Reliability of Isometric Strength Measurements in Trunk and Neck Region: Patients With Chronic Neck Pain Compared With Pain-Free Persons
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CEOPS- Center of Excellence for Orthopedic Pain management Speising
Abstract
Scheuer R, Friedrich M. Reliability of isometric strength measurements in trunk and neck region: patients with chronic neck pain compared with pain-free persons.
Objective
Evaluation of reliability of isometric strength measurements in the neck and trunk region and comparison of these measurements between patients with chronic neck pain and pain-free subjects.
Design
Nonrandomized controlled trial.
Setting
Institutional practice.
Participants
Patients with neck pain (n=53) and pain-free persons (n=42) (mean age ± SD, 49.7±10.74 vs 48.7±12.02; women, 73% vs 71%).
Interventions
Strength of flexion, extension, and lateral flexion in the neck and trunk were measured. Each participant underwent 2 measurement passes on each of 2 examination days; 3 were performed by the same investigator, 1 by a second.
Main Outcome Measures
Intrarater (short-term and long-term) and interrater reliability, differences in strength between both groups of probands.
Results
Reliability in both groups ranged from substantial to almost perfect (intraclass correlation coefficient: patients, .76–.89; control group, .80–.88). The patients' strength in the neck and trunk was significantly below that of the control group (P<.002).
Conclusions
Isometric strength measurement is a reliable and feasible way to estimate the possible benefit of specific strengthening programs. Patients with chronic neck pain showed strength deficits in all measured regions.
A LONG WITH LOWER-BACK pain, neck pain is one of the most common musculoskeletal complaints in patients in the industrialized world. A Scandinavian study shows that the point prevalences of chronic lower-back pain and chronic neck pain were 16% and 17%, respectively. Fifty-one percent of the subjects had both back and neck pain.1 A Canadian study corroborates these results, finding that the age-standardized lifetime prevalence of neck pain is 66.7% and the point prevalence is 22.2%.2 In Austria, a recent survey found that 36.4% of respondents had experienced musculoskeletal pain in the preceding 3 weeks, with pains in the spinal region the most prevalent. The complaints of the severest pain commonly were indicated in the neck.3
The fact that no other underlying condition was found in a large proportion of patients with chronic spinal pain suggests that both psychosocial factors and posture problems followed by painful muscle tension play a key role in the pathogenesis of these so-called nonspecific pains.4 Posture problems are by definition correctable, by such means as targeted strengthening and stretching of the muscles. Even before the onset of pain, many of the affected patients were not in good physical condition, which only further deteriorated through their avoidance of movements that were painful or demanding. Neck pains are often associated with decreased strength in the musculature surrounding the spine,5 with women displaying a higher incidence of neck pain6 and taking longer to recover from whiplash.7
Numerous studies over the measurement of maximum muscle strength in the trunk8, 9 and neck region10, 11, 12, 13, 14, 15, 16support the reliability of these findings. However, in most of them, the apparatus used for measurement was self-made. Because of the differences in the construction of the machine—that is the measuring devices as well as the way the subject is belted—the reproducibility of the results is obviously affected. Furthermore, in many cases, only movements in a single direction in either the neck or lower back were measured, so the relative strengths of opposing muscle groups were scarcely calculated. This may be a result of the infeasibility of taking additional measurements under the limitations of the available equipment. Finally, to our knowledge, the muscle strength of patients with pain in the spinal region is rarely studied, while the investigation of the muscles surrounding the spine is of particular interest in these cases.
The large groups of muscles in the trunk and neck play a key role in the positioning of the spine, so imbalances in these muscles could cause posture problems. As a consequence, the following hypothesis can be posted: Measurements of the relative strengths of muscles in the spinal region of neck and trunk provide a means to calculate the risk of misalignment. Thus, in this study, we investigated the data from isometric measurements of maximum muscle strength in patients with neck pain and pain-free probands, as well as the reliability of these data.
This study is therefore assessed according to the following primary outcome criteria:
- •Are isometric measurements of maximum muscle strength reliable in persons with chronic neck pain? Are there differences in variability compared with pain-free subjects?
- •Are there significant strength deficits in the cervical and/or lumbar region in patients with chronic neck pain?
The main outcome measures of the study are the results of the isometric measurements of maximum muscle strength in the neck and back.
The secondary outcome criterion was as follows:
- •Are patients with chronic pain able to tolerate the isometric measurement of maximum muscle strength?
