-Blair and Coppen (1939a) Coppen (1942)

Jones and Hunter (1990)

Harper (1952)

Mioche at al (1991)

Sheppard and Scott-Blair (1952) Mioche et al (1991)

Scott-Blair and Coppen (1939a)

Sheppard and Scott-Blair (1952) Sheppard and Scott-Blair (1952)

Scott-Blair (1954)

Sheppard and Scott-Blair (1952)

Experience

Experience

Experience Experience

Experience

Occupation

Occupation Occupation Occupation

Occupation

Occupation Stiffness magnitude Stiffness magnitude Stiffness magnitude Stiffness magnitude

Stiffness magnitude Method of handling Method of handling Method of handling Method of handling

Method of handling Method of handling

Experience judging cheese firmness led to enhanced discrimination that was not generalisable to non- cheese stimuli Contrast and series effects less marked with experienced judges Experience did not enhance discrimination Experience did not enhance reliability on an absolute judgement task Expert cheese graders less reliable in ranking the stiffness of visco-elastic materials Dairy workers and a blind masseuse had superior discrimination Routine analysts have superior discrimination Routine analysts have superior discrimination Routine analysts more consistent in ranking the stiffness of visco-elastic materials Scientific training had no effect on the consistency in ranking the stiffness of visco-elastic materials Scientific training had no effect on discriminability Discriminability greater for softer stimulus pairs

Discriminability deteriorates at extremely low stiffness values (e l N/mm) Discriminability deteriorates at extremely high stiffness values Discriminability deteriorates at extremes of stiffness; optimal stiffness level is dependent on method of handling stimulus Discrimination the same across the range 13.5-30.5 N/mm Teeth more discriminative than fingers

The subject selected method was most discriminative The subject selected method was most discriminative Testing the stimuli twice provides better discrimination than three times which is better than once The dominant and non-dominant limbs have equal discriminability The dominant and non-dominant limbs have equal discriminability

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physiotherapists how to perform the test. This approach is currently being investigated by the author to see if it will enhance the reliability of clinical assessment of PA stiffness.

WHAT FACTORS ENHANCE THE PERCEPTION OF STIFFNESS?

Maitland (1986) provides a fairly detailed de- scription of how best to assess PA stiffness and his suggestions provide a useful starting point in seeking to establish the most effective method to test PA stiffness. For example, Maitland (1986) advocates the use of the area of the ulnar border of the hand between the pisiform and hook of the hamate as the point of contact when per- forming the PA pressure in the lumbar spine. The technique thus makes use of a more rigid area of the hand which, according to the theoretical work of Peleg (1980a, 1980b) and the investigation of Mioche et a1 (1991), results in enhanced stiffness discrimination. Maitland (1986, p. 76) also advocates the use of two or three oscillatory movements when judging PA stiffness. This suggestion is consistent with the work of Sheppard and Scott-Blair (1952), who demonstrated that stiffness discrimination was greatest when the stimulus was compressed two or three times rather than only once. Therefore, Maitland’s suggestions, presumably based on clinical experience, have a firm theoretical basis.

However, an overly strict adoption of Mait- land’s (1986) protocol that forbids the physiotherapist from adapting Maitland’s methods to their own body type may be counterproductive. Two studies have shown that allowing subjects to use their natural method of assessing stiffness stimuli results in enhanced discrimination (Scott- Blair and Coppen, 1939a; Sheppard and Scott- Blair, 1952). Physiotherapists should thus be guided by Maitland’s suggestions but allow some latitude in performance of the test, particularly if the variant feels more comfortable or natural to them.

Maitland (1986) suggests that the degree of feel afforded by the pisiform grip is lost when stronger pressure needs to be applied and so the therapist should change to the pisiform grip

version of the PA pressure. This suggestion is consistent with the results of Mioche et a1 (1991), who demonstrated that the method of handling the stiffness stimuli can affect the sensitivity of stiffness discrimination, with the superior method dependent on the stiffness magnitude. At present, the discriminability of each version of the PA pressure, however, has not been investigated and so Maitland’s opinion awaits empirical support.

It may be possible to enhance the ability of student physiotherapists to judge stiffness intensity through a training programme. A recent study by Keating, Matyas and Bach (1993) showed that the ability of physiotherapists to apply specified forces during PA central pressure testing was improved by a training programme that utilised quantitative feedback on performance. At present, the feedback that is generally provided to physiotherapists learning to use the test is, at best, qualitative in nature. Novices are generally asked to test the spine and compare their stiffness estimate to that of the teacher. Given the previous research that has consistently shown that manual assessment yields unreliable estimates of PA stiffness, and the perceptual literature that suggests that experience per se does not result in enhanced stiffness discrimination (see Table 2), the teacher’s feedback may in fact be incorrect and so not assist learning. A better approach may be to get the novice to practise rating stiffness stimuli of known magnitude. These stimuli could be patients who have had their PA stiffness measured with an objective measurement device such as that developed by Lee and Svensson (1 990), or the stimuli could be provided through a simulated spine such as that developed by Trott et a1 (1989). By providing novices with immediate quantitative feedback on their performance, it may be possible to enhance their ability to judge stiffness magnitude.

It is important to recognise that situational factors apart from stimulus intensity can exert a powerful effect on the perceived magnitude of stimuli and create what are known as sensory illusions, a well-known example being the size- weight illusion. With this illusion the larger of two objects of identical weight is consistently reported as heavier when lifted. Importantly, this illusion is not extinguished when vision is

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removed, and is present in congenitally blind subjects and so the illusion appears to be a haptic (manual) phenomenon (Ellis and Lederman, 1993). While stiffness illusions have not been formally recognised, certainly a number of non- stiffness factors have been shown to affect stiffness perception (e.g. temperature and stimuli di- mensions) and so these may come to be regarded as stiffness illusions. The critical issue is that the non-stifhess factors that affect stiffness perception need to be identified so that their potentially confounding influence may be controlled during the clinical assessment of PA stiffness.

WHAT DO PHYSIOTHERAPISTS MEAN BY STIFFNESS?

The last factor that may contribute to the unreliability of clinical assessment of PA stifiess is that there may be confusion between raters on the meaning of ‘stiffness’ or how ‘stiffness’ relates to other terms physiotherapists use to describe what they feel when they assess the spine, such as ‘end-feel’ or ‘resistance’. While stiffness has a clear unambiguous meaning in mechanics, it needs to be established that this is also what physiotherapists mean by stiffness. While this may initially seem an odd remark, there is evidence within the physiotherapy literature to suggest that what the physiotherapist perceives as stiffness (i.e. sensory stiffness) may not be equivalent to mechanical stiffness.

Certainly, some authors have fostered the view that sensory stiffness is equivalent to mechanical stiffness. For example, Jull and Bullock (1987, p. 72) suggest that ‘. . . manual examination be regarded as a basic in uiuo assessment of the load-displacement characteristics of the particular direction of movement at a segment’. Additionally, movement diagrams (Maitland, 1986) can look very similar to the force- displacement curves that are used to describe the stiffness properties of musculoskeletal soft tissues. However, Maitland and other advocates of manual stiffness tests, at times use mechanical terms imprecisely. For example, Maitland (1 986, p. 35 1) uses the terms ‘resistance’ and ‘stiffness’ in- terchangeably, which suggests that perhaps sens-

ory stiffness may not be equivalent to mechanical stiffness. Examination of the original mQvement diagrams (Hickling and Maitland, 1970) reveals force-displacement curves that are essentially linear with no toe region and so quite unlike the force-deformation curves obtained when testing musculoskeletal soft tissues in uitro. It seems that over time, as physiotherapists have become fa- miliar with the literature on soft tissue mechanics, the movement diagram has evolved to its present form.

This potentially erroneous use of mechanical terms to describe sensory judgements was first described by Katz (1938), who found that the judgements of dough ‘body’ or ‘spring’ made by bakers did not correspond to the physical quantities of viscosity or elasticity. More com- pelling is the work of Scott-Blair and Coppen (1940, 1942, 1943), who found that human subjects are able to compare bitumen and rubber for ‘firmness’ when the physical properties of the soft materials differ dimensionally. From a consideration of mechanics it would be im- possible to compare two objects of different di- mensions (it would be like comparing velocity and displacement); however, the subjects in Scott- Blair and Coppen’s studies had no difficulty comparing the bitumen and rubber for ‘firmness’.

Similar confusion arises when subjects rating food texture use the term ‘hardness’ to describe the property a materials scientist would label as stiffness (Boyd and Sherman, 1975). This am- biguity in the use of terms to describe food texture attributes led the food industry to develop a standard lexicon for texture attributes (Szczesniak, 1963). Current practice within the food industry when rating food texture is to clearly define and reference the textural attributes to be judged. In this regard, the reference stimuli probably provide a dual function, in that they direct the judge to the attribute under consideration and also provide a basis for judging the relative magnitdde of that attribute. Such practice results in judgements of food texture that correlate highly with instrumental measures of texture (Finney and Abbott, 1972; Perez et al, 1993). These methods could be readily adapted to assessing the PA stifiess of the spine and may

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well enhance the objectivity and accuracy of manual PA stiffness assessment.

The last piece of evidence that suggests that PA stiffness is not equivalent to mechanical stiffness is provided by studies that have shown that humans can discriminate mechanical stiffness quite well (see Table 2) and scale it acceptably (Finney and Abbott, 1972; Mioche et al, 1991; Munoz et al, 1986; Perez et al, 1993; Sheppard, 1953), yet physiotherapists seem unable to judge PA stiffness reliably (Matyas and Bach, 1985; Binkley et al, 1992; Maher and Adams, 1994). The simplest explanation for the difference in results is that the clinical concept of ‘PA stiffness’ is not equivalent to mechanical stiffness.

CONCLUSION

While the manual assessment of PA stiffness is a frequent clinical procedure, its value remains unclear. Most physiotherapy studies that have investigated this procedure have been reliability studies, the majority of which have demonstrated that the procedure yields unreliable estimates of PA stiffness. The results suggest that manipulative physiotherapists need to develop more appropriate methods for assessing PA stiffness in clinical practice. In this regard, the food industry provides a useful model, as it has recognised the limitations of using touch to determine the mechanical properties of materials. Over 50 years of collaborative research between rheologists who developed methods to measure the mechanical properties of food, and psychologists who developed methods to measure the subjective re- sponses of the people grading the food, has resulted in greatly improved methods of assessing the mechanical properties of food. It is likely that a similar research effort by physiotherapists could result in more objective assessment of PA stiffness.

Possible approaches to enhancing the value of stiffness assessment include the use of measurement tools such as those developed by Lee and Svensson (1 990) and the refinement of current manual methods of assessing PA stiffness. These refinements include a clear definition of the nature of PA stiffness, identification and then control of the factors in the clinical environment that

affect stiffness perception, the implementation of training programmes utilising quantitative feedback to improve the reliability and accuracy of manual stiffness assessment, and a change in current clinical practice so that stiffness is rated relative to reference stiffness stimuli rather than the physiotherapist’s expectation of ‘normal’ stiffness.

Acknowledge men t

The author would like to thank Jane Latimer for her con- structive comments on an earlier draft of the manuscript.

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