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Reproducibility and Reference Values of Inclinometric Balance and Isometric Trunk Muscle Strength Measurements in Finnish Young Adults

Paalanne, Niko P1,2; Korpelainen, Raija1,3,4; Taimela, Simo P5; Remes, Jouko6; Salakka, Matti7; Karppinen, Jaro I2,6

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Journal of Strength and Conditioning Research: August 2009 - Volume 23 - Issue 5 - p 1618-1626
doi: 10.1519/JSC.0b013e3181a3cdfc
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There is a need for objective methods to measure functional capacity. These needs include purposes such as quantifying lumbar mobility, trunk muscle performance, postural stability, and cardiovascular fitness (21). Decreased muscle strength and impaired postural control have been connected to numerous pathological conditions in young adults. These include low back pain (LBP) (4,5,16) and lumbar discectomy (7), neuromuscular diseases such as multiple sclerosis (29) and amyotrophic lateral sclerosis, and vestibular deficits (1). Also, survivors of childhood malignancies have decreased maximal muscle strength (31). Muscle strength assessment may provide physicians and physical and occupational therapists with important clinical information about weakness that may relate to functional limitations in activities of daily living. Clinical decisions about patients who have sustained physical or neurologic injuries or patients recovering from surgery are frequently made also taking into account muscle strength. It is therefore important to have accurate and reliable methods for assessment and to know the biological variability of the said measurements. Isometric measurement has proved to be a reliable method to assess trunk muscle strength (27). Various positions, equipment, and procedures have been used for these measurements (5,9,21,26,27). The existing reference values are either for older people (9) or based on relatively small groups of subjects of certain ages (2,26). The need for a simple quantified test to screen for balance disorders is widely recognized. Inclinometric method for assessing body sway is a new technique to evaluate postural stability, and its reproducibility and validity have been studied previously in elderly people and in patients with Parkinson disease (14,32). However, there are no reference values for adolescents or young adults. The underlying assumptions of the present study were that reproducibility of the isometric trunk muscle testing and inclinometric measurement of body sway must be sufficient to justify their use and that there is additionally a need for reference values for young adults to justify their use in clinical practice. The present study was conducted to evaluate the reproducibility of these tests and also to set reference values for this age group.



The reference value data were collected in a subcohort of the Northern Finland Birth Cohort 1986, which includes data on 9,479 children born in northern Finland between July 1, 1985, and June 30, 1986, from birth to 18 years. This cohort was originally collected to study health-related factors in northern Finnish population. The cohort has been followed since birth and the follow-up continues. The subcohort was formed from all the 2,969 cohort members who were living within 100 km radius from the city of Oulu. A postal questionnaire was sent to all cohort members in 2003-2004. Nonrespondents were sent 2 reminders, and finally, a short interview was conducted with the nonrespondents who could be contacted by phone. The final response rate was 68% with 1,987 young adults agreeing to participate. The participants were invited to a clinical examination in summer 2005. By the end of April 2005, 874 young adults (44% of those who were invited) were examined. Mean age of the subjects at the time of the examination was 19.0 years (SD 0.2). The characteristics of the cohort participants are presented in Table 1.

Table 1
Table 1:
Characteristics of the total study population.*

The intrarater reproducibility was assessed in 19 volunteers, who belonged to the cohort and participated in the physical examination. The interrater reproducibility was examined in 15 volunteer healthy students with a mean age of 22.7 years (SD 2.9, range 19-30 years).

The study was approved by the Ethics Committee of Northern Ostrobothnia Hospital District, Finland (34/2005).

Experimental Approach to the Problem

Reference value data and reproducibility data were both collected with the same equipment. Maximal isometric trunk muscle strength was measured with a computerized strain gauge dynamometer (NewTest, Co., Oulu, Finland) in standing position. When measuring flexion and rotations, resting pads were placed against the popliteal fossa, lumbar spine, and on scapular level and a padded sternal pad was positioned against chest at the same level with the scapular pad. When measuring extension, pads were placed against proximal part of tibia, pelvis, and sternum. Scapular pad was positioned at the height of the spines of the scapulae. Subjects were instructed to perform maximal push against the pad for about 3 seconds. Each measurement was performed 3 times, and the best performance was recorded. Results that exceeded the second best result of the same subject by more than 5 kg were excluded. In this case, one new repetition was performed. If the best value still exceeded others by more than 5 kg, the second best result was recorded. Ratios between extensor and flexor strength and between rotation strengths were calculated for all subjects.

