Intratester and Intertester Reliability of Neck Isometric Dynamometry

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Intratester and Intertester Reliability of Neck Isometric Dynamometry Nikolaos Strimpakos, MSc, PT, PEd, Vasiliki Sakellari, PhD, PT, Georgios Gioftsos, PhD, PT, Jacqueline Oldham, PhD ABSTRACT. Strimpakos N, Sakellari V, Gioftsos G, Oldham J. Intratester and intertester reliability of neck isometric dynamometry. Arch Phys Med Rehabil 2004;85: 1309-16. Objective: To evaluate the reproducibility of measurement for maximum voluntary isometric contractions of the cervical musculature in different movements. Design: Repeated test-retest measurements. Setting: A department of physiotherapy. Participants: Thirty-three healthy subjects (17 men, 16 women; age range, 19 – 63y) for the intraexaminer study and 10 healthy subjects (4 men, 6 women; age range, 20 –37y) for the interexaminer study. Interventions: Maximum isometric strength in sitting and standing for flexion, extension, lateral flexion, and rotation using a custom isomyometer device. Three tests, performed 5 to 8 days apart, to assess intraexaminer reliability. Two examiners, each performing 1 trial, measuring on the same day to assess interexaminer reliability. Main Outcome Measures: Intraexaminer and interexaminer reliability of neck muscle strength. Results: The standing position showed better reproducibility than the sitting position. The intraclass correlation coefficient (ICC1,3) was above .84 for all tests in any movement and position and above .93 when the first test was excluded. The standard error (SE) of measurement (⬍16.5N; ⬍.13N-m for rotation) and smallest detectable difference (SDD) (⬍20.1%) were also small. For interexaminer reliability, the ICC2,1 ranged from .88 to .94 and the SE from 10.7 to 20.8N (⬍1.15N-m for rotation); the SDD was less than 29.8% (except right rotation, which was 38.8%). Conclusions: A reliable protocol for measuring neck strength has been developed. Standing position and a full practice session produces more reliable measurements. Key Words: Cervical vertebrae; Muscle contraction; Neck muscles; Rehabilitation; Reliability and validity. © 2004 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation

From the Centre for Rehabilitation Science, University of Manchester, Manchester, UK (Strimpakos, Oldham); and Department of Physiotherapy, TEI Lamias, Lamia, Greece (Strimpakos, Sakellari, Gioftsos). Presented in part at the European College of Sport Science’s 7th Annual Congress, July 25, 2002, Athens, Greece. Supported by the Greek Scholarship Foundation. No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit on the author(s) or on any organization with which the author(s) is/are associated. Reprint requests to Nikolaos Strimpakos, MSc, PT, PEd, Centre for Rehabilitation Science, University of Manchester, Central Manchester and Manchester Children’s Hospital’s NHS Trust, Oxford Rd, Manchester M13 9WL, UK, e-mail: [email protected]. 0003-9993/04/8508-8159$30.00/0 doi:10.1016/j.apmr.2003.08.104

USCLE FORCE TESTING is important in estimating the M extent of disability and as an indicator of neck dysshow reduced muscle function. Cervical spine studies 1,2