Methods
We recruited subjects from the general population according to the following eligibility criteria:
- •Men or women
- •18–70 years old
- •Informed consent
- •Among patients with neck pain:
- Pain that had lasted at least 12 weeks
- Neck pain both with and without emanation into the back of the head, shoulder girdle, and proximal upper arm
- •Among the pain-free control group:
- No pains originating in the spinal region in the previous 12 months
The risk to the participants was minimized by the exclusion criteria provided by the manufacturer of the testing apparatus, which mainly exclude persons with clinically significant cardiovascular diseases and musculoskeletal pathologies.
All procedures followed protocol and accorded with the ethical standards of the responsible ethics committee.
Because the isometric measuring procedure has certain advantages for persons with musculoskeletal pains,17 and the standing position supports physiologic lumbar lordosis,13 the following apparatus turned out to be well suited for this study. It is furthermore able to measure many directions of movement.
The Back Check 607a (fig 1) is commercially available equipment that has 3 adjustable arms for fastening the subject (seefig 1A) as well as 2 adjustable measuring mechanisms (see fig 1B).
Fig 1
Back Check 607—lumbar lateral flexion left. (A) Three adjustable arms for fastening the subject. (B) Two adjustable measuring mechanisms.
For the measurements in the neck region, we fastened the subject into the apparatus at scapula level,18 with the measuring mechanism positioned just above eye level. In measurements of the lower back, we fastened the subject into the apparatus at the pelvis, with the measuring mechanism positioned at sternum level. The measurements were taken in kilograms.
We measured the strength of extension, flexion, and lateral flexion in the neck and lower back. As figure 2 shows, on each of 2 separate days (see fig 2: day 1 + 2) within a 5-day span, we took 2 measurements (see fig 2: t1-4). As far as possible, we arranged the examinations of each subject at the same time of both days. The first 3 measurements were taken by the same male investigator (see fig 2: R1), while the last was conducted by a female investigator (see fig 2: R2). The second investigator was chosen to be female because we assumed this could affect probands' (male and female) behavior and ambition doing strength measurements more than another male investigator.
Fig 2
Work flow diagram for each subject. Abbreviations: R1, male investigator; R2, female investigator; t1–4, 4 measurement passes (time points).
Thus, we assessed short-term, long-term, and interrater reliability: short-term comparing t1 and t2, long-term comparing t1 and t3, and interrater comparing t3 and t4.
After each measurement, we asked subjects to quantify pain they experienced meanwhile on the 11-level numeric rating scale. We also performed 2 measurements of 30% to 50% strength before measuring the maximum muscle strength, which allows the required muscle groups to warm up.
The duration of the examinations was measured using a stopwatch, including the time needed for adjusting and fastening the subject into the apparatus.
Statistical Analyses
We analyzed test groups, measurements of muscle strength, pain intensity during the examinations, and the length of the examinations using descriptive statistics (mean, SD, maximum, minimum).
Furthermore we calculated an ANOVA for repeated measurements for each variable, accounting for the fixed-effects group, sex, measurement pass (4 time points) (see fig 2: t1-4), age, height, and weight and with the random effect test subject in order to investigate the difference in the measured target variables between the test groups. There are 9 degrees of freedom for the model, 89 for the patients, and 282 for the residuals.
In order to assess reliability, we chose the following approach. Analysis of variance components were performed separately for the control group and test group to determine the intersubject, interrater, long-term, short-term, and residual variability. The ICC was calculated as the ratio of the intersubject component of the total variance and is therefore a measure for the reliability of the investigations. The higher the ICC, the greater the variability between patients compared with the variability within the patient—that is, the higher the ICC, the lower the variability within the patients.
According to the guidelines provided by Landis and Koch,19 in interpreting the degree of agreement, .21 to .40 is seen as a fair level of agreement, .41 to .60 as moderate, .61 to .80 as substantial, and greater than .81 as almost perfect. Although these divisions are clearly arbitrary, they do provide useful benchmarks.
We assessed long-term, short-term, and interrater reliability by using variance component analysis in which the ratio of short-term, long-term, and interrater variability to the overall variability was determined, that is, the lower the value, the higher the respective reliability.
To investigate whether there is a difference in the variance over the 4 time points between the 2 test groups, we computed F tests for equality of variances.
As the level of significance, we chose a P value of .05. In order to correct for multiplicity, all P values less than .002 were considered as statistically significant (Bonferroni correction).
Results
We tested 53 patients with neck pain and 42 control subjects (mean age ± SD, 49.7±10.74y vs 48.7±12.02y; women, 73% vs 71%) for this study in the year 2008 (table 1, fig 3). As the P values of the t tests attest, the test groups were well matched (table 2).
| Group | Men | Women |
|---|---|---|
| Patients n (%) | 14 (26.42) | 39 (73.58) |
| Controls n (%) | 12 (28.57) | 30 (71.43) |
Fig 3
Flow chart of group formation: Recruitment of interested volunteers and assessment for eligibility.
| Group | Variable | Mean ± SD | Minimum | Maximum | P |
|---|---|---|---|---|---|
| Patients | Age (y) | 49.72±10.74 | 24.00 | 67.00 | .669 |
| Height (cm) | 168.13±8.76 | 150.00 | 187.00 | .952 | |
| Weight (kg) | 71.32±14.39 | 48.00 | 107.00 | .997 | |
| Controls | Age (y) | 48.71±12.02 | 24.00 | 67.00 | .669 |
| Height (cm) | 168.24±8.08 | 150.00 | 186.00 | .952 | |
| Weight (kg) | 71.31±12.36 | 45.00 | 95.00 | .997 |
The assessment of reliability of the isometric strength measurements in the trunk and neck region showed ICC values between .76 and .89 (table 3).
| Patients With Chronic Neck Pain (n=53) | Pain-Free Subjects (n=42) | ||
|---|---|---|---|
| Lumbar | Flexion | .88 | .80 |
| Extension | .89 | .88 | |
| Lateral flexion left | .82 | .87 | |
| Lateral flexion right | .85 | .87 | |
| Cervical | Flexion | .85 | .84 |
| Extension | .76 | .81 | |
| Lateral flexion left | .80 | .86 | |
| Lateral flexion right | .87 | .81 |
The comparison of the mean variance of the 2 test groups by means of the F test showed that, after the Bonferroni correction, there was no significant difference between them (P>.554).
Long-term, short-term, and interrater variability are very low (table 4) —that is, the lower the variability, the higher the respective reliability.
| Variables | Patients With Neck Pain (n=53) | Pain-Free Subjects (n=42) | |||||||
|---|---|---|---|---|---|---|---|---|---|
| Variability | Error | Variability | Error | ||||||
| Interrater | Long-Term | Short-Term | Interrater | Long-Term | Short-Term | ||||
| Lumbar | Flexion | .00 | 1.12 | 0.00 | 10.54 | .00 | 7.21 | 3.09 | 10.12 |
| Extension | .00 | 1.20 | 0.00 | 10.29 | .00 | 1.73 | 0.38 | 10.19 | |
| Lateral flexion left | .36 | 1.49 | 0.00 | 16.29 | .00 | 3.43 | 0.29 | 8.87 | |
| Lateral flexion right | .05 | 1.35 | 0.41 | 12.74 | .00 | 3.67 | 0.88 | 8.68 | |
| Cervical | Flexion | .00 | 1.39 | 0.00 | 13.97 | .00 | 7.21 | 0.79 | 8.10 |
| Extension | .00 | 3.62 | 0.00 | 20.16 | .46 | 6.47 | 1.61 | 10.75 | |
| Lateral flexion left | .32 | 2.99 | 1.59 | 15.12 | .00 | 3.41 | 1.08 | 9.78 | |
| Lateral flexion right | .00 | 2.14 | 0.01 | 10.64 | .00 | 9.07 | 1.30 | 8.72 | |
The ANOVA demonstrated a significant effect between the patient and control groups. In the control group, a significantly higher value for muscle strength (P<.002) was obtained across the 4 measurement passes. Patients' strength deficits were especially remarkable in the neck region, as table 5 shows. Furthermore, a significant effect between men and women was observed: the measurements of muscle strength in women were significantly lower (P<.001).
| Patients With Chronic Neck Pain (n=53) | Pain-Free Persons (n=42) | P | ||
|---|---|---|---|---|
| Mean ± SD | Mean ± SD | |||
| Cervical | Extension | 10.4±5.0 | 14.8±6.2 | <.001 |
| Flexion | 6.5±3.3 | 10.3±5.2 | <.001 | |
| Lateral flexion right | 8.5±4.2 | 11.7±5.5 | <.001 | |
| Lateral flexion left | 7.8±3.8 | 10.6±5.6 | <.001 | |
| Lumbar | Extension | 29.1±16.0 | 42.8±18.9 | <.001 |
| Flexion | 17.4±11.2 | 23.1±13.6 | <.002 | |
| Lateral flexion right | 19.6±12.5 | 29.1±16.0 | <.001 | |
| Lateral flexion left | 18.2±10.7 | 27.4±14.6 | <.001 |
A significant increase in maximum muscle strength could be observed over the course of the 4 measurement passes (P<.001) for both groups.
As ANOVA shows, age, height, or weight appeared to have no significant influence on muscle strength in trunk and neck (P=.055–.948).
In the control group, there was no indication of pain during the measuring procedure. Three persons in the patient group experienced pain between 2 and 8 on the 11-level numeric rating scale. However, according to their subjective indications, they claimed that they had been able to use 65% to 100% of their maximum muscle strength.
The mean duration of the procedure for the muscle groups relevant to this study was 12 minutes, 7 seconds ±115s over 182 recorded examinations.
Discussion
The results of the variability analyses showed that there was no significant difference in the mean variance of maximum muscle strength values between the patient and control groups. Moreover, the ICC values of the 2 groups showed good to very good correspondence over the 4 measurement passes, with respective values between .76 and .89.
The consistent reliability demonstrated in studies of maximum muscle strength, which were performed on neck and trunk with other measuring apparatuses, was corroborated by this study, and seems to be independent of the apparatus. The main difference to existing studies is the investigation not only of healthy subjects but also of patients with chronic pain. Furthermore, neck and trunk muscle strength was investigated in the sagittal as well as frontal directions of movement, while existing studies only investigated neck or trunk region and mostly only with regard to sagittal movements. Another difference is the use of commercially available equipment, which makes the measurements reproducible and useable for others.
As expected, the patient group demonstrated muscle weaknesses compared with the control group. However, these weaknesses affected different groups of muscles in various ways and were especially remarkable in the neck region. In the neck, there were significant weaknesses in every direction of movement measured: the musculature of the neck was not able to provide adequate support. Because of the large amount of leverage the neck muscles need to support the head, this weakness of the neck muscles can lead to a heightened risk of injuries caused by even slight impacts, which could cause neck pain to become chronic.
Muscle weaknesses were likewise observed in the lower back. However, these muscles need not provide as much leverage as those of the neck.
The observed increase in strength over the course of the 4 measurements taken suggests that it might be advisable to take more than 1 measurement during each session and record the highest. However, in this case, a corresponding recovery period should be planned. We also performed 2 measurements of 30% to 50% strength before measuring the maximum muscle strength, which is quite advisable: it allows the required muscle groups to warm up and creates a certain learning effect.8, 13
In addition, it can be observed that the isometric measuring procedure is commonly a less strenuous test of maximum muscle strength for patients. Altogether, the measuring procedure was tolerated very well.
The strength measurements performed in this study have proved to be methods feasible in practice, as the average duration (approximately 12min) of the procedure shows.
The objective evidence of training successes could motivate patients to maintain physical exercise programs.20, 21
Study Limitations
Limitations of this study were the limited possibilities of fastening the subject in the Back Check system compared with other measuring devices, which measure only 1 direction of movement. Another limitation is the impossibility of measuring the strength of rotation. In addition, the measurement of individual muscle groups is not possible, although the movements measured in this study are perhaps more relevant in everyday life. The use of auxiliary musculature can be prevented by experienced researchers. Finally, the pains in the patient group were not always limited to neck pain but also included lower back pain in about 44%, which are generally quite often accompanied by each other .1, 3
Conclusions
The findings of this study suggest that measurements of muscle strength in the trunk and neck can provide a reliable and useful diagnostic tool in everyday practice for patients whose complaints of nonspecific pains in the spinal region cannot be traced back to a clear underlying cause. Muscle weaknesses are very probable in patients with chronic pains, although this is not always a necessary precondition. Therefore, by means of an assessment as conducted in this study, it is possible to quantify these weaknesses without great effort and thus assess the possible benefit of muscle-strengthening exercises. Furthermore, the objective evidence of training successes could motivate patients to maintain physical exercise programs.
Supplier
aDr. Wolff Sports & Prevention GmbH, Bachumer Weg 70, D - 59757 Arnsberg, Germany.
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