Postural sway measurements were performed under standardized conditions using an inclinometry-based method (Body Sway Measurement System; Crea Research, Co., Oulu, Finland) as described earlier (13). In short, the device consisted of a belt fastened firmly at the level of the iliac crest, an inflexible measuring rod, an inclinometric module, a joint structure lying on the ground, a power unit, and a personal computer. The deviating movement of the measuring rod was calculated separately for the side-to-side and forward-backward directions at the individually selected calculation height (h = 0.55 × body height), and the recorded sway parameters were the total path length, area, and speed of postural sway. The path length was obtained by calculating the distance between the sequential location points of each sample, and after that, summing the values. After that, the algorithm of the software approximated the outlines of the measured sway and calculated the assessed area. During the measurement, the subjects were instructed to remove their shoes and stand with their feet together and arms down their sides. They were told to gaze at a fixation point located on a wall 2.5 m ahead of them. Each test lasted 60 seconds, and each subject performed the tests twice with eyes open and twice with eyes closed. The better result was recorded from each test.

In the intrarater reproducibility study, the same tester performed postural sway and strength measurements under standardized conditions. The test was repeated within 7 days, and the subjects were told to keep their daily routines as similar as possible and to avoid, for instance, alcohol or heavy physical exercise. The actual maintenance of the routines was controlled with a questionnaire. When assessing interrater reproducibility, the same subject was tested twice and another tester repeated the test within 30 minutes after the first measurement.

Statistical Analyses

Chi-square test was used to evaluate selection bias of the study, and t-test was used to compare the strength values between the genders. P values ≤0.05 were considered as significant. Reference values were estimated following the recommendation of The International Federation of Clinical Chemistry, and the values were calculated using the RefVal program (28). The program was used to detect and eliminate outliers. Fit to the Gaussian distribution was also tested with the program.

Reproducibility of the measures was evaluated by calculating the intraclass correlation coefficient (ICC) (25), which has been recommended for tests of reliability (18,20). Intraclass correlation coefficient is the proportion of variability in the observations, which is due to the differences between pairs. Calculations were carried out with SPSS for Windows version 14.0. Intrarater reproducibility was calculated with a 1-way random model, which uses ICC (1,1) equation. Interrater reproducibility was calculated using 2-way mixed model, which uses ICC (3,1) equation (25).

As ICC does not indicate absolute differences between measurements, the method of analysis described by Bland and Altman to assess agreement between 2 measurements was used (6). The method includes plotting the difference between the values of the first and second measurements against the mean of the 2 values, which allows the size and the range of the differences to be easily seen (18). The Bland-Altman plot shows the systematic error between the pairs and allows evaluation of the extent to which the observations in a pair disagree, depends on the magnitude of the measurement. In addition, 95% limits of agreement (LOA), the mean of differences (d), SD of the difference (SDDiff), and SE of the mean were calculated.


Selection Bias

The subjects included in our study were similar to the rest of the cohort with regard to the self-reported weight and height. Six-month prevalence of LBP at the age of 16 years was a little higher in men participating in our study compared with the rest of the cohort (68 vs. 60%, p = 0.05). Men participating in the study were physically more active than the rest of the cohort: 42% of the participants and 35% of the nonparticipants exercised 4 times a week or more (p = 0.03). Male participants also smoked less than the rest of the cohort (p < 0.001). In women, there were no significant differences in the prevalence of LBP, physical activity, or smoking. Women were more active to participate in our study than men (45.7% of the invited vs. 42.0%) However, the difference was not statistically significant (p = 0.10).

Intrarater Reproducibility

The results of the intrarater reproducibility tests for trunk muscle strength measurements and the results of the tests for body sway measurements are presented in Table 2. The ICC values of the muscle strength measurements varied between 0.84 and 0.95 indicating good or excellent reliability. Mean difference values (d) were negative (range from −2.35 to −0.16) indicating a slight systematic improvement between measurements.

Table 2
Table 2:
Intrarater reliability of isometric trunk muscle strength measurements and body sway measurements (n = 19).*

The ICC values for body sway measurements ranged from 0.39 to 0.74 indicating poor to moderate test-retest reproducibility between measurements. Mean difference values (d) ranged from −0.67 to 0.10 staying close to zero and indicating a slight systematic improvement between measurements. Standard deviation values (SDDiff) of the body sway measurements ranged from 0.09 in body sway velocity eyes open to 6.70 in body sway path length eyes closed.

Interrater Reproducibility

The results of the repeated muscle strength tests and the results of the repeated body sway measurements are presented in Table 3. The ICC values in the repeated trunk testing varied between 0.84 and 0.88, suggesting high reproducibility. However, the results revealed quite wide range of measurement error and poor agreement between the raters especially in repeated trunk flexion testing (mean difference −5.62, 95% LOA −29.40 to 18.16). The Bland and Altman analysis of the trunk extension is presented in Figure 1, which shows that the magnitude of the differences is not dependent on the magnitude of the measurement value. The figure also indicates systematic bias as almost all values are scattered above zero.

