DepartmentVeterans fairs

Journal of Rehabilitation Research andDevelopment Vol . 34 No . 4, October 1997Pages 427-439

The use of surface ENIG power spectral analysis in theevaluation of back muscle function

Anne F. Mannion, PhD ; Beth Connolly, MSc ; Kherrin Wood, MSc; Patricia Dolan, PhDComparative Orthopaedic Research Unit, Dept of Anatomy, University of Bristol, Bristol BS2 8EJ, UK

Abstract The rate of decline in the median frequency (MF)of the surface EMG power spectrum (MFgrad) has been evalu-ated for its use in monitoring the fatigability of the erectorspinae muscles during the performance of submaximal isomet-ric contractions . MFgrad consistently displayed a highly sig-nificant relationship with endurance time for the task (p<0 .05),giving a slightly better correlation for contractions performedin lordotic postures, with the back muscles in a shortened posi-tion, than in flexed postures . No significant difference existedbetween mean MFgrad values recorded from the right and lefterector spinae muscles, at either thoracic or lumbar levels . Theback extensor muscles of men were more fatigable than thoseof women, and this has been discussed in relation to corre-sponding differences in muscle fiber type relative size . Greatererector spinae muscle fatigability was associated with both theexistence of, and the risk of developing, serious low back pain.Possible mechanisms for the association, from retrospectiveand prospective points of view, have been advanced.

This material is based upon work supported by the Medical ResearchCouncil of the United Kingdom, the Arthritis and Rheumatism Council ofthe United Kingdom, and Remedi of the United Kingdom.Address all correspondence and requests for reprints to : Dr. A.F. Mannion,Comparative Orthopaedic Research Unit, Department of Anatomy, Universityof Bristol, Southwell St, Bristol BS2 8EJ, UK ; email : a.f.mannion@bris .ac .uk.

Key words: electromyography, endurance, erector spinae,fatigue, median frequency.

INTRODUCTION

Over the last two decades, many studies have iden-tified an association between low back pain (LBP) andhighly fatigable back muscles (1-9) . The majority ofthose examining the association retrospectively haveemployed the technique of surface electromyography inthe assessment of fatigability to negate the influence ofsubject motivation and/or the need for persons to performa contraction to their limit of endurance . Examination ofthe change in the frequency distribution of the elec-tromyographic (EMG) signal has been widely promotedas a valuable, noninvasive technique by which the devel-opment of local muscle fatigue can be evaluated duringthe maintenance of an isometric contraction. During afatiguing contraction, a compression of the power spec-trum to lower frequencies is typically observed and thiscan be tracked by monitoring a characteristic statisticalcomponent of the spectrum, such as its mean or medianfrequency (MF) during the contraction (10) . This change

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in the EMG power spectrum with fatigue has been verywell documented for a number of different musclegroups, but its cause and relationship to other physiolog-ical/metabolic properties of the muscle are less wellunderstood . We have previously demonstrated that therate of decline in the EMG power spectrum MF shows ahigh correlation with the endurance time for the task (11),and with the reduction in force-generating capacity of themuscle (12), indicating that—even if the commondenominator remains elusive—there exists uo!oue asso-ciation between myoelectric and mechanical indices offatigue for specific tasks.

During performance of the back muscle fatigue testsin many of the aforementioned studies, subjects werepositioned in either an upright (standing) co prone pos-ture, in which the lumbar spine is lordotic and the exten-sor muscles shortened . Tests performed in these posturestake advantage of the fact that the back muscles areresponsible for generating almost the entire extensormoment acting on the lumbar mpinc, because the sur-rounding passive tissues, such as the lumbodorsal fascia,intervertebral ligaments, and even the disc itself, whichmight otherwise make a significant contribution to theextensor moment, are untensioned when the lumbar spineis lordotic (13) . Nonetheless, functionally, the erectorspinae muscles perform most of their work in a length-ened position, extending the upper body from flexed pos-tures or (eccentrically) controlling the movement of thedescending trunk against gravity. Therefore, it is also ofinterest to investigate the fatigue behavior of these mus-cles in postures resembling those in which the musclesmost commonly function.

The work described in this paper is directed towardthe examination of back muscle function using surfaceEMG techniques . Specifically, we will consider our vari-ous methods of measurement, discuss the results ofexperiments that have sought to evaluate gender differ-ences in back muscle fatigability, and present preliminarydata from a small retrospective study, and a large scaleprospective study, of the association between highly fati-gable back muscles and LBP.

METHODS

ElectromyographyIn each of the studies described, surface EMGtech-

niques were used to objectively monitor fatigue during

performance of the given exercise tasks . Followingappropriate skin preparation (abrasion with Cardioprepskin preparation pads (MSB Ltd, Ramsbury,Marlborough, Wilts, UK), followed by cleaning withalcohol) to reduce skin impedance, pairs of 10 mm sur-face electrodes (Bioiruce TM Bio-Adhesive neonatal/\g/AgCl ECG electrodes on 2Q><38 mm self-adhesivegel backing ; MSB Ltd) were attached to the skin overly-ing the erector spinae at the levels of the 10th thoracic(7`10) and 3rd lumbar (L3) vertebrae, approximately 3–4cm from the midline of the back and with an interelec-trode spacing for each pair of approximately 20 to 25mm. Electrodes were positioned either unilaterally orbilaterally, depending on the particular experiment . A ref-erence electrode was positioned on the skin overlying thesternum, and the impedance between each recording elec-trode and the reference electrode was always less than 10M . The raw EMG signals were band-pass filteredbetween 5–300 Hz, amplified, and analogue to digital (A-D) converted at a sampling rate of 1024 }{z . 7buEM(3power spectral density was computed for 1 s samplingperiods at fixed intervals throughout the test, using a FastFourier Transform algorithm . MF was defined as the fre-quency that divided the spectrum into two regions con-taining equal power.

Exercise TasksRelative Submaximal Forces (at Fixed PercentMaximum Voluntary Contraction)

IIM(S power spectral parameters (14) and the rela-tionship between force and total EMG activity (15) areboth greatly affected by erector spinae muscle length . Themuscle length, in turn, is largely determined by the cur-vature of the lumbar spine in the sagittal plane, and thiswas measured and controlled in our experiments usingthe 3-Space ImotoU«System (Po1hcnnom, Colchester, VT).The Isotrak is comprised of a source of pulsed e!ecUn-ozugoehovvuven and a sensor that detects them, attachedto the skin surface overlying the spinous processes of thefirst lumbar (L.!) and the first sacral (S 1) vertebrae : theangle between the source and sensor indicates the lumbarcurvature, and this is sampled at 60 Hz and input to amicrocomputer . The range of lumbar flexion was estab-lished for each subject by measuring lumbar curvature inerect standing and in full forward flexion while the sub-ject attempted to touch the toes . This then enabled subse-quent postures, and hence relative muscle lengths, to bedefined in terms of the percentage of the full range of

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flexion of the lumbar spine . Isotrak measurements areaccurate and reproducible if the device is mounted on theskin correctly (16).

The 60% MVC TestFor determination of isometric maximum voluntary

contraction of the back extensors, subjects stood in awooden frame, strapped to a cushioned crossbar with thefront of the hips/thighs resting against it (Figure 1).While leaning forward in the frame, subjects wererequired to pull upward on a handlebar attached by achain to a floor-mounted load cell . The handlebar wasconstrained by the frame to move only in the verticaldirection, and its height was adjusted so that the lumbarspine of the subject was flexed by 60–70 percent of therange of flexion, as measured by the Isotrak . Subjectspulled up with maximum effort for 3 .3 s while the outputfrom the load cell was A-D converted at 60Hz and inputto a microcomputer. To avoid the effects of spurious hightransient forces resulting from jerking movements, theforce data were averaged over 0.33 s time periods.Between three and five attempts at generating a maxi-mum were usually required (with 2–3 min rests betweeneach) to obtain a consistent maximum (within 5–10 per-cent of each other).

The total extensor moment generated by the backmuscles was calculated with a knowledge of the externallever arm (i.e ., the perpendicular distance from the pointof force application at the load cell to the estimated cen-ter of the L5-S I disc), the sacral angle, and an estimate ofthe internal lever aim, which is dependent on the build orframe size of the individual. An estimate of the extensormoment exerted on the lumbar spine, due to superincum-bent body weight, was added to the value generated dur-ing the maximum voluntary contraction, estimated as 60percent body weight, acting on a lever aun of 60 percentof the perpendicular distance from the estimated center ofthe L5-S1 disc to the load cell (17,18), to give the totalmaximal extensor moment generated (=MVC, Nm) . Themaintenance of fixed submaximal force outputs were per-formed in the same testing apparatus, during which sub-jects observed their efforts on a digital voltmeter or chartrecorder in an attempt to keep the output from the loadcell as close to the designated target force as possible.The test was terminated by the investigators if the outputappeared to have declined, consistently, to less than 90percent of the target force . Subsequent analysis of thedata allowed calculation of the exact time for which theforce output was not below 90 percent of the target for

Load cell

Figure 1.Apparatus used for determination of maximal isometric back extensorstrength: (top) frontal view showing handlebar constrained by side-runners to move only in the vertical direction and attached to load cell(L) by means of a chain; (bottom) lateral view showing Isotrak (I) inposition on the lumbar spine.

more than two consecutive measurements, and this valuewas used as the endurance time for the task. Verbalencouragement was given throughout all tests.

Body Weight-dependent Task (ModifiedBiering-Sorensen Test)

Subjects were placed in a prone position on anexamination couch with the lower body, from the superi-or border of the iliac crest downward, strapped to thecouch (1) . With hands touching the ears, elbows out to the

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side and level with the trunk, and the head in a neutralposition, subjects were required to maintain the unsup-ported upper body in a horizontal position until, due tofatigue, they were no longer able to overcome the force ofgravity. As soon as the upper body ceased to be horizon-tal, and subjects were unable to rectify the situation, thetest was terminated . Verbal encouragement was giventhroughout, and the endurance time was recorded to thenearest whole second.

StatisticsThe number of subjects involved in each experiment

is described in the appropriate section . Data are present-ed as mean -!- standard deviation (SD) unless otherwiseindicated . Statistical analysis of the data described wascarried out using the Student's t-test (paired andunpaired), analysis of variance and covariance, and sim-ple and multiple linear regression, as appropriate.Repeated measures analysis of variance with calculationof the intraclass correlation coefficient (R) was used toassess the reliability of measures . The data from theprospective study were analyzed using forward condi-tional logistic regression analysis . Statistical significancewas accepted at the 5 percent level.

RESULTS

EMG MF Changes During FatigueThe typical temporal changes in MF recorded during

the course of a fatiguing isometric contraction are shownin Figures 2 and 3 . In order to assess which statisticalfunction most accurately described the change in MFwith time, we subjected our data to both linear and expo-nential regression analyses, with "goodness of fit" beingdetermined by the coefficients of determination (R 2 ).Within the data of any given group of subjects there wasno significant difference in mean R 2 values between thetwo functions, but the linear model generated R2 valuesslightly higher than or equivalent to those obtained withthe exponential function in the majority of individualcases, and was, therefore, considered the more appropri-ate general model with which to quantify individualchanges in MF with time (11) . The intercept of the regres-sion for any given data set yields the "initial median fre-quency" (MFinit), from which the decline in MF withtime can be expressed as a percent per second (MFgrad).We have found the latter measure to be more reproduciblethan the absolute value, in Hz/s .

Time (s)

Figure 2.Typical changes in MF with time at left thoracic (top) and right tho-racic (bottom) regions of the erector spinae during the maintenance of60% MVC to fatigue.

Reliability of MeasuresIn order to assess the reliability of measures

(endurance time and EMG power spectral parameters)during the performance of submaximal isometric contrac-tions, 10 subjects with no history of back pain performedboth a 60% MVC and a Biering-Sorensen test, twice, witheach test being carried out on a separate day. There was nosignificant difference between the repeated trials for theMFgrads and intercepts recorded at each region or for thetest endurance time, and, in each case, the intraclass relia-bility coefficient, R, was high (Table 1).

Right-left Differences in EMG MF ParametersThe data from 34 subjects, with no history of LBP,

were examined for differences in MF characteristics

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Figure 3.Typical changes in MF with time at left lumbar (top) and right lumbar(bottom) regions of the erector spinae during the maintenance of 60%MVC to fatigue.

between the right and left erector spinae muscles at agiven vertebral level during the performance of two sub-maximal fatigue tests . The mean±SD age, height, andbody mass of the subjects were 23 .0±4.3 years,1 .77±0.07 m, 75 .8±9.7 kg, respectively, for the males(n=17) and 29.2±10.7 years, 1 .64±0.05 cm, 60.6±6 .9kg, respectively, for the females (n=17).

Bilateral recordings were made both during a fatigu-ing contraction performed at 60% MVC, and during per-formance of the Biering-Sorensen test, each carried outon a separate day. No significant differences wereobserved between the MFgrad obtained from the rightand left sides of the erector spinae at a given vertebral

level of the spine (Table 2). The same was true forMFinit, except for the L3 site during performance of theBiering-Sorensen test. This was also the case when thedata of the left-handed subjects (n=2) were removedfrom the analysis . The correlation coefficient betweenMF intercept or MFgrad values for the right and left sideranged between r=0 .62 and r=0.86, depending on theprecise location and the exercise test employed (Table 2).The right-left differences in MFgrad tended to be slightlymore pronounced for the 60% MVC test.

The Relationship between EMG MF Changes andTime to Fatigue

Regression analysis revealed significant relation-ships (p<0 .05) between the rate of change in MF at anygiven recording site along the back extensors and the timefor which the fatiguing contraction could be maintained(Table 3) . In each case, the logarithmically transformedvalues for both the MF gradient and endurance time wereused, as these gave the highest correlation between thetwo variables . The relationship was better for perform-ance in the Biering-Sorensen test, where the lumbar spineis lordotic and the back muscles are shortened, than in the60% MVC endurance test, where the lumbar spine isflexed and the back muscles are considerably lengthened.In the 60% MVC test, the MFgrad data from the left sidegave better correlations with endurance time than didthose of the right side; there was a tendency for the oppo-site to be true for the Biering-Sorensen test, although thedifferences were slight . The best predictor of endurancetime was given either by the greatest MF gradientobtained at any of the chosen recording levels or by thecombined data from all recording sites, using either themean value for all sites, or by entering the data from eachsite into a multiple regression (Table 3) . These findingsemphasize the importance of recording from more thanone site along the erector spinae muscles.

Pooling the data from men and women, as there wasno significant gender difference in the slope or the inter-cept of the relationship for either fatigue test, analysis ofcovariance was used to compare the relationship betweenendurance time and greatest MFgrad for the Biering-St rensen and 60% MVC fatigue tests (p>0 .05) . No sig-nificant difference was observed between the slopes ofthe regression for the two different fatigue tests(p=0.899), but the elevation was significantly greater forthe Biering-Sorensen test (p=0 .0001 ; Figure 4) . In otherwords, there was no difference between the tests in the

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Table 1.Reliability of measures for endurance time, MFgrade and MFint for 60% MVC and Biering-Sorensen fatigue tests (n=10).

60% MVC test Biering-Sorensen test

Parameter P R P R

Endurance time 0 .70 0.87 0 .80 0 .98

T10 L MFgrad 0 .10 0.74 0 .08 0 .97

T10 R MFgrad 0 .19 0.73 0 .79 0 .99

L3 L MFgrad 0 .83 0.84 0 .31 0 .98

L3 R MFgrad 0 .13 0 .96 0 .93 0 .97

Mean T10 MFgrad 0 .45 0 .72 0 .20 0 .99

Mean L3 MFgrad 0 .37 0 .95 0 .89 0 .98

Greatest MFgrad 0 .64 0 .94 0 .70 0 .97

T10 L MFint 0 .28 0 .97 0.24 0 .76

T10 R MFint 0 .37 0 .82 0.23 0 .59

L3 L MFint 0 .13 0 .80 0 .59 0 .94

L3 R MFint 0 .13 0 .98 0 .88 0.87

p<0.05 = significant difference trial 1 versus trial 2 ; R = intraclass coefficient (the closer to 1 .0 the better the reliability) ; T10 = 10th thoracic vertebral level;

L3 = 3rd lumbar vertebral level ; L = left, R = right ; greatest = highest value recorded from any of the four regions.

Table 2.Right-left differences in MF variables derived from 60% MVC and Biering-Sorensen fatigue tests (n=17 men, 17 women).

60% MVC Biering-Sorensen test

MFInitHz

MFgrad%a Is

MFInitHz

MFgrad%Is

ThoracicRight

mean 55 .3 -0.470 60 .4 -0.186

SD 9.1 0 .281 8 .9 0 .086

Left mean 56 .0 -0.446 60 .5 -0.194

SD 8 .9 0 .290 7 .1 0 .084

r 0.73 0 .80 0 .78 0 .84

LumbarRight mean 46 .6 -0.391 66 .1 -0.258

SD 7 .4 0 .236 10 .8 0 .109

Left mean 47 .1 -0.427 71 .2* -0.267

SD 8 .6 0 .258 12 .7 0 .119

0 .83 0 .64 0.62 0 .86

r = correlation coefficient, right vs . left; * p< 0 .05 difference between left and right sides.

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Table 3.Relationship between median frequency changes and endurance time to fatigue.

Endurance Time (s)

60% MVC Biering-Sorensen test

r p r p

Left T10 MF grad (%/s) 0 .762 0 .0001 0 .764 0 .0001Right T10 MF grad (%/s) 0 .412 0 .0210 0 .771 0 .0001Left L3 MF grad (%/s) 0.681 0 .0001 0 .755 0 .0001Right L3 MF grad (%/s) 0.388 0 .0283 0 .783 0 .0001

Mean (R&L) T10 MF grad (%/s) 0.541 0 .0017 0 .795 0 .0001Mean (R&L) L3 MF grad (%/s) 0.426 0 .0150 0 .805 0 .0001Mean all 4 sites MF grad (%/s) 0.524 0 .0017 0 .851 0 .0001

Max all 4 sites MF grad (%/s) 0.738 0 .0001 0 .826 0 .0001

All 4 sites MF grad (%/s) 0.810 0 .0001 0 .881 0 .0001

(multiple regression)

Data are for 17 men, 17 women, aged 19 to 48 yrs . Recordings were made bilaterally from the erector spinae muscles at the levels of T10 and L3 . Prior to correla-tion, both variables `MFgrad' and `endurance time' were logarithmically transformed (see text for details) . r = correlation, p = significance of the correlation coef-ficient being greater than 0 (equivalent to the significance of the variance in endurance time explained by the particular independent variable under investigation).

log 10 [decline in median frequency (%/s)]

Figure 4.Relationship between endurance time and the rate of change in MF(greatest value obtained from either right or left, thoracic or lumbarregions of erector spinae) during isometric contraction to fatigue : datafrom 60% MVC test (open circles) and Biering-Sorensen test (closedcircles) . Log values of each parameter are used for the regression . Onthe x axis the range 0 to -1 covers a range of MF decrements from 1to 0 .1 %/s.

change in endurance time associated with a given changein the rate of decline in MF, but a given MFgrad wasassociated with a systematically longer endurance timeduring performance of the Biering-Sorensen, comparedwith the 60% MVC, test .

Gender Differences in EMG MF Changes with FatigueDuring performance of the modified Biering-

Sorensen test, a significantly greater back muscle fatiga-bility was observed in men than women, which corre-sponded with a shorter task endurance time for the men.Qualitatively, the differences were retained during per-formance of a fatigue test at a fixed submaximal forceoutput (60% MVC) although the gender difference didnot always reach significance (Table 4).

The Relationship Between Maximum Strength andFatigability

The mean maximum extensor moment generated bywomen was significantly lower than that of the men,whether expressed in absolute terms, or relative to totalbody mass or fat-free mass, as shown in Table 5 (19).Considering the data for men and women separately, nosignificant relationship (p>0 .05) was observed betweenstrength per unit body mass (or per unit fat-free mass) andMFgrad (individual regions, the mean or the greatest)during performance of either the Biering-Sorensen test or60% MVC to fatigue.

Back Muscle Fatigability as a Risk Factor for theDevelopment of First-time LBP

In a prospective study of 403 young, nonimpairednurses who previously had never suffered from serious

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Table 4.Gender differences in EMG median frequency changes with fatigue.

Men (n=17) Women (n=17)Signif

Variable Mean SD Mean SD p value

60% MVC testEndurance time 61 .6 15 .7 78 .4 50 .5 0 .200MFGRADT (%/s) -0.557 0 .270 -0.363 0 .222 0 .035

*MFGRADL (%/s) -0.440 0.239 -0.351 0 .218 0.274MFGRADmean (%/s) -0.499 0 .225 -0.355 0 .175 0.050

*MFGRADgreatest (%/s) -0.665 0 .291 -0.496 0 .185 0.057

Biering-Sorensen testEndurance time 164 .5 48 .7 220 .4 88 .5 0.029

*MFGRADT (%/s) -0.227 0 .078 -0.153 0 .069 0.006MFGRADL (%/s) -0.303 0 .105 -0.222 0.102 0.028

*MFGRADmean (%/s) -0.265 0 .082 -0.187 0 .079 0 .008

*MFGRAD greatest (%/s) -0.340 0 .100 -0.241 0 .102 0 .007

*

*p < 0.05 difference between men and women . Recordings were made bilaterally from the erector spinae muscles at the levels of T10 and L3 . MFGRADT = meanvalue from both right and left thoracic regions ; MFGRADL = mean value from both right and left lumbar regions ; MFGRADmean = mean value from all four regions;MFGR Dgreatest = greatest value obtained at any of the four regions.

Table 5.Gender differences in maximal voluntary contraction of the back extensors.

Men (n=17) Women (n=17)Signif

Variable Mean SD Mean SD p value

Absolute strength (Nm) 440.2 139 .8 248 .8 41 .2 0.0001Strength/body mass (Nm. kg- l ) 5 .76 1 .39 4 .12 0.59 0.0001Strenght/fat-free mass (Nm .kg I) 6 .72 1 .71 5 .60 0.85 0 .0211

Fat-free mass (FFM) _ (100 - % body fat) x body mass . % fat estimated from the sum of four skinfold thicknesses, according to the method of Dumin andWomersley (19).

LBP (i .e ., requiring medical attention or time off work),the fatigability of the back muscles was monitored duringthe maintenance of 80 percent MVC for 28 seconds andduring performance of the Biering-Sorensen test tofatigue (the tests were conducted as part of a comprehen-sive assessment of spinal function) . Individuals were fol-lowed up by postal questionnaire at 6-month intervalsfollowing their initial assessment, in order to investigateany subsequent attacks of serious LBP. Preliminaryanalysis of the results from the first 200 women(mean-!-SD) age, height, and body mass : 26.414.9 yr,1 .65±0.06 m, 61 .7±8.6 kg, respectively, followed up for12 months, revealed that 13 percent of the group devel-oped serious LBP for the first time in the interveningperiod. The greater EMG MFgrad (observed at either the

thoracic or lumbar region of the back muscles) displayedduring performance of the Biering-Sorensen test was sig-nificantly associated with the development of first timeattacks of LBP (p=0 .03, R=0.13) . Greater risk was asso-ciated with a steeper rate of decline in MF (i .e ., with morefatigable back muscles).

Interestingly, the simple measure of endurance timehad no predictive value for the development of LBP. Themean values for the two groups (those that went on todevelop serious LBP and those that remained pain-free),analyzed with univariate analyses, are shown in Table 6.A higher rate of decline in MF during performance of the80% MVC test had no predictive value for first timeattacks of serious back pain.

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MANNIOV et al . Surface EMG and Back Muscle Function

Table 6.Results from prospective study : mean values for back muscle fatigability in the group that developed serious first-ti

backpain and the gorup that remained pain-free.

no LBP (n=174) LBP (n=26)Signif

Variable Mean SD Mean SD p value

Biering-Sorensen testEndurance time 141 .7 52 .1 123 .0 60 .6 0 .109MFGRADT (%/s) -0.233 0 .098 -0.284 0 .142 0 .026

*MFGRADL (%/s) -0.267 0 .117 -0.311 0 .186 0 .119MFGRADgreater (%/s) -0.290 0 .113 -0.350 0 .167 0 .023

80% MVC for 28s

MFGRADT (%/S) -0.753 0 .517 -0.830 0 .451 0 .495MFGRADL (%/s) -0.631 0 .667 -0.534 0 .781 0 .522

MFGRAn greater = greater value obtained from thoracic or lumbar recording sites.

Back Muscle Fatigability Associated with the Existenceof LBP and Physical Activity Levels

A retrospective study was carried out to examine thedifference in fatigability of the back muscles of twogroups of men, one of which had never experienced anyLBP (control, n=10) and a second which had experi-enced either repeated episodes of nonspecific LBP or atleast one episode that they considered severe or verysevere, sufficient to have disrupted their work and/orleisure activities, or caused them to be out sick (LBP,n=12). The mean±SD age, height, body mass, bodymass index, and percent body fat of the subjects were30.8±4.8 years, 1 .76±0.06 m, 73 .2±5 .0 kg, 23 .8-!-1 .8kg/m-2 and 16 .5±2 .5 percent respectively for the controlgroup and 33 .6±6.9 years, 1 .79±0.05 m, 77 .7±6.7kg,24.4±2.9kg/m-2 and 18 .5±4.0 percent, respectively, forthe LBP group . There was no significant differencebetween the two groups for any of these physical charac-teristics (p>0 .05).

Fatigability of the back muscles was monitored,unilaterally, during performance of the Biering-Sorensentest for 60 s . The greater MFgrad (observed at eitherthoracic or lumbar regions of the back muscles) washigher in the LBP group, but the difference just failed toreach significance : LBP -0 .396 ±0 .137 percent/s, con-trols -0.282±0.180 percent/s ; p=0.10. Interestingly,these mean values for the greater MFgrad were similar tothose shown in Table 6 for the LBP and no-LBP groupsin the prospective study.

Questionnaire analysis of physical activity levelsrevealed that both groups were quite active, with 45 46

percent of the individuals exercizing several times perweek. The LBP group exercized slightly more often thanthe control group, which may have provided them with animproved endurance capacity. In order to investigate thismore fully, the LBP and control groups were subdividedagain on the basis of whether they exercized once/weekor more, or never/only occasionally . The results (meanvalues) are shown in Table 7. while the group numbersare too low for statistical analysis, it is of interest to notethat the MFgrad for LBP exercisers was very similar tothat of the nonexercisers with no LBP.

DISCUSSION

The results of this study confirm our previous find-ings that, during the maintenance of fixed-load isometric

Table 7.Greater MFgrad obtained in either thoracic or lumbar regionsof the erector spinae in LBP and control groups separated byexercise habits . (Mean (SD))

Exercise Low back pain

Yes

No MeansYes -0.368 (n = 10) -0.231 (n =6) -0.317 (n=16)

(0 .129) (0 .152)No -0.537 (n=2) -0.358 (n= 4) -0.418 (n = 6)

(0 .475, 0 .600) (0 .202)Means -0.396 (n=12) -0.282 (n=10)

(0 .137) (0 .180)

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contractions, the rate of decline in the MF of the erectorspinae EMG power spectrum shows a high correlationwith the time for which the task can be performed (11).This is found to be the case for back muscle fatigue testsperformed with the lumbar spine in either flexed or lor-dotic postures, although the relationship is more highlysignificant for the latter. This may be because in the lor-dotic position typical of the Bielng-Sorensen test theback muscles assume greater responsibility for generat-ing the extensor moment across the spine, receiving littleor no assistance from passive tissues . The regressiondescribing the relationship between MFgrad andendurance time displayed the same slope for both types offatigue test, although a given MFgrad was associatedwith a shorter endurance time in the 60% MVC test thanin the Biering-Sorensen test. In performing the 60%MVC test, the musculature of the shoulders, aims, andlegs contribute toward stabilizing the body, and it is pos-sible that these muscles could fatigue earlier than theback extensors, causing the activity to cease prior to thedevelopment of substantial back muscle fatigue . It is alsoconceivable that there may be a real, physiological dif-ference in the relationship between EMG changes andendurance time when the muscles operate at differentlengths. At present, we cannot account for this differencebetween the two tests.

No significant differences were observed betweenthe mean rate of change in MF for the right and left erec-tor spinae muscles in this predominantly right-handedgroup of subjects, although occasional individual differ-ences were observed . This contrasts with the results ofMerletti et al . (20), who observed greater myoelectricmanifestations of fatigue in the back muscles of the upperlimb dominant side. The reason for this inconsistent find-ing is unclear, although it is possible that differences inthe mode of stimulation of the muscle, voluntary in thepresent study and electrically evoked in Merletti et al .,elicited differing fatigue responses, as has been observedbefore (21) . The right and left MFgrad values tended tobe better correlated with each other during the Biering-Sorensen test than during the 60% MVC test . In the lat-ter, it is possible that individuals pulled with a slightunevenness on the bar, causing an unequal distribution ofthe load. This is, perhaps, a result of their particular hand-edness or side-dominance . This may also explain thesuperior ability of the MFgrad recorded from the erectorspinae on the left side to predict 60% MVC endurancetime, compared with the corresponding value from theright side . We would recommend always recording from

more than one or two sites on the back extensors, as thisprovides for greater sensitivity in quantifying the suscep-tibility of an individual to fatigue.

In performing the two isometric endurance tests, asignificant gender difference in fatigability was observed,characterized by a shorter time to fatigue and a greaterrate of decline of MF in the men . Similar sex-differencesin endurance of the back extensors have been reportedbefore (2,11,22) . Preliminary data from muscle biopsyanalyses of the thoracic and lumbar regions of the backmuscles suggest that the gender differences in fatigabili-ty might arise as a result of differences in the relative sizeof the two main muscle fiber types . In both regions of theerector spinae the mean type I:II fiber size ratio is signif-icantly greater in women than men (p<0 .05), andaccounts for 80 percent and 60 percent of the variance inMFgrad (%/s) in thoracic (n=13) and lumbar (n=8)regions, respectively, during performance of the modifiedBiering-Sorensen test".

The metabolic and physiologic profile of the type I(slow twitch) fiber provides it with a higher oxidativepotential and with the capacity to sustain an isometric con-traction with a greater economy of tension maintenancethan the type H (fast twitch) fiber (23) . Accordingly, a con-traction which employs predominantly type I fibers, inpreference to type II, will result in a lower rate of accumu-lation of metabolic by-products such as hydrogen ions, lac-tate, ADP, and H2PO4, which have previously been shownto correlate with the decline in the power spectrum MFduring fatigue (24-26) . In heterogeneous skeletal muscles,the critical factor controlling the recruitment of motor unitsis motor unit type, proceeding in the order of increasingcontraction strength and fatigability (27) . Since the forceoutput of a muscle is proportional to its cross-sectionalarea, which, in turn, correlates with the mean cross-sec-tional area of the individual fibers (28-30), it could behypothesized that the larger the relative mean cross-sec-tional area of the muscle fibers in the early recruited motorunits, the less do additional (faster, more fatigable) unitsneed to be recruited to achieve the required submaximalforce. This may explain why the possession of a greatermean type I relative to type II fiber size is associated withless rapid MF changes during fatigue.

'Mannion AF, Dumas GA, Stevenson JM, Espinosa FJ, Faris MW, Cooper RG.Association between muscle fiber size and type distribution and electromyo-graphic indices of fatigue in the erector spinae muscles . Preliminary data pre-sented at the International Society for the Study of the Lumbar Spine, Helsinki,Finland, June 1995 .

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In contrast to another study involving back-pain-freesubjects (9), a greater back extensor strength (expressedeither in absolute terms or relative to body mass or fat-free mass) was generally not, or only weakly, associatedwith a higher fatigability of the muscles . If it is, indeed,the muscle fiber type size and distribution that determinesthe fatigability of a muscle, this tends to suggest thatthose same factors do not govern the maximum force-generating capacity of the muscle ; although, strictly, thecorrelation should be performed between fatigability andstrength per unit total muscle cross-sectional area to ver-ify this . Some studies in the literature support this notion(31,23), while others maintain that the type II fiber iscapable of generating a greater force per unit area thanthe type I (32).

The preliminary results of our prospective study arethe first to show, using objective indices of fatigue, thathighly fatigable back muscles predispose to first-timeattacks of serious LBP. A similar conclusion, for men butnot for women, has been reached previously (1), althoughin that study back muscle endurance capacity wasassessed on a simple "time to fatigue" basis . Endurancetime, per se, had no predictive value in this and in a pre-vious study carried out on female nurses (33), emphasiz-ing the importance of making objective measurements offatigability that do not rely on psychological factors, suchas subject motivation . There are a number of possibleexplanations for the observed association between fatiga-ble back muscles and the development of LBP. Duringrepeated bending and lifting activities, a reduction in theforce-generating capacity of the back muscles, subse-quent to the development of fatigue, might demand thatthe passive tissues of the spine resist proportionatelymore of the forward-bending moment, rendering themmore susceptible to injury . Studies from our laboratory,which have shown a progressive increase in the degree towhich the lumbar spine is flexed during repetitive lifting,would tend to support this argument (34) . There are alsoreports that fatigue diminishes the control and coordina-tion of trunk muscular activity (35) and significantlyreduces precision in trying to match a required force (36),each of which could leave the lumbar spine more vulner-able to injury, leading to the development of LBP.

An interesting finding from our preliminary analysisof the prospective study data was that fatigue during themaintenance of a fixed 80% MVC had no predictivevalue for the subsequent development of LBP. It thereforeseems that the Biering-Sorensen test, which requires thatthe individual support his/her own upper body weight

against the force of gravity, may be the better test withwhich to identify individuals at risk. Any unfavorablebiomechanics that prevail during performance of theBiering-Sorensen test are also likely to exert an effectduring everyday activities, because in tasks that involveprolonged bending and lifting, the upper body mustalways be supported in addition to any external load . Thiscan be expected to be particularly relevant in occupationssuch as nursing (from which our subjects for the prospec-tive study were drawn) where considerable periods oftime are spent in the forward bent position . The Biering-Sorensen test may have better reflected the response ofthe muscles to the demands placed upon them duringdaily life, and therefore proved to be the more sensitiveindicator of "functional" fatigability.

Supporting the results of many other studies in the lit-erature (1-9), our retrospective study suggests that theerector spinae muscles of individuals with LBP are morefatigable than those of healthy controls . In attempting toexplain the results of retrospective studies it is always dif-ficult to establish cause and effect . It is conceivable that themuscles of persons with LBP were particularly fatigableprior to their development of LBP (and, in view of theabove, may even have contributed to its development).Equally feasible is the possibility that the reduction in backextensor endurance is secondary to changes in the size andstructure of the back muscles following pain-induced inac-tivity and disuse, or reflex inhibition of the musculature . Inother musculoskeletal systems, it has been shown thatinjury to a joint is associated with a reduction in alpha-motoneurone activity to the muscles surrounding that joint,leading to chronic disuse atrophy, sometimes selectivelyfor a particular muscle fiber type (37) . It is also known thatcessation of normal repetitive low-level activity patternsand stretch, which otherwise serve to maintain the size ofthe slow twitch (type I) fiber, results in a transformation ofthe muscle toward a faster, more fatigable type (38) . Inrelation to LBP, there is still much controversy in the liter-ature: while all the research using physical assessment sug-gests that the back muscles of LBP subjects are morefatigable than controls, all but one (39) of the reports onmuscle histomorphometry suggest that LBP subjects dis-play particularly marked type II fiber atrophy of the erec-tor spinae (40-42) . These two conclusions appear to be atvariance with what is known of the relationship betweenfatigue and muscle fiber type, and further investigation isclearly required to resolve the differences.

The results of this and other (4,5) retrospective stud-ies, and also those of a number of training studies

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(3,9,43,44), suggest that regular exercise and condition-ing of the back muscles may assist in decreasing the fati-gability otherwise observed in LBP sufferers . It istherefore recommended that exercises specificallydesigned to improve the muscular endurance capacity ofthe back extensors be included in any functional restora-tion programs for LBP subjects.

CONCLUSION

Measurement of the rate of decline in MF of the sur-face EMG power spectrum (MFgrad) provides an excel-lent technique for objectively monitoring the fatigabilityof the erector spinae muscles during the performance ofisometric contractions . Its superiority over simple "timeto fatigue" measures is demonstrated by its sensitivity togender differences in fatigability as well as differencesbetween individuals with and without LBP and those atrisk of developing serious first time LBP.

It would appear that good fatigue-resistance of theback extensors may protect against both first-time andrecurrent attacks of LBP, and, as such, training forimproved muscular endurance should be included in anypreventive or restorative exercise programs for low backpain.

ACKNOWLEDGMENT

We would like to thank Chris Standell for her technicalassistance with these experiments.

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