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strength in patients with neck pain, headache, and other neckshoulder disorders. However, it is often difficult to distinguish whether the muscular weakness is the cause of acute or recurrent injury and pain or is a result of the pain itself. Debate continues regarding the correlation between pain and strength measurements. Many data exist on improved neck strength and reduced neck pain after rehabilitation.3,4,6,12-14 Others9,15 insist that it is not valid to quantify spinal disease through strength measurements, because strength correlates poorly with pain and disability and strength measurements are not very reproducible in patients. Patients’ fear of evoking their pain during neck measurements often makes their tests invalid, even though no serious adverse effects have been noted in patients or healthy subjects after maximum voluntary isometric contractions3,6,12 (MVIC). Despite conflicting opinions about strength and pain correlations, there is a general consensus that strength measurements are of clinical value, at least for determining training dosage and for documenting rehabilitation efficacy. Unfortunately, existing studies have failed to give reliable and conclusive recommendations for clinical usage. A recent review on the reliability and validity of neck strength measurements showed that many studies are flawed, mainly in their methods, instruments, and statistical analysis.16 Only 1 article published before 2001 reported strength measurements for all neck movements17; most studies evaluated strength in 1 plane—namely, the sagittal plane. Two other studies, by Moroney18 and Vasavada19 and colleagues, used instruments able to measure in all directions. However, neither study reported on reliability and Moroney noted study limitations related to problems with instrumentation. Only a few studies6,17-20 used instruments able to measure rotation. Problems concerning the axis of rotation and the construction of strength measurement devices with 3 degrees of freedom and sufficient stabilization have kept researchers away from this measurement. Reliability and validity estimates were confounded also by the variety of methods and protocols. Initial body position (ie, standing, sitting, lying) seems to play a crucial role in determining maximum strength values. Indeed, peak extension values published from studies that used different examination positions varied from 22 to 245N.20-24 Most studies measured strength from a sitting position; thus, information on the standing position (a functional position adopted in many occupational settings) is sparse. Other problems related to insufficient information about neck strength stability over time and lack of consensus on the most appropriate number of repetitions within the same experiment. Further, warm-up and pretesting efforts are needed to protect the subjects from injury and to provide clinical stability, yet these factors are rarely described.16 Diversity of statistical analysis techniques hinders comparison between studies. No single coefficient is adequate for a thorough and valid reliability analysis, and many researchers Arch Phys Med Rehabil Vol 85, August 2004

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CERVICAL SPINE MUSCLE STRENGTH, Strimpakos

These points could be adjusted for each subject without affecting the moment arm, provided that the rotational axes of the apparatus and cervical spine remained in the same vertical plane (fig 2). The moments for axial rotation were revolved around the vertical axis at a radius of 50mm. All adjustable parts of this apparatus (seat, frames, transducer) were labelled in centimeters for reproducibility on subsequent measuring occasions. The data were captured by using the LabVIEW, version 5.1, software program.c

Fig 1. Measuring neck strength during flexion in sitting.

failed to consider the sample size of their study in the interpretation of their results.16 The aim of our study was, therefore, to evaluate the testretest reliability of a new protocol for measuring neck muscle strength using a custom device that enables the following factors to be taken into account: measurement of neck strength in all primary directions, the number of within-session repeats, the effect of initial body position on neck strength, and the statistical analyses used. The study hypotheses were that (1) neck muscle strength could be measured reliably over time in all primary movements in a healthy population, (2) neck strength measurements would yield a lower intratester reliability from a sitting than a standing position in a healthy population, and (3) men would yield higher peak values for neck muscle strength than women. METHODS Thirty-three healthy subjects were recruited for intraexaminer reliability from a local physiotherapy department. Sex, age, and anthropometric characteristics (height, weight, body mass index [BMI]) were recorded. Selection criteria were ability to give informed consent, aged between 18 and 65 years with the same ratio between men and women, and ability to write and comprehend Greek or English. Subjects had to be healthy without history of neck pain, headache, injury, or surgery in the cervical spine as determined by a self-completed questionnaire. All subjects gave full informed consent before the experiments. A subset of 10 subjects was selected for the interexaminer study. Apparatus A custom-made isomyometer devicea was used for these measurements. The device consisted of a 50-kg load cellb and a stabilization system with 2 frames: one at the level of the chest, the other at the lumbar spine (fig 1). The load cell forms part of a fixed-length strut or tie that was stationary for all occasions. The forces were always axial to the transducer. A separate part of the apparatus (using the same load cell) measured axial rotation in relation to 4 fixation points on the skull. Arch Phys Med Rehabil Vol 85, August 2004

Fig 2. Measuring neck strength during rotation in standing.

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CERVICAL SPINE MUSCLE STRENGTH, Strimpakos Table 1: Anthropometric Characteristics of Subjects Subjects

Intraexaminer Study Men

17

Women

16

Total

33

Interexaminer Study Total (N⫽10; 4 men, 6 women)

Age (y)

Height (cm)

Weight (kg)

BMI (kg/m2)

Mean ⫾ SD Range Mean ⫾ SD Range Mean ⫾ SD Range

23.5⫾5.3 19–37 28⫾11.9 20–63 25.7⫾9.3 19–63

179.5⫾7.6 169–192 165.9⫾7.8 150–182 172.9⫾10.2 150–192

82.8⫾10.8 64–105 60.9⫾6.9 52–70 72.1⫾14.1 52–105

25.7⫾3.8 21.2–35.5 22.3⫾3.0 19.0–29.8 24.0⫾3.8 19.0–35.5

Mean ⫾ SD Range

25.1⫾6.9 20–37

170.7⫾8.6 160–184

68.9⫾11.7 52–91

23.6⫾2.9 19.0–26.7

n

Abbreviation: SD, standard deviation.

Procedure The test procedure was explained to subjects before the experiments. After a warm-up session,3,6,20 neck movements in all primary planes (sagittal, vertical, horizontal) were examined in the same order (extension, flexion, right lateral flexion, left lateral flexion, right rotation, left rotation) from 2 initial positions, first sitting and then standing. The direction of movements and the positions were not randomized to eliminate the possibility of ordering effect on the test-retest measures. For flexion, subjects faced toward the instrument, and for all other movements they faced away. The head and the trunk were adjusted to the upright neutral position by ensuring that the line between the nasion and the opisthion was horizontal.20,25 When sitting, subjects relaxed the shoulders, arms, and legs, and the angle of the hips and knees was set at 90°. In both positions, the arms were hung at the side and when standing the subjects were measured with bare feet. Subjects were required to push isometrically on the load cell, which was placed against the occiput while testing extension, midway between the inner canthus of both eyes during flexion, and above the external auditory canal while testing side flexion.20,26 The rotational center of the apparatus in the side view was also placed above the external auditory canal to align it with the rotational center of the cervical spine that runs through the external auditory meatus. Subjects were required to tuck in their chins slightly during flexion because this was more comfortable and very important for measuring all neck flexors.7,27 Subjects were instructed to make 3 practice contractions at different force levels, each lasting for 5 seconds. Three or more MVIC attempts of 5 seconds each12,20-23,25,28 with 1 minute between each10,23,29,30 were then requested until the efforts were within 10% of each other,6,10 and the best effort was taken as the MVIC.5,12,21,22,25,28,31-34 Subjects were verbally encouraged (with standard commands) to give maximum effort. Subjects were given about 3 minutes to rest between different movements and 15 minutes between the sitting and standing positions. To assess intraexaminer reliability, the tests were repeated on 3 occasions (5– 8d apart) by the same investigator20,23,35 at the same time of day to minimize any effect of diurnal variation. A full verbal report of factors that could influence neck strength over this period was obtained, including possible pain or injury of the neck, drugs for other conditions, specific neck exercises, or other unusual activities. For the women, the measurements were taken within 1 full menstrual cycle.36,37 Interexaminer Study To evaluate the intertester reliability of the measurements, 10 of the 33 subjects were examined on the same day by a

second blinded, independent investigator after a 15- to 30minute rest. The evaluations of each subject were performed from the more reliable standing position and one after the other in random order, so that the same investigator was not always the first to perform the testing. Data Analysis Recorded strength data (newtons, newton meters) were transferred to SPSS statistical programd for subsequent analysis. Because all data (anthropometric and strength values) were initially tested for normality of distribution according to the Kolmogorov-Smirnov test, parametric tests were used. For analysis of intraexaminer and interexaminer reliability, we used repeated-measure analysis of variance (ANOVA). The intraclass correlation coefficients (ICC1,3, ICC2,1 for intraexaminer and interexaminer study, respectively); the standard error (SE) of measurement, defined as the square root of the within-subject mean squared error from the repeated-measures ANOVA; and the smallest detectable difference (SDD) (SDD⫽1.96公2⫻SE expressed as a percentage of parameter’s grand mean) reliability indices were each calculated. Discrepancies between trials in the interexaminer study were assessed using the technique described by Bland and Altman,38 which presents visual interpretations of the amount of agreement of the means of 2 trials against the difference between the trials. The use of 95% confidence intervals (CIs) of the range of differences between the 2 trials shows how closely the measurements agree on different occasions. We performed an independent-samples t test to understand the effect of sex on strength values. Paired-samples t tests were used to identify between-sides differences for side flexion and rotation and between-positions differences (sitting vs standing) for each day and to identify significant differences between the 2 examiners for the interexaminer study. The level of significance for all calculations was set at the 5% confidence level. RESULTS Anthropometric Characteristics Table 1 shows the anthropometric characteristics of the subjects measured in the intraexaminer and interexaminer study. Intraexaminer Study All reliability indices for the intraexaminer reliability study are in table 2. The ICCs between the 3 trials among all subjects for both positions ranged between .84 and .96 for all 6 movements. For sitting, the values ranged from .84 to .90 and for Arch Phys Med Rehabil Vol 85, August 2004

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CERVICAL SPINE MUSCLE STRENGTH, Strimpakos Table 4: Peak Strength Values From Tests 2 and 3 for All Subjects, by Position

Table 2: Intraexaminer Reliability and Clinical Applicability Results for Neck Strength in All Directions From 2 Initial Positions for All Test Occasions and for Tests 2 and 3 (All Subjects) Sitting Movement

Flexion All tests Tests 2, 3 Extension All tests Tests 2, 3 Right flexion All tests Tests 2, 3 Left flexion All tests Tests 2, 3 Right rotation All tests Tests 2, 3 Left rotation All tests Tests 2, 3

Sitting

Standing

Movement

Mean ⫾ SD

Mean ⫾ SD

P Value

Flexion Extension Right flexion Left flexion Right rotation Left rotation

166.6⫾75.8 241.7⫾80.8 179.6⫾67.3 178.3⫾66.1 10.68⫾5.22 10.85⫾5.37

153.8⫾74.9 218.4⫾74.6 169.1⫾62.5 168.8⫾64.3 10.68⫾5.37 10.84⫾5.05

.000* .000* .000* .000* .998 (NS) .998 (NS)

Standing

ICC1,3

SE

SDD (%)

ICC1,3

SE

SDD (%)

.89 .97

18.9 14.2

33.25 23.64

.96 .99

12.6 7.7

23.38 13.91

.84 .94

27.5 20.9

33.16 23.99

.95 .95

16.3 16.5

21.04 20.99

.88 .97

17.0 11.2

27.77 17.32

.93 .96

14.9 12.3

25.04 20.18

.88 .93

19.3 17.4

31.42 27.11

.94 .97

13.1 10.4

22.19 17.02

.90 .97

1.4 1.0

37.59 24.85

.92 .98

1.2 0.7

33.22 18.24

.87 .97

1.5 0.9

39.84 23.41

.89 .97

1.3 0.8

33.93 21.27

NOTE. SE of measurement values for flexion, extension, and right and left flexion are in newtons; for rotation, values are in newton meters.

standing from .89 to .96. When the first-day results were excluded, the ICCs exhibited higher scores, ranging in sitting from .93 to .97 and in standing from .95 to .99. For male subjects, the ICCs ranged from .76 to .93 in sitting and from .87 to .97 in standing for tests 2 and 3. The female subjects generally yielded higher values, ranging between .86 and .94 in sitting and .89 to .96 in standing (table 3). The SE of measurement ranged from 11.2 to 27.5N in sitting and 7.7 to 16.5N in standing for the flexion, extension, and side flexion for all 3 tests and 11.2 to 20.9N in sitting and 7.7 to 16.5N in standing for the second and third tests only. For

NOTE. Values for flexion, extension, and right and left flexion are in newtons; for rotation, values are in newton meters. Abbreviation: NS, not significant. *Significant difference between positions.

rotation movements, the SE ranged from .9 to 1.0N-m in sitting and .7 to .8N-m in standing for the last 2 measurements for all 33 subjects (see table 2). The male subjects yielded higher SE estimates than the female subjects because they had higher maximum strength values (see table 3). The SDD indices varied from 17.3% to 39.8% in sitting and from 13.9% to 33.9% in standing for all tests. When we excluded the first test, the SDD estimates were lower (ⱕ27.1% in sitting, ⱕ21.27% in standing). Flexion from standing had the lowest SDD value (13.91%) and left rotation in sitting had the highest SDD value (39.84%). With the exception of rotation, women’s SDD estimates (12.5%–18.2%) were generally lower than those for men (13.1%–21.3%) (see table 3). When we compared peak strength values (table 4), we found that extension in sitting yielded the greatest mean strength (241.7N) and flexion in standing the lowest (153.8N). When the results from the first day measurements were removed, the differences between means were not significant (P⬎.05). In relation to each subject’s maximum extension strength, the flexion-to-extension ratio averaged .69 and side flexion-toextension ratio averaged .74. In general, the standing position yielded significantly lower peak strength values than sitting (P⬍.05) with the exception of rotation, for which the differ-

Table 3: Male-Female Intraexaminer Reliability Values for Tests 2 and 3 Movement Subjects

Flexion Male Female Extension Male Female Right flexion Male Female Left flexion Male Female Right rotation Male Female Left rotation Male Female

Sitting

Standing SDD (%)

Mean ⫾ SD

ICC1,3

19.0 5.4

23.2 15.1

213.3⫾55.1 90.6⫾21.0

.97 .96

10.0 4.0

13.1 12.6

.83 .91

27.4 10.9

25.1 17.1

273.4⫾59.5 159.9⫾31.9

.87 .95

20.9 7.2

21.3 12.5

233.4⫾45.5 122.5⫾26.5

.92 .89

13.4 7.0

15.9 15.8

217.9⫾44.2 117.0⫾25.8

.89 .92

15.8 7.7

20.1 18.2

231.4⫾45.6 121.9⫾23.0

.76 .86

23.4 7.5

28.1 17.2

218.0⫾46.0 116.0⫾27.7

.92 .96

13.4 5.5

17.1 13.1

14.4⫾4.5 6.7⫾1.95

.93 .91

1.22 0.60

23.5 24.5

14.47⫾4.8 6.66⫾1.7

.96 .93

0.9 0.4

16.9 17.7

14.8⫾4.5 6.6⫾2.06

.93 .94

1.2 0.5

22.5 20.6

14.67⫾3.9 6.77⫾1.9

.94 .89

0.99 0.65

18.8 26.6

Mean ⫾ SD

ICC1,3

228.7⫾50 100.7⫾24.4

.87 .94

302.0⫾62.7 178.0⫾35.6

SE

SE

SDD (%)

NOTE. SE of measurement values for flexion, extension, and right and left flexion are in newtons; for rotation, values are in newton meters.

Arch Phys Med Rehabil Vol 85, August 2004

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side flexion, whereas the SE in rotation ranged from 0.83 to 1.25N-m. The SDD was less than 29.4% for all movements, except the right rotation for which the SDD was 38.8%. No significant difference was observed between the 2 examiners (P⬎.05). The levels of agreement plots for each movement are in figures 4 and 5. These figures show that there was no systematic bias between examiners. Moreover, any difference between the examiners was not related to the magnitude of the measured strength. DISCUSSION The method and the device used in this study gave easily reproducible measurements by the same or a different examiner. The ICC values in the intraexaminer study were almost excellent (⬎.93) for all movements in both positions, and the SE of measurement was quite low (⬍21N), indicating small variation among subjects. These results are similar to those described by Vernon,17 Ylinen,20 and Peolsson27 and their colleagues. However, different methods and statistical techniques make any direct comparison impossible. When we excluded the first test and took measurements in the standing position, the SDD was less than 21% for all movements—a result similar to or lower than that from some other studies.17,20,39 Despite the general improvement in the SDD compared with other studies, the value is still quite high and represents the percentage change that would have to be overcome to show that any intervention, for example, had had an effect.

Fig 3. Mean strength for all 3 trails (A) in sitting, (B) in standing, and (C) for rotation in both positions.

ences between both positions were not significant (P⫽.998). Mean strength for all trials is presented in figures 3A, 3B, and 3C. For all subjects in both positions, the paired-samples t test showed that the maximum strength values did not differ statistically between right and left side flexion or between right and left axial rotation (see table 4). As for sex differences, men were 42% to 58% stronger than women in all movements in both positions (P⬍.05) (see table 3). Interexaminer Study All data from the interexaminer study are summarized in table 5. The correlation between the 2 examiners was very high (ICC2,1 range, .88 –.94) for all 6 movements. The SE of measurement ranged from 10.7 to 20.8N in flexion, extension, and

Effects Practice. When we excluded the first test, all reliability estimates were better and the peak strength values also were greater. One practice or familiarization test has been used by several investigators in both cervical and lumbar spine measurements6,40 and seems to be needed to achieve more reliable and accurate results. This familiarization should be routine in strength measurements, to eliminate fear, increase confidence, and facilitate the compliance of neck soft tissue.3,6,35 Position and movement. The initial body position for measuring neck muscle strength seems to be very important for the magnitude of the results. Different initial body positions yielded different strength values for both patients and healthy subjects.17,21,23,24 Only 1 study previously compared 2 positions with the same instrument,21 although in that study the mean values for neck extension differed from all other published values, making the results questionable and incomparable. In our study, standing position produced higher ICC estimates with lower SE of measurement and SDD than sitting (see table 2); however, the maximum strength values in standing were significantly lower (P⬍.05) for all motions but rotation (see table 4). One main reason for these results seems to be the stabilization system and the compensation from parts of the

Table 5: Interexaminer Reliability and Strength Values Movement

1st Examiner (mean ⫾ SD)

2nd Examiner (mean ⫾ SD)

ICC2,1

SE

SDD (%)

P Values

Flexion Extension Right flexion Left flexion Right rotation Left rotation

153.6⫾45.8 219.7⫾71.8 167.7⫾59.1 161.8⫾46.3 8.93⫾3.7 8.81⫾3.5

147.4⫾43.8 208.2⫾61.9 165.6⫾40.6 156.6⫾38.5 8.89⫾3.6 8.47⫾3.3

.92 .90 .88 .94 .88 .94

13.0 20.8 17.6 10.7 1.25 0.83

23.9 27.01 29.4 18.7 38.8 26.6

.310 (NS) .249 (NS) .797 (NS) .305 (NS) .944 (NS) .385 (NS)

NOTE. Values for flexion, extension, and right and left flexion are in newtons; for rotation, values are in newton meters.


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Fig 4. Interexaminer differences between trials where the difference between 2 trials is plotted against the mean of 2 trials in (A) flexion, (B) extension, (C) right flexion, and (D) left flexion. The dashed lines are 95% CIs.

body other than the cervical spine. Many subjects reported using their trunk and legs while pushing the transducer in sitting, which resulted in greater strength values. Another problem when using the sitting position was the difficulty subjects had in establishing and maintaining the upright neutral position, despite corrections by the examiners. The standing position is more functional, with fewer factors that must be controlled by the investigators. The only comparisons that can be made using data from other studies are those for sitting, where the flexion and lateral flexion ratio relative to extension peak values (.69, .74, respectively) are similar. Vasavada et al19 reported for both the flexion/extension and lateral flexion/extension ratios a value equal to 0.7, whereas Kumar et al24 reported ratios of .73 and .75, respectively. Some older studies4,41 have shown ratios

Fig 5. Interexaminer differences between trials where the difference between 2 trials is plotted against the mean of 2 trials in (A) right rotation and (B) left rotation. The dashed lines are 95% CIs.


similar to those found in the current study. However, several researchers6,20,27,42 have published a lower flexion/extension ratio in the range of 0.4 to 0.6. On the other hand, other studies17,27 found a lateral flexion/extension ratio that was quite high (0.95–1.00). The discrepancy between published estimates may be due to the different methods and instruments used, the position of the head during measurements (offering physiologic and mechanical advantage), and the population studied. The Vasavada study19 also yielded strength moments in axial rotation (mean, 15N-m for men; mean, 6N-m for women) that were similar to those found in our study for both sexes. The slight differences between the 2 studies may be due to different study sample characteristics and the smaller sample size of Vasavada’s study. The values reported by Ylinen et al20 (mean,


8.7N-m) are lower than those of our study, but these investigators did not publish details of their sample, so any conclusions and comparisons are not possible. Sex. In our study, women yielded higher ICCs with lower SEs of measurement and SDD estimates than men in all movements except rotation (see table 3). One possible reason for these differences is that women produced lower forces than men, thus influencing the statistical analyses. The maximum strength generated by the women ranged between 42% to 58% of the men’s peak strength. These findings agree with most studies2,3,19,27 but contrast with other studies24,39 that presented a range of 20% to 40% strength difference between sexes. This finding may be the result of differences in subject characteristics, an effect of training,39 and/or differences in stabilization and isolation of the cervical spine. Interexaminer Study Although the sample size for the interexaminer study was small, reliability estimates were very high (ICC2,1 range, .88 – .94) with a small SE of measurement (ⱕ21N). These values were similar to or better than those from many other studies,23,27,29,42,43 which ranged between .54 and .95. The SDD was higher than the intraexaminer study (18.7%–38.8% vs 13.9%– 21.3%), as expected from the literature.16 Comparison with other studies was not possible because of the lack of published estimates. Only Agre et al43 reported a coefficient of variation of 17%, but their sample consisted of only 4 subjects. Despite the directions and the standard protocol, errors could be present because of the slightly different neutral position and differences in the way the 2 examiners gave commands. Another more likely explanation for the quite high SDD values in our study is the small sample size, because an outlier in a small sample can easily affect the reliability estimates. Clinical Implications and Recommendations Establishing valid and reliable measures of neck function will enable clinicians to monitor intervention effects over time within groups of patients. It will also permit comparison of between-group differences (ie, healthy vs pathologic conditions). The use of these measures in this capacity now requires further exploration. CONCLUSIONS The value of measuring neck muscle strength has been reported in several studies.3-7,12,13,17,29 Measuring neck strength using the technique and methods described in the present study appears to be both safe and reliable, although the value of the measures in terms of detecting a clinically significant change requires further exploration. Acknowledgments: We thank John Daly and Dave Kruup for their technical help. References 1. Triano J, Schultz AB. Correlation of objective measure of trunk motion and muscle function with low-back disability ratings. Spine 1987;12:561-65. 2. Staudte HW, Duhr N. Age- and sex-dependent force-related function of the cervical spine. Eur Spine J 1994;3:155-61. 3. Highland T, Dreisinger T, Vie L, Russell G. Changes in isometric strength and range of motion of the isolated cervical spine after eight weeks of clinical rehabilitation. Spine 1992;17(Suppl 6): S77-82. 4. Jordan A, Mehlsen J. Cervicobrachial syndrome: neck muscle function and effects of training. J Musculoskel Pain 1993;1:283-8.

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38. Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1986; 327:307-10. 39. Jordan A, Mehlsen J, Bulow P, Ostergaard M, DanneskioldSamsoe B. Maximal isometric strength of the cervical musculature in 100 healthy volunteers. Spine 1999;24:1343-8. 40. Graves J, Pollock M, Foster D. Effect of training frequency and specificity on isometric lumbar extension strength. Spine 1990; 15:504-9. 41. Franco J, Herzog A. A comparative assessment of neck muscle strength and vertebral stability. J Orthop Sports Phys Ther 1987; 8:351-6. 42. Seng KY, Lee Peter VS, Lam PM. Neck muscle strength across the sagittal and coronal planes: an isometric study. Clin Biomech (Bristol, Avon) 2002;17:545-7. 43. Agre J, Magness J, Hull S, et al. Strength testing with a portable dynamometer: reliability for upper and lower extremities. Arch Phys Med Rehabil 1987;68:454-8. Suppliers a. Medical Engineering and Maintenance, Design and Development Section, St. Mary’s Hospital, Manchester, Whitworth Park, Manchester, M13 0JH, UK. b. Tedea-Huntleigh; Vishay Transducers, 37 Portmanmoor Rd, Cardiff, CF24 5HE, UK. c. National Instruments Corp, 11500 N Mopac Expwy, Austin, TX 78759-3504. d. SPSS Inc, 233 S Wacker Dr, 11th Fl, Chicago, IL 60606.