Table 3
Table 3:
Interrater reliability of isometric trunk muscle strength measurements and body sway measurements (n = 14).*†
Figure 1
Figure 1:
Bland and Altman analysis for the maximal isometric extension (kilograms). Data from interrater measurements (n = 14).

In the repeated sway path and length measurements, ICC values ranged from 0.61 to 0.85 indicating acceptable reproducibility. Intraclass correlation coefficient values for the sway area remained slightly under acceptable level. Mean difference (d) between the 2 measurements was close to zero except in sway path length in the eyes closed test, where the value was −2.74 cm. Figure 2 illustrates the scattering of the eyes open sway area values. The mean difference was zero indicating no systematic bias. However, SDDiff was 0.5 indicating quite wide LOA.

Figure 2
Figure 2:
Bland and Altman analysis for the body sway area (square centimeters) eyes open. Data from interrater measurements (n = 14).

Isometric Strength of the Trunk Muscles

The 95% normative values and quartiles of the isometric trunk strength measurements are presented in Table 4. Men performed significantly better than women in all muscle strength tests. Back extensor muscles were generally stronger than back flexor muscles. Extension to flexion ratio was higher in women (mean 1.63, SD 0.44) than in men (mean 1.37, SD 0.33) (p < 0.001).

Table 4
Table 4:
Normative values (95%) and quartiles for maximal isometric strength in young adults (N = 874).

Body Sway

The normative values and quartiles of the body sway measurements are presented in Table 5. The mean values in women were significantly better than in men for all the sway measurements. The average sway path length with eyes open was 17.9 cm (SD 4.8) in women and 19.3 cm (SD 7.7) in men (p = 0.001). The average path length with eyes closed was 33.5 cm (SD 9.9) in women and 37.8 cm (SD 11.1) in men (p < 0.001). The average velocity with eyes open was 0.30 cm·s−1 (SD 0.09) in women and 0.32 cm·s−1 (SD 0.09) in men (p = 0.001) and with eyes closed 0.56 cm·s−1 (SD 0.17) in women and 0.63 cm·s−1 (SD 0.19) in men (p < 0.001). The average area for body sway with eyes open was 1.07 cm2 (SD 0.55) in women and 1.23 cm2 (SD 0.68) in men (p < 0.001). With eyes closed, the values were 2.17 cm2 (SD 1.30) in women and 2.66 (SD 1.50) in men (p < 0.001).

Table 5
Table 5:
Normative values (95%) and quartiles for body sway in young adults (N = 874).


The present study was conducted in order to evaluate the reproducibility of the maximal isometric trunk muscle testing and inclinometric method to measure body sway and to establish reference values for maximal isometric trunk muscle strength and body sway in young adults. The variability between measurements at the individual level was high, but on average, there was only a slight systematic difference between measurements. The reference values indicate that both physical measurements have a remarkable biological variation.

Men included in this study were slightly more physically active and had a little higher prevalence of LBP at the age of 16 years than the rest of the cohort. They also smoked less than the nonparticipants. This should not, however, have a major impact to the generalizability to our findings because the difference was rather small. Moreover, smoking or LBP did not have significant effects on physical capacity measured in this study (data not shown). As subjects participating in the intrarater reproducibility testing were randomly selected from the study population, they should be representative of the whole study population. Volunteers who participated in the interrater reproducibility tests were slightly older than rest of the study subjects. This may have a slight effect on the ICC values of the interrater measurements.

As there are no definite cutoff values for ICC (3), many different interpretations have been presented. Some studies have adopted classification for κ (15), where values below 0.20 indicate poor agreement, values between 0.21 and 0.40 fair, values between 0.41 and 0.60 moderate, values between 0.61 and 0.80 substantial, and values over 0.81 almost perfect agreement. Scientific Advisory Committee of The Medical Outcomes Trust considers ICC values 0.70 and higher as satisfactory for group comparisons (24), and this view has been adopted by some authors (8). In some studies (3,18,20), values between 0.80 and 0.90 have been considered as good and values over 0.90 as excellent. In this study, we considered values over 0.70 as acceptable.

In our study, the intrarater reproducibility of the isometric trunk muscle strength measurements was very good, as ICC values ranged from 0.84 to 0.94. Interrater reproducibility was also good, with ICC values from 0.84 to 0.88. Our results are in agreement with the results of a previous study (12), where the reliability of the isokinetic trunk muscle strength measurement was evaluated. Our results show a somewhat higher reproducibility than those by Moreland et al. (19), who studied the interrater reliability of the abdominal and trunk extensor dynamic endurance, handheld dynamometry of isometric flