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Decreased Isometric Neck Strength in Women With ChronicNeck Pain and the Repeatability of Neck StrengthMeasurements

Jari Ylinen, MD, Petri Salo, PT, Matti Nykanen, MD, Hannu Kautiainen, BA, Arja Hakkinen, PhD

ABSTRACT. Ylinen J, Salo P, Nykanen M, Kautiainen H,Hakkinen A. Decreased isometric neck strength in women withchronic neck pain and the repeatibility of neck strengthmeasurements. Arch Phys Med Rehabil 2004;85:1303-8.

Objectives: To evaluate neck flexion, extension, and, espe-cially, rotation strength in women with chronic neck paincompared with healthy controls and to evaluate the repeatabil-ity of peak isometric neck strength measurements in patientswith neck pain.

Design: Cross-sectional.Settings: Rehabilitation center and physical and rehabilita-

tion medicine department at a Finnish hospital.

Participants: Twenty-one women with chronic neck painand healthy controls matched for sex, age, anthropometricmeasures, and occupation.

Interventions: Not applicable.Main Outcome Measures: Peak isometric strength of the

cervical muscles was tested in rotation, flexion, and extension.Results: Significantly lower flexion (29%), extension (29%),

and rotation forces (23%) were produced by the chronic neckpain group compared with controls. When the repeated testresults were compared pairwise against their mean, consider-able variation was observed in the measures on the individuallevel. Intratester repeatability of the neck muscle strength mea-surements was good in all the 4 directions tested in the chronicneck pain group (intraclass correlation coefficient range, .74.94). The coefficient of repeatability was 15N, both in flexion

and extension, and 1.8Nm in rotation. On the group level,improvement up to 10% due to repeated testing was observed.Conclusions: The group with neck pain had lower neck

muscle strength in all the directions tested than the controlgroup. This factor should be considered when planning reha-bilitation programs. Strength tests may be useful in monitoringtraining progress in clinical settings, but training programsshould be planned so that the improvement in results is wellabove biologic variation, measurement error, and learning ef-fect because of repeated testing.

Key Words: Muscles; Neck; Rehabilitation; Torque. 2004 by the American Congress of Rehabilitation Medi-

cine and the American Academy of Physical Medicine andRehabilitation

CHRONIC NECK PAIN IS a common musculoskeletaldisorder in the working-age population in modern indus-

trialized countries.1-3 It has considerable economic impactthrough sick leave and the increased use of primary health careservices, medication, specialist care, laboratory tests, diagnos-tic imaging, and therapeutic interventions.4,5

Muscle function is an important factor in understanding neckpain. Chronic neck pain is thought to be more common amongwomen, because their muscle strength is lower than that ofmen.6 The sustained muscle contraction required to hold thehead in various positions and the fatigue because of muscularweakness are suspected of being causative factors in chronicneck pain.7,8 Gogia and Sabbahi9 found greater neck muscle

fatigue in patients with chronic neck pain than in healthycontrols in the electromyographic analysis. Several studieshave found weakness in the cervical flexor and extensor mus-cles in patients with chronic neck pain compared with healthycontrols,10-14 but we did not find anyone to report weakness inthe rotator muscles.

The reliability of isometric neck strength measurements hasbeen found to be good both in healthy subjects and in patientswith chronic neck pain.11-24 However, the statistical methodsused in earlier studies do not make visible the possible varia-tion in the results between repeated tests in individual casesbecause of biologic variation and measurement error.

The main aim of our study was to evaluate whether patientswith chronic neck pain had lower neck strengthin particular,isometric rotation strengththan healthy controls. A second-

ary objective was to evaluate, by using appropriate statisticalmethods, the repeatability of isometric neck strength tests inwomen with chronic neck pain.

METHODS

Participants

The subjects, women with nonspecific chronic neck pain forat least 6 months, were recruited through local occupationalhealth care services in eastern and southern Finland. The otherinclusion criteria were age 25 to 53 years, full-time officeworker, and permanently employed. In most patients withchronic neck pain, no specific neck pain etiology is generallyrevealed by clinical examination, laboratory tests, or medical

imaging techniques. Such patients are commonly defined ashaving mechanical neck pain or nonspecific neck pain. Theexclusion criteria in our study were the presence of specificdiseases known to cause chronic neck pain, such as nerve rootcompression, a previous cervical operation, peripheral nerveentrapment, spinal stenosis, known radiologic instability,anomaly of the cervical spine, spasmodic torticollis, disorder ofthe shoulder, fibromyalgia, rheumatoid arthritis, or other in-flammatory joint diseases, as well as diseases and conditionspreventing physical loading, such as pregnancy, severe depres-sion, mental illness, a recent major operation, or acute infec-tion. These states were assessed mainly on the basis of thepatients medical history and the clinical examination. All the

From the Department of Physical and Rehabilitation Medicine, Jyvaskyla CentralHospital, Jyvaskyla (Ylinen, Salo, Hakkinen); Punkaharju Rehabilitation Center,Punkaharju (Nykanen); and Rheumatism Foundation Hospital, Heinola (Kautiainen),Finland.

Supported in p art by the Finn ish Social I nsurance Ins titution a nd Jyvaskyl a Cent ralHospital.

No commercial party having a direct financial interest in the results of the researchsupporting this article has or will confer a benefit upon the author(s) or upon anyorganization with which the author(s) is/are associated.

Reprint requests to Jari Ylinen, MD, Jyvaskyla Central Hospital, Dept of Physicaland Rehabilitation Medicine, Keskussairaalantie 19, 40620 Jyvaskyla, Finland,e-mail: [email protected].

0003-9993/04/8508-8685$30.00/0doi:10.1016/j.apmr.2003.09.018

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participants gave written consent before entering the study. Thelocal ethics committee approved the study.

The patients who volunteered to participate in the study were21 middle-aged women (table 1). They filled out a set ofquestionnaires assessing their neck pain and level of disability.Mean neck pain standard deviation (SD) was 5422mm onthe visual analog scale.25 The mean Oswestry26 and Vernon27

disability indices were 138 and 135, respectively. Thewomen had been suffering from neck pain for an average of96 years.

The healthy controls were recruited from among officeworkers (n263) at Jyvaskyla Central Hospital via an e-mailmessage describing the study. The control group finally con-sisted of 21 women, none of whom had had any neck painduring the previous 6 months or any of the other contraindi-cations listed above. None of the controls had any backgroundin strength training, and they were only infrequently engaged inphysical exercise. The control subjects were matched with thepatients for occupation (office work), sex (female), age (5y),weight (5kg), and height (5cm), to eliminate major con-

founding factors (table 1).Hand-grip strength is commonly used to assess the func-tional ability of the upper extremities, and it has also beenshown to correlate with overall muscle strength of the body,sex, age, anthropometric parameters, health, and remaining-lifetime in the aged.28,29 Thus, this measure was chosen as apractical method of comparing the general strength levels ofthe subjects with the controls (table 1). Grip strength wasmeasured by a Jamar dynamometer.30,a The subjects performed3 maximal efforts at 45-second intervals in a sitting positionwith the wrist straight and the elbow at a 90 angle.

Experimental Procedure

The neck strength measurement systemb was used to test theselected parameters of isometric neck strength with subjects

seated in a standard position, and the methodology followedthe same method used in testing healthy subjects in the reli-ability study reported earlier (fig 1).24 The measurement systemwas calibrated with standard weights (5kg, 10kg, 15kg, 20kg).Isometric rotation strength was measured first. Torque was notmeasured from the head supports but from the shaft of theapparatus to which the head supports were attached. The rota-tional axis of the upper cervical column, which runs throughthe center of the dens,31 was brought into the same line as theshaft of the measurement device. The openings of both earcanals are located directly above the dens. Thus, the head waspositioned by looking at the opening of both ear canals from 2sights running parallel to the shaft of the measurement device.

The head was centralized by a screw system traveling the samedistance from both sides and tightening the support against thehead. Then the subjects were instructed to turn the head to theright and to the left against the head supports.

During the flexion and extension measurements, the subjectspushed directly forward or backward against the padded straingauge of the neck strength measurement system. Verbal en-couragement was given in a steady loud voice without shout-ing. Changing position took about 2 minutes. Three submaxi-mal warm-up efforts with gradually increasing force wereperformed in each direction, to acquaint subjects with thetesting position and with the neck strength measurement sys-tem. In the test trials, each subject was told to perform at least3 maximal efforts lasting 5 seconds each, at 45-second inter-vals. If the third result showed an improvement of more than5%, additional efforts were performed until the improvement instrength remained under that. The best result was used in thefinal analysis.

