The Reliability of Measuring Neck Muscle Strength with a Neck Muscle Force Measurement Device

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1 Original Article The Reliability of Measuring Neck Muscle Strength with a Neck Muscle Force Measurement Device J. Phys. Ther. Sci. 15: 7 12, 2003 ASGHAR REZASOLTANI, Ph D, PT 1, 2), AMIR AHMADI, B Sc, PT 1), AZADEH JAFARIGOL, B Sc, PT 1), VEIKKO VIHKO, Ph D 2) 1) Department of Physiotherapy, Faculty of Rehabilitation, Medical University of Shaheed Beheshti: Damavand Ave. across from Bo-Ali Hospital: Tehran, Iran. TEL FAX rezasoltani@medscape.com 2) LIKES- Research Center for Sports and Health Sciences, Rautpohjankatu 10, JKL, Finland. Abstract. The purpose of this study was to examine the reliability of an isometric neck muscle force measurement device. Thirty-five healthy non-athlete subjects (18 males and 17 females, aged 19 30) participated in this study. The maximal isometric muscle strength of the neck extensor and flexor muscles were measured at different times and on different days using a newly designed neck muscle force measurement apparatus. On each occasion, subjects performed three maximum voluntary contractions in each direction of neck extension and flexion movement. The interclass correlation coefficient (ICC) and standard deviation within subjects (Sw) were computed to evaluate the reliability and magnitude of measurement error between measurements. The results of the intra-rater, test-retest, and inter-rater reliability (ICCs 0.94, Sws 4.9 N) indicate that the neck muscle force measurements were highly repeatable and varied little between measurements. There were no statistically significant differences in neck muscle force measurements between times, between days and between raters. Maximal isometric muscle strengths were significantly higher in males than in females (p < 0.00). Women s neck muscle strengths were 55% and 54% of men in cervical extension and cervical flexion, respectively. The isometric muscle force measurement device which was used in this study appears to be a reliable and useful method for measuring the force of the neck extensor and flexor muscles. It could also be suitable for the assessment of a physiotherapeutic or rehabilitation program. Key words: Reliability, Neck, Force (This article was submitted Jul. 17, 2002, and was accepted Sep. 30, 2002) INTRODUCTION Strength is regarded as the peak torque developed during maximal voluntary contraction (MVC) 1). The strength of a group of muscles can be measured by a muscle strength test, employing a force measuring instrument. Force measuring devices have been used to determine and monitor the effects of training programmes on back extensor muscles in healthy subjects 2). It was shown that a certain isometric back exercise training programme effectively improved muscle strength in back extension. By employing an isometric cervical muscle force measurement device Pollock et al. (1993) reported a significant improvement in isometric cervical extension force after a full range of cervical motion training 3). In comparative studies, other researchers used an isometric cervical

2 8 J. Phys. Ther. Sci. Vol. 15, No. 1, 2003 Table 1. The characteristics of the subjects studied (n=35) Groups n Age Weight (kg) Height (cm) BMI Males ± ± ± ± Females ± ± ± ± BMI = Weight/height 2. muscle force measurement device and measured isometric cervical strength in males and females, showing that the strength of the cervical flexor and extensor muscles in females were 61% and 44% respectively of that in the male subjects 4, 5). In another study, the effect of the aging processes on cervical muscle strength was evaluated by a similar appratus in both males and females 6). It was shown that the cervical muscle strength may decrease by 20 to 25 percent more in males than females in both flexion and extension. In the field of rehabilitation, neck muscle force measuring instruments have been used to evaluate neck muscle strength and the effectiveness of strength training in relieving pain and improving cervical muscle function in cervicalgia 7 9). Silverman et al. (1991) showed that patients suffering from neck pain and disability had weaker cervical flexors than the controls 7). Levoska and Keinanen-Kiukaanniemi (1993) applied two different kinds of physiotherapeutic methods to occupational cervicobrachalgia 8). In a one-year follow-up study of the effects of dynamic exercise therapy, they found a significant increase in cervical extension strength. However, while their method involved using a variety