International Journal of Osteopathic Medicine 9 (2006) 113e119

www.elsevier.com/locate/ijosm

Research report

Algometer reliability in measuring pain pressure threshold overnormal spinal muscles to allow quantification of

anti-nociceptive treatment effects

Louise Potter a,*, Christopher McCarthy b, Jacqueline Oldham a

a Centre for Rehabilitation Science, University of Manchester, Central Manchester and Manchester Children’s University Hospital’s NHS Trust,

Oxford Road, Manchester M13 9WL, UKb Medical School Building, University of Warwick, Coventry, CV4 7AL, UK

Received 12 March 2006; received in revised form 20 November 2006; accepted 23 November 2006

Abstract

Background: Algometry has been shown to be an effective way of quantifying pressure pain threshold (PPT), although its reliability

in assessing spinal muscle pain (excluding trigger points) has not been robustly analysed.Objectives: Intra-rater PPT assessment by algometry over the belly of four pairs of spinal muscles (iliocostalis, multifidus, gluteusmaximus and trapezius) in a healthy sample was analysed.

Methods: Healthy subjects had their PPT measured twice (within 5 min) on three occasions (separated by a week). Intra-class cor-relation coefficients and the smallest detectable difference were calculated to analyse the reliability of the measurements and 95%limits of agreement plots were drawn to assess systematic difference.Results: Assessments revealed good within-session reliability (80 assessments) (ICC> 0.91) and good between-session reliability

(ICC> 0.87), with a moderate measurement error (approximately 3 kg/cm2) and no systematic difference within-session orbetween-sessions.Conclusions: PPT assessment by algometry is a reliable, both within-session and between-sessions, measure of a subject’s pain. This

study provides further validity to the use of this measure as a suitable, convenient method of monitoring treatment effects.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Algometry; Pain pressure threshold; Reliability; Treatment effect

1. Introduction

Back problems are hugely problematic and costly andas yet there is no clear consensus on treatment ap-proach.1 It is widely accepted that back pain isa multi-factorial, heterogeneous problem and that ithas biopsychosocial components,2 making it difficultto assess individual treatment effects. Elements of the

* Corresponding author. Tel./fax: þ44 0161 276 6672.

E-mail address: louise.j.potter@student.manchester.ac.uk

(L. Potter).

URL: http://www.medicine.manchester.ac.uk/crs/.

1746-0689/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ijosm.2006.11.002

physical component of back pain are physiological tis-sue changes which provoke nociceptive impulses andcause the perception of pain. One aim of manipulativeinterventions is to attenuate the nociceptive componentof back pain3 thus establishing a valid and reliablemethod of assessing treatment-induced changes in noci-ceptive pain would be advantageous.

Nociceptive pain is that evoked by a noxious stimulusand as such can be assessed by an externally appliednoxious stimulus, as is used in algometry. Algometryhas long been used to measure soft tissue pain associatedwith trigger points4e6 and has been used to assess the ef-fectiveness of treatments which attempt to alleviate these

114 L. Potter et al. / International Journal of Osteopathic Medicine 9 (2006) 113e119

specific tender spots.7 The algometer has been shown tobe an effective way of quantifying the pressure painthreshold (PPT)6 relating well to other clinical pain mea-sures and there is evidence to support the reliability ofalgometry to measure the PPT of trigger points.4 How-ever, there is little evidence for the test-retest reliabilityof the algometer to record the PPT over muscle not hav-ing trigger points. Reeves’ (1986) early study showed thealgometer to be reliable in determining the pain pressurethreshold of active trigger points in the head and neck,in a small group of headache patients. The study byFischer (1987) is much cited as providing further evi-dence of the reliability of the algometer; however, hisstudy collected only one set of measurements for eachsubject and the results were said to demonstrate reliabil-ity on the basis that there was no significant differencedetected in the PPT taken from one side of the bodycompared to the other. Other studies have assessed reli-ability of measuring trigger points in shoulder and neckproblems.4,8 However, there is little evidence for the re-liability of algometry to measure PPT in spinal musclesthat are not exhibiting trigger points.

If the algometer is to be used to assess the effect of an in-tervention it must first be shown to be unequivocally reli-able, with the instrument making equivalent recordingson the same subject on repeated occasions. Thus thepurpose of this study was to assess the within-session andbetween-session intra-rater reliability of the algometer, inthe measurement of pain pressure thresholds over non-trigger but consistent muscle points in healthy subjects.A robust analysis of algometry measurement reliabilityof these muscles has not been conducted previously.

2. Methods

2.1. Subjects

This study was conducted on a convenience sample ofuniversity staff (n¼ 10), one-half of the sample weremale and one-half female (a grouping that would con-trol for gender differences in PPT),9 a total of eight mus-cles were tested per person which generated a sample of80 assessments. Ethical approval for the study wasgranted from North Manchester Local Research andEthics Committee. Each volunteer gave informed con-sent. An asymptomatic subject group (subjects with nocurrent or recent history (<6 months) of spinal pain)was chosen for this study. The subjects attended forthree sessions separated by the period of at leasta week. On each session the experimental procedurewas repeated twice to allow the determination ofwithin-session and between-session reliability. Testingwas performed by one examiner (a registered osteopathwith 15 years of experience). Muscles were tested ina random order to reduce the influence of the memory

of initial recordings on subsequent recordings. A re-search assistant made note of each measurement andthe rater was blinded to the previous measurements.

2.2. Equipment

The pressure algometer used was a pain threshold me-ter, model PTH-AF 2, commercially available through

Fig. 1. Surface electrodes attached over iliocostalis and the superim-

posed crosses show the point that the algometer was applied to the

subject.

Fig. 2. Algometer being applied to a subject to record the PPT.

that the change is due to true difference and not

115L. Potter et al. / International Journal of Osteopathic Medicine 9 (2006) 113e119

the Pain Diagnostic and Treatment Corporation (GreatNeck, NY 11021, USA). The device is a force gauge fittedwith a disc-shape rubber tip bearing a surface of exactly1 cm2. The range of gauge is 0e10 kg, divided into one-tenth of a kilogram. All readings are expressed as kilo-grams per square centimetre (kg/cm2).

2.3. Procedure

This experiment was part of a pilot for a multifacetedelectromyography randomised controlled trial, for whicheach subject had four sets of surface electrodes attachedover the following superficial muscle pairs: upper trape-zius, iliocostalis, multifidus and gluteus maximus. Theelectrode’s position was recorded on acetates alongwith other surface markers, such as moles or scars to en-able exact replication of electrodes’ placement at eachvisit. The test sites for the algometry measurementswere just medial to the joining point of the two electrodes(see Fig. 1), except for the trapezius muscle electrodewhere the testing point was just superior. This ensuredthat testing was approximately in the centre of the mus-cle belly and produced a method for ensuring that mea-surements were taken over a consistent point on eachtesting occasion. This method ensured exact replicationof testing site both intra-session and between-session.

Once the point to be tested was identified the pressuregauge was applied perpendicular to the surface of thesubject’s body (see Fig. 2). Pressure was applied at aconstant rate of approximately 1 kg/s.6 Subjects were

Table 1

Mean of all tests and visits for individual muscle testing points

Muscle Mean

(kg/cm2)

Standard

deviation

(kg/cm2)

Iliocostalis right 8.28 3.33

Multifidus right 8.99 3.65

Gluteus maximus right 8.52 3.27

Gluteus maximus left 7.92 3.11

Multifidus left 9.45 3.64

Iliocostalis left 9.38 3.85

Trapezius left 3.94 1.87

Trapezius right 4.23 1.96

instructed to say ‘yes’ as soon as the sensation of pres-sure changed to pain. It was carefully explained toeach subject that the purpose of the experiment was totest his or her pain threshold and that it was not a mea-sure of his or her pain tolerance. The subjects attendedon three occasions (hereafter referred to as a visit 1, 2or 3) and at each visit the muscles were tested in thesame order on two occasions (hereafter referred to astest 1 and 2) 5 min apart. Subjects had a practice sessionon the back of each hand before the main testing com-menced. Overall each subject had six tests of eight mus-cles making for a total of 48 measurements per subject.

2.4. Data analysis

Intra-class correlation coefficients (ICCs) were calcu-lated to examine the strength of agreement betweentests. The ICC represents the proportion of variance inthe data that is due to the true score variance.10 ICCswere calculated using a two-way random effectsmodel,11 as each subject was assessed by the same rater.This allows for the generalisation of the results to otherraters. ICC model (2,1) was used for within-session anal-ysis and (2,k) for between-session analysis, where an av-erage of scores across trials was used. ICCs werecalculated using the Statistical Package for the SocialSciences (SPSS v11, Chicago, Illinois, USA).

The ICC, which is a relative measure of reliability,can be affected by between subject variability so the re-test reliability was also considered in the units of mea-surement (kg/cm2), by calculating the standard errorof the mean (SEM) which is an absolute index of reli-ability, Eq. (1).

SEM¼ SDffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffið1� ICCÞ

pð1Þ

where SD is the standard deviation of the grand mean.The smallest detectable difference (SDD) was derived

from the SEMand described as a percentage of the param-eter’s grand mean ðSDD ¼ 1:96

ffiffiffi2p

SEMÞ. The SDD pro-vides a clinically relevant assessment of the change that isrequired in a measurement to be sure (with 95% certainty)

Table 2

Intra-rater, within-session reliability of PPT measurements of individual muscle points

Muscle ICC (CI)

Test 1/2 at visit 1 Test 1/2 at visit 2 Test 1/2 at visit 3

Iliocostalis right 0.85 (0.52e0.96) 0.90 (0.66e0.97) 0.80 (0.42e0.95)

Multifidus right 0.92 (0.74e0.98) 0.70 (0.20e0.91) 0.90 (0.67e0.97)

Gluteus maximus right 0.91 (0.70e0.98) 0.93 (0.76e0.98) 0.88 (0.61e0.97)

Gluteus maximus left 0.93 (0.77e0.98) 0.89 (0.65e97) 0.92 (0.71e0.98)

Multifidus left 0.90 (0.68e0.97) 0.95 (0.82e0.99) 0.92 (0.74e0.98)

Iliocostalis left 0.80 (0.41e0.95) 0.87 (0.57e0.96) 0.98 (0.92e0.99)

Trapezius left 0.84 (0.50e0.96) 0.96 (0.86e0.99) 0.90 (0.67e0.97)

Trapezius right 0.96 (0.86e0.99) 0.93 (0.74e0.98) 0.96 (0.85e0.99)

This table shows the ICC and 95% confidence intervals of the individual muscle point’s algometry measurements.

116 L. Potter et al. / International Journal of Osteopathic Medicine 9 (2006) 113e119

attributable tomeasurement error.Finally to check for sys-tematic error 95% limits of agreement plots were drawn.

3. Results

All recruited subjects (n¼ 10) completed the trial at-tending for all three sessions. The mean PPT for eachmuscle pair is presented in Table 1. The test-retest reli-ability results, within-session, are presented in Table 2.As the range of ICCs for the individual muscle points(0.80e0.99), presented in Table 2, signified excellent re-liability the individual muscle point data were pooled foreach trial, producing the overall picture as presented inTable 3. ICC values show good (>0.91)12 within-sessionreliability. SDDs (that ranged from 2.48 kg/cm2 to3.12 kg/cm2) are also presented in Table 3, showingthat a PPT change of approximately 3 kg/cm2 (i.e. 35e40%), as measured by the algometer, could be attribut-able to real change with 95% confidence intervals.

Similarly, the between-session analysis showed goodreliability with an ICC of 0.87 between sessions 1 and2 and 0.95 between sessions 2 and 3. The calculatedSDD showed 3.00 kg/cm2 of change in PPT, similar tothat of within-session measures that could be attributedto real change, with 95% confidence intervals. Table 4displays the individual muscle point’s ICC revealing

Table 3

Intra-rater, within-session reliability of PPT measurements of the

pooled data

ICC(2,1) CI SEM

(kg/cm2)

SDD

(kg/cm2)

SDD %

grand

mean

Within-session

test 1/2 visit 1

0.91 0.87e0.94 0.89 2.48 38

Within-session

test 1/2 visit 2

0.91 0.87e0.94 1.13 3.12 39

Within-session

test 1/2 visit 3

0.93 0.90e0.96 1.02 2.83 34

This table shows the intra-rater, within-session ICCs with confidence

intervals, the calculated SEM, SDD and the SDD presented as a ratio

of the parameter’s grand mean, of PPT measurements using a handheld

algometer.

Table 4

Intra-rater, between-session reliability of PPT measurements of indi-

vidual muscle points

Muscle ICC (CI)

Mean test 1/2 for

visit 1/2

Mean test 1/2 for

visit 2/3

Iliocostalis right 0.66 (0.27e0.92) 0.91 (0.67e0.98)

Multifidus right 0.81 (0.29e0.95) 0.91 (0.65e0.98)

Gluteus maximus right 0.82 (0.34e0.96) 0.96 (0.86e0.99)

Gluteus maximus left 0.88 (0.56e0.97) 0.97 (0.89e0.99)

Multifidus left 0.89 (0.57e0.97) 0.90 (0.62e0.97)

Iliocostalis left 0.68 (0.20e0.92) 0.93 (0.72e0.98)

Trapezius left 0.93 (0.74e0.98) 0.97 (0.88e0.99)

Trapezius right 0.91 (0.66e0.98) 0.97 (0.90e0.99)

a slightly wider range (0.66e0.97) although only twoof the 16 scores (those for the muscle iliocostalis) areoutside of the range classified as demonstrating good re-liability (>0.75).

Iliocostalis as an individual muscle point was less re-liable; data from this point were excluded and the anal-ysis was re-run, this is also presented in Table 5 andshows an improved reliability and smaller measurementerror (<3 kg/cm2).

Fig. 3 shows the within test differences (tests 1 and 2)plotted against the mean of score of the two tests, sepa-rately for week 1, 2 and 3 assessment visits. There is nosystematic difference observable between tests. The dif-ferences between sessions (visits 1, 2 and 2, 3) plottedagainst the mean score of tests 1, 2 and 2, 3 (using themean of tests 1, 2 calculated for each visit) of the testsare presented in Fig. 4. Again, there is no systematic dif-ference between visits. It should be noted from Figs. 3and 4 that the data displayed a heteroscedastic distribu-tion with the degree of variability tending to become fargreater for the higher PPT scores. This has implicationsin the interpretation of variability statistics (SD) thatmay be an overestimate for low scores and underesti-mate for high scores.

The PPT values for muscles of the lower spine werehigher than those muscles more cranially, shown inTable 1. T-tests revealed a statistically significant( p¼ 0.01) difference between the values for the uppertrapezius and iliocostalis muscle testing sites, both onthe left and right sides.

4. Discussion

This study shows that algometry is a stable method forevaluating the PPT over spinal muscles in a healthy

Table 5

Intra-rater between-session reliability of PPT measurements of the

pooled data

ICC(2,k) CI SEM

(kg/cm2)

SDD

(kg/cm2)

SDD %

grand

mean

Between-session

Visit 1/2

0.87 0.79e0.91 1.22 3.38 47

Between-session

excluding

iliocostalis

0.91 0.85e0.95 1.01 2.80 40

Between-session

Visit 2/3

0.95 0.93e0.97 0.82 2.29 28

Between-session

excluding

iliocostalis

0.96 0.93e0.98 0.76 2.12 28

This table shows the intra-rater, between-session ICCs with confidence

intervals, the calculated SEM, SDD and the SDD presented as a ratio

of the parameter’s grand mean, of PPT measurements using a handheld

algometer.

117L. Potter et al. / International Journal of Osteopathic Medicine 9 (2006) 113e119

Intra-visit algometry test1/2 visit 1

Mean algometry test 1/2 visit 1(kg/cm2)1614121086420

Diff

eren

ce te

st 1

/2 v

isit

1 (k

g/cm

2 )

4

2

0

-2

-4

+2 SD

-2 SD

Mean

2.109

-0.379

-2.868

Intra-visit algometry test1/2 visit 2

Mean test 1 test 2 visit 2 (kg/cm2)

Mean test 1 test 2 visit 3 (kg/cm2)

1614121086420

Diff

eren

ce te

st 1

test

2 v

isit

2 (k

g/cm

2 ) 4

2

0

-2

-4

+2 SD

-3.344

Mean

3.197

-.073

-2 SD

Intra-visit algometry test1/2 visit 3

181614121086420

Diff

eren

ce te

st 1

test

2 v

isit

3 (k

g/cm

2 ) 4

2

0

-2

-4

+2 SD

-2 SD

Mean

2.729

-3.127

-.199

Fig. 3. The intra-session differences between algometry measurements for each week, plotted against the mean of the measurements. The mean is

represented by the solid line and 95% limits of agreement by the dashed lines (n¼ 80).

population and that it has a relatively small measurementerror. No previous studies have robustly analysed, thetest-retest reliability or themeasurement error of the algo-meter, for evaluating the PPT over spinal muscles and sothis study provides new evidence, that adds support to

earlier reliability studies.4,13 However, this study is in dis-agreement with that of Hogeweg et al.,14 who reporteda significant difference in healthy volunteers PPTwithin-session, but not between-session, after measuringparaspinal muscle points. The authors suggested that it

118 L. Potter et al. / International Journal of Osteopathic Medicine 9 (2006) 113e119

Between session visit 1/2

Mean visit 1 visit 2 (kg/cm2)1614121086420

Diff

eren

ce v

isit

1 vi

sit 2

(kg/

cm2 )

6

4

2

0

-2

-4

-6

-8

-1.51Mean

+2 SD2.42

-2 SD-5.44

Between session visit 2/3

Mean visit 2 visit 3 (kg/cm2)1614121086420

Diff

eren

ce v

isit

2 vi

sit 3

(kg/

cm2 )

6

4

2

0

-2

-4

-6

-8

Mean0.36

+2 SD3.65

-2 SD-2.92

Fig. 4. The differences between algometry measurements for the average of the tests for visits 1/2 and 2/3 plotted against the mean of the measure-

ments. The mean is represented by the solid line and the 95% limits of agreement by the dashed line (n¼ 80).

might be due to sensitisation or habituation, which is notsupported by the results of our study.

Based on the results of this study a change in PPTof ap-proximately 3 kg/cm2 (95%CI2.28e3.38)wouldbeneces-sary to be sure that the outcome was due to a treatmenteffect and could not be attributable to chance or measure-ment error ( p< 0.05). Previous studies have reported a de-crease in PPT of between 1.1 kg/cm2 and 2.6 kg/cm2 asrepresenting a significant change between groups in inter-vention studies.6,7,15 These studies have tended to look atmyofascial points in the head and neck, but even so the re-sults shouldbe interpretedwith cautionas it is possible thatthe observed changes in PPTmight have been due to mea-surement erroralone.Thisworkwould suggest theamountof PPT change required, to be confident of measuringa true change and not simply measurement error, may beslightly higher than previously cited but still of a levelthat does not render the measure as clinically unhelpful.

This work has found an increase in PPT from lowervalues cranially, to higher values over more caudal mus-cle points. The PPT of iliocostalis and gluteus maximus

points were significantly higher ( p< 0.05) than the up-per trapezius points, consistent with the findings ofFischer (1987) and Hogeweg et al. (1992).13,14 Therefore,this study may have a degree of criterion-related validityvia its similarity in findings with these previous studies.The explanation for this regional difference is unclear.One plausible theory is that this phenomenon may bedue to the higher number of proprioceptors in the cervi-cal region as compared with the lumbar region.8 Thismight also account for the poorer PPT reliability ob-served in the iliocostalis muscle points.

It would be expected that in a symptomatic popula-tion greater reliability, of the algometer, would beachieved as it would be less likely that higher, more vari-able PPT readings, would be recorded. There was no de-monstrable systematic difference seen either betweentests or between sessions indicating that there was notraining effect. This suggests that the algometer couldbe used as a single pre-test and post-test measure.

Normally mechanical nociceptive impulses are gener-ated from intense pressure being applied through the

119L. Potter et al. / International Journal of Osteopathic Medicine 9 (2006) 113e119

skin activating small diameter thinly myelinated Ad fi-bres, which conduct impulses to lamina I andV in the dor-sal horn, of the spinal cord.16 Tenderness occurs when thenon-noxious stimulus is sufficient to stimulate nociceptiveimpulses in muscle mechanoreceptors, in which thresh-olds have been lowered. It is thought that segmentaldysfunction causes a lowering in the threshold of nocicep-tors17 thus enabling a normally non-noxious stimulationsufficient to pass threshold level and cause a nociceptiveimpulse to be generated. Manual treatments are thoughtto alter the mechanoreceptor activity in the soft tissuesand thereby decrease tissue tenderness.18 Since PPT issynonymous with tenderness, evaluating PPT providesa method for measuring the anti-nociceptive effect ofmanual treatments.

Only two papers report using algometry to measuretreatment effects for therapeutic interventions in lowback pain.19,20 Hsieh and Lee19 conducted a study com-paring the effect of one-shot percutaneous electricalnerve stimulation and transcutaneous electrical nervestimulation using changes in VAS, body surface scorein addition to PPT. Cote et al.20 assessed the PPT pre-and post-spinal manipulation. Neither of these studiesreported a significant change in PPT following the inter-vention. Despite this algometry does appear to offera simple, practical method of analysing treatment effect.

In conclusion within-day and between-day reliabilityof the algometer in quantifying the PPT of normal spi-nal muscles was good with moderate measurement er-ror. Furthermore the technique is easy to perform andthus makes it an ideal tool for quantifying, an objectivemeasure of a subject’s pain, allowing researchers and cli-nicians to monitor treatment effects.

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