Fig 1. Neck strength measurement system consists of 2 partshaving strain gauges of their own. Rotator muscle strength is mea-sured with the part above the head, which is attached on both sidesof the head by 4 pads. Flexor and extensor muscle strength ismeasured with the pad, which is now supporting the head, butwhich is lifted up to the level of the forehead and occiput on thesetests.

Table 1: Demographic and Clinical Data of the Women WithChronic Nonspecific Neck Pain and Healthy Controls

Neck Pain Group

(n21)

Control Group

(n21)

Demographic

Age (y) 446 448

Weight (kg) 6810 6913

Height (cm) 1666 1655

BMI (kg/m2

) 253 254Clinical

Grip strength, right (N) 32459 31469

Grip strength, left (N) 29469 29449

NOTE. Values are mean SD.Abbreviation: BMI, body mass index.

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In the chronic group with neck pain, the strength tests wererepeated at the same time on the following day by the sametester, to evaluate repeatability. The tests were conducted by 2physiotherapists, both of whom had considerable experience instrength testing with the neck strength measurement system.

Statistical Procedures

The results are expressed as means with SDs and with 95%confidence intervals (CIs). Intraclass correlation coefficients

(ICCs) and coefficients of repeatability were calculated be-tween 2 repeated trials for each strength parameter tested. Ananalysis described by Bland and Altman32 was done in whichdifferences between 2 strength measurements were plottedagainst the corresponding mean for each patient, to show thevariability of the results at the individual level. Statisticalcomparisons between the groups were performed using the2-tailed unpaired t test and the multivariate Hotelling T2 test.33

Hommel adjustments were used to correct levels of signifi-cance for multiple testing. Correlation coefficients were calcu-lated by the Pearson method. The normality of variables wasevaluated by the Kolmogorov-Smirnov test with Lilliefors sig-nificance or by the Shapiro-Wilk test. The level was set at .05for all tests. Statistical packages SPSS, version 11.0,c andSTATA, version 8.0,d were used for the analysis.

RESULTS

The distributions of age, anthropometric measures, and gripstrength were similar in both groups (table 1). There was asignificant difference in peak force production between thepatient group and the control group in both the multivariate testand the univariate t test (table 2). Peak force production in thepatient group was 29% lower in both flexion and extension and23% lower in rotation than in the healthy control group. Theindividual variation in the peak force produced was consider-able in the patient group and even greater in the control group.No significant difference emerged in the neck flexion/extensionstrength ratio between the patient (.41) and control (.40)groups.

There was no significant difference in the peak force pro-

duced in rotation to right and left within either of the groups.Pearson correlation coefficients between the different force

parameters within groups were slightly better for the controlgroup than for the patient group (table 3).

In the control group, 40% of subjects made 4 efforts inrotation and 12% made a fifth effort, whereas 29% made 4efforts in flexion and 19% made 4 efforts in extension. In thepatient group, 48% of subjects made 4 efforts in rotation, 14%made 4 efforts in flexion, and 29% made 4 efforts in extension,but none had to perform a fifth effort. On the same testoccasion, the second or third attempt in each direction mea-

sured gave the best result in 70% to 80% of subjects in bothgroups. The first attempt produced the highest result only in10% of subjects. The same proportion of subjects attained theirbest result on their fourth attempt. Although 20% of the sub-jects performed 5 attempts in at least 1 direction, only 1 womanin each group produced her best result on the fifth attempt in 1direction. The second effort was on average 8%, and the thirdeffort 10%, better than the first.

Intratester repeatability was analyzed by using the ICC,which varied from .74 to .98, depending on the directionstested (table 4). The coefficient of repeatability was 15N inflexion and extension and 1.8Nm in rotation. The differences inpeak isometric strength between the first and second measure-ments were plotted against their means; figures 2 and 3 showthe considerable variation between the results of consecutive

tests on the individual level. The results of the second testoccasion were often greater than the results of the first for thesame patient.

There were no major complications, despite the fact that thesubjects were instructed to perform the neck strength tests withmaximal effort within the limits of pain that they were able tobear.

DISCUSSION

Peak isometric neck strength values were statistically signif-icantly reduced in the women with chronic neck pain comparedwith healthy controls in all the directions tested. Kraut andAnderson10 found, by manual testing, that neck flexor musclestrength was significantly lower in a chronic neck pain popu-lation than in healthy controls. Silverman et al11 and Barton and

Hayes12 showed this with objective measurements. Jordan etal13,18 further confirmed this finding and found lower neck

Table 3: Correlation (Pearson) Coefficients With 95% CIs Between Different Parameters of Isometric Neck Strength Within Groups

Neck Pain Group (n21) Control Group (n21)

Rotation,

Right

Rotation,

Left Flexion

Rotation,

Right

Rotation,

Left Flexion

Rotation, left .80 (.56.92) .89 (.74.96)

Flexion .61 (.24.82) .57 (.19.81) .68 (.35.86) .62 (.26.83)

Extension .65 (.31.85) .62 (.26.83) .51 (.10.77) .70 (.38.87) .71 (.40.87) .62 (.26.83)

Table 2: Peak Isometric Neck Strength Values in Women With Chronic Neck Pain and Healthy Controls

Neck Pain

Group Control Group

Mean (95% CI) Difference

Between Groups* P Value

Rotation (Nm)

Right 5.81.2 8.02.4 2.2 (1.03.4) .001

Left 6.11.6 7.42.3 1.3 (0.12.6) .032

Flexion (N) 53.818.3 75.723.5 21.8 (8.735.0) .002

Extension (N) 132.138.5 187.139.2 54.9 (30.779.1) .001

NOTE. Values are mean SD unless otherwise noted.*Multivariate P value (Hotelling T2): P.001.Univariate, t test.

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against their means, considerable variation and a tendencytoward better results on the second test occasion were found in

all the parameters tested. The reason may be that on the secondtest occasion the subjects had already learned the test procedureand were familiar with the neck strength measurement systemand could thus concentrate better on force production. Theimprovement in forces varied between 10% and 13% in rota-tion and flexion but was only 3% in extension. Thus, extensionstrength seems to be more stable than the other forces mea-sured. This may be partly because of differences in the anat-omy and function of the muscle groups tested.

We did not find any previous studies of neck strength in-volving the analysis recommended by Bland and Altman.However, Rantanen37 used this analysis to evaluate the repeat-ability of isometric trunk strength measurements and foundwide intrasubject variation, which is in line with the results ofour study. Ylinen et al24,35 showed that repeated neck strength

testing without any strength training improved the results onaverage by 3% to 8% on the second test occasion on the sameday in healthy individuals and by 7% to 10% at 12-monthfollow-up in women with chronic neck pain. These resultsshould be borne in mind when assessing improvements instrength test results in the clinic because a small improvementmay not be the result of effective training but may be becauseof biologic variation, measurement error, and a learning effectdue to repeated testing.

Because of the wide range of peak neck strength results inboth the patient and control groups, those with neck paincannot be differentiated from healthy persons by strength tests,even though the patients with chronic neck pain were signifi-cantly weaker than the healthy controls at the group level. Wehave found no studies evaluating low neck strength as a pos-

sible risk factor for neck pain. However, strength testing maybe useful in testing the basic strength level of the individualpatient, setting goals for strength training, and monitoringprogress of training, provided that the training method is ef-fective enough to improve results well above biologic varia-tion, measurement error, and test effects, as shown in otherstudies.16,35,38-40

Patients with chronic neck pain may have low neck strengthin flexion, extension, and rotation, or in any combination ofthese. Knowledge of the different parameters of neck strengthis essential in planning individually tailored neck strengthtraining programs. Strength testing may also be useful to mo-tivate training, but it is of paramount importance that the

training intensity is monitored and its effectiveness followedup.

CONCLUSIONS

Neck strength in all directions was significantly lower inpatients with neck pain than in controls. Different parametersof neck strength should be tested separately, to obtain fullinformation about neck strength. On the same testing occasion,at least 3 maximal attempts in each direction should be testedto obtain a true peak strength result. When neck strength testingis used to follow up the training of individual patients in theclinic, measurement error, biologic variation, and improvementin results because of repeated testing should be kept in mind.However, when the strength training methods used are effec-tive, the results should be on a considerably higher level, andtherefore we recommend isometric neck strength measure-

ments for clinical settings.Acknowledgments: We thank Kai Viertola for technical assis-

tance and expertise with the neck strength measurement system.

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