of therapeutic techniques, they observed that a specific active physical exercise therapy programme seemed to work better than all the others in the treatment of neck disorders. Ylinen and Ruuska (1994) used a certain rehabilitation programme to treat cervical disorder patients and revealed a significant improvement in neck muscle force by a cervical muscle force measurement instrument 9). They also showed that this improvement in cervical muscle force correlated significantly with a lessening of pain and disability. The aim of this study was to test the reliability of a newly designed isometric cervical muscle force measurement device, with testing parameters of flexion and extension. The major criticism of this study was to detect the extent of any real changes in cervical muscle force measurement while repeating the measurements. SUBJECTS AND METHODS The experiment included a study of 35 healthy volunteers (18 males, aged years and 17 females, aged years) in the inter-trial reliability. Twenty seven subjects (16 subjects and 11 subjects) were randomly selected for the testretest and the inter-rater reliability studies. All participants were students or office workers in the city of Tehran, Iran. The exclusion criteria were: having any history of hearing disabilities, temporomandibular joint dysfunctions, neuromuscular neck disorders such as limited range of motion, muscular soreness, pain or spasm, spinal abnormalities, muscle tonicity affected by medication and subjects having had any sports or fitness program involving the neck and shoulder muscles within the previous three months. In order to exclude any cases with systemic diseases which may affect normal muscle action, such physical tests as the temporomandibular joint test, the neck distraction and compression tests, the sensation test and the test for neuromotor reflexes of upper limbs, the range of motion test and the manual neck and upper limbs muscle test were all performed. Height and weight were measured with the usual anthropometrics measurement procedures. Body mass index (BMI) was computed as weight/height 2. The mean, SD and range of the age and anthropometric characteristics of the subjects are presented in Table 1. The force generated by the cervical extensor and flexor muscles was measured with a neck muscle force measurement apparatus. The device was made of two parallel bars which were fixed to the wall. The cell load was mounted in a box and its level was horizontally and vertically adjustable. There were two thorax and pelvic stabilizers which

3 9 trunk, upper and lower limbs. Then, they were instructed to perform three MVCs of neck extension and flexion each lasting for 3 4 seconds, with 5- minute intervals between MVCs. The order of testing was similar for all subjects. In the inter-trial and test-retest reliability, Maximal isometric muscle strengths were measured twice a day with a one hour interval, on two successive days with a one week interval at the same time each day. Repositioning of the subjects was carried out between each set of testing. Testers were two physiotherapy students (male and female) who were adequately trained and educated by the same instruction. STATISTICS Fig. 1. Subject s position during isometric muscle testing. were also adjustable according to the body size of the subjects. The calibration of the device which was tested by a standardized load remained unchanged in each day prior to testing. Subjects sat with the head and neck in a neutral position (Fig. 1). While the load cell was against the occipital bone to measure cervical extension force, it was against the frontal area for cervical flexion force measuremement. In the all force measurements both hands were on the thighs and arms close to the body, both hips were in adduction and 90 degrees of flexion with both knees in 90 degrees of flexion and both feet were on the floor. The sternal notch, chin and tip of the nose were kept in a vertical line, and the line between the base of the nose and occiput was horizontal. The seat surface was horizontal. The thorax and pelvis were tightly fixed by two straps at the level of spine of scapula and iliac spine. In order to warm up, subjects performed a few (2 3) submaximal cervical muscle contractions before each set of maximum voluntary contractions (MVCs). The subjects were instructed to relax their In all the reliability studies, the intraclass correlation coefficient (ICC), the coefficient of variation (CV%), and the standard deviation within subjects (Sw) were computed. The ICC was calculated from one-way analysis of variance 10). This method is recommended in the literature as a relatively dependable way of evaluating the reliability between two or more sets of measurements 11). The CV% was calculated as the mean derived by the equation [(SD/mean) 100] across trials for inter-trial, test-retest and inter-rater reliability. The Sw was used as a measure of absolute reliability by which the variability of the scores gained from measurement to measurement was examined 11). The Sw, given in metric measures, expresses the magnitude of the measurement error in the same units (Sw = SD 1ICC ). For each Sw, a 95 percent confidence interval was taken into account in the calculations (1.96 Sw) 12). In order to make it easier to interpret the Sw, the smallest detectable difference (SDD) was also computed as Sw ). Paired t-tests were used to compare the differences between maximal isometric muscle strength measurements measured at different times, on differet days and by the first and second rater and to detect the difference in cervical extension and flexion force between males and females. Pearson s product moment, scatter plotting and R squared were used to determine the association between MVC measurements measured at two different times, on two different days and by two different testers. The SPSS statistical software

4 10 J. Phys. Ther. Sci. Vol. 15, No. 1, 2003 Table 2. The intraclass correlation coefficient (ICC), the percentage of the coefficient of variation (CV%) and the standard deviation within subjects (SW with 95-percent confidence intervals) for the intrarater (intertrial), test-retest and inter-rater reliabilities of the measurements of the maximal isometric neck extension and flexion forces. No statistically significant differences were found between measurements taken at different times of the day, on different days, and between raters The reliability measurements n ICC CV% Sw (N) SW (N) (95-% CI) SDD (N) Inter-trials Extension ± Flexion ± Test-retest Extension ± Flexion ± Inter-raters Extension ± Flexion ± CV% = (SD/MEAN) 100. Sw = SD ( 1 ICC ). SW, 95-CI (confidence interval) = Sw SDD (smallest detectable difference) = Sw package (version 10.0 for Windows) was used for all statistical analyses. Statistical significance was set at a 5 percent error level (p < 0.05). RESULTS Intrarater reliability Table 2 presents the results concerning the intertrial and test-retest reliability of measurements taken by the same rater. There were no significant differences in muscle force measurements taken at different times of the day and on different days. The consistency of the three repeated measurements at the same time by the first and second rater were ICC=0.95 and ICC=0.94. Inter-rater reliability Table 2 demonstrates the results of the inter-rater reliability of the measurements. The ICC, CV% and Sw indicate a high degree of repeatability of the cervical muscle force measurements between raters. There were no statistically significant differences between cervical muscle force measurements as measured by the first and the second examiner. The correlations between MVC measurements measured at two different times, on two different days and by different testers were significantly high for both cervical extension and flexion forces (0.92 < r < 0.97, p < 0.00). Figure 2 represents the results of the scatter plotting and R squared for both measurements of the neck extension and flexion forces. Male subjects appear to have significantly higher cervical extension and cervical flexion forces than female subjects (p < 0.00). The ratio of the neck flexion force on neck extension force measurements was not different between males and females. The results of the cervical extension and flexion force measurements are provided in Table 3. DISCUSSION In this study, different statistical methods were used to determine the reliability of a newly designed cervical muscle force measurement instrument for force measurements of the two cervical extensor and flexor muscle groups. In all the reliability studies undertaken here, the results of the ICC and CV% were in concordance with the Sw measurements of force measurements. The standard deviation within subjects (Sw) is recommended in the literature as the most important index of reliability, which quantifies errors; a smaller Sw indicating a higher degree of reliability 12). In a follow-up training programme the Sw can isolate the true measurement from an error resulting from an intervention 14). In rehabilitation assessment the results of Sws are very informative. For example, by computing the SDD an examiner who wants to estimate a patient s progress will know what differences need to be measured to allow a valid conclusion that real change has occurred and exclude the possibility that the results are due to measurement error. This means that the smallest detectable difference must be small enough to indicate real changes during the course of the intervention. The SDD in muscle force measurements in healthy subjects showed that only changes larger than N (for time, day and examiner effects) can be regarded as real changes

5 11 Fig. 2. The scatter-plotting and the R squared results of the relationship between: a) the isometric neck muscle force measurements at different times (time1 and time2, n=35) and on separate days (day1 and day2, n=16); and b) the isometric neck muscle force measurements by two raters (n=11). Table 3. The isometric neck extension and felxion forces and the ratio of the neck flexion on extension force in males (n=18) and females (n=17) Groups n Extension (N) Flexion (N) Flex./Ext. Males ± ± ± Females ± ± ± when measuring cervical muscle force after any muscle training or therapeutic programmes. In the reliability measurements of this study, the biological variation between measurement occasions was small because the reliability was high in all cases (ICC 0.94). The results of the calculations of the CV% values were also between the accepted values for a biological system (10 15%) 15). Inter-rater reliability is regarded as a basic criterion for objective measurements. The results of this study showed no significant difference in cervical muscle force measurements between two raters. The high inter-rater reliability indexes indicated no large differences in the muscle force measurement between raters with different levels of experience. In this study, the statistical outcomes derived by the calculation of inter-trial, test-retest and interrater reliability revealed that the measurements of the neck muscle force were highly reproducible. This consistency raises the possibility of the use of the isometric neck muscle test device in the field of physiotherapeutic and rehabilitation assessments. ACKNOWLEDGMENTS The authors would like to thank the subjects for

6 12 J. Phys. Ther. Sci. Vol. 15, No. 1, 2003 their participation in this study and Dr Veikko Vihko, LIKES- Research Center for Sports and Health Sciences, JKL, Finland for his scientific support. REFERENCES 1) Miller DI, Nelson RC: Biomechanics of Sport. Philadelphia: Lea & Febiger, 1973, pp ) Graves JE, Pollock ML, Foster D, et al.: Effect of training frequency and specificity on isometric lumbar extension strength. Spine, 1990, 15: ) Pollock ML, Graves JE, Bamman MM, et al.: Frequency and volume of resistance training, Effect on cervical extension strength. Archives of Physical Medicine Rehabilitation, 1993, 74: ) Laubach L: Comparative muscular strength of men and woman: A Review of the Literature. Aviation, Space and Environmental Medicine, 1976, 47: ) Staudte HW, Duhr N: Age and sex-dependent forcerelated function of cervical spine. European Spine Journal, 1994, 3: ) Jordan A, Mehlsen J, Bulow PM, et al.: Maximal isometric strength of the cervical musculture in 100 healthy volunteers. Spine, 1999, 13: ) Silverman JL, Rodriques AA, Agre JC: Quantitative cervical flexor strength in healthy subjects and in subjects with mechanical neck pain. Archives of Physical Medicine and Rehabilitation, 1991, 72: ) Levoska S, Keinanen-Kiukaanniemi S: Active or passive physiotherapy for occupational cervicobrachial disorders? A comparison of two treatment methods with a 1-year follow-up. Archives of Physical Medicine and Rehabilitation, 1993, 74: ) Ylinen J, Ruuska J: Clinical use of neck isometric strength measurement in rehabilitation. Archives of Physical Medicine and Rehabilitation, 1994, 75: ) Haas M: Statistical methodology for reliability studies. Journal of Manipulative and Physiological Therapeutics, 1991, 14: ) Domholdt E: Physical Therapy Research: Principle and Application. Philadelphia: W.B. Saunders Company, 1993, pp ) Bland JM, Altman DG: Measurement error. British Medical Journal, 1996, 313: ) McNemar Q: Physiological Statistics. 3th edition, New York: John Wiley and Sons Inc., 1962, pp ) Hinderer SR, Hinderer KA: Quantitative methods of evaluation. In: J.A. Delisa and B.A. Gans (Eds.), Rehabilitation Medicine, Principle and Practice. Philadelphia: J.B. Lippincott Company, 1993, p ) Stokes M: Reliability and repeatability of methods of measuring muscle in physiotherapy. Physiotherapy Practice, 1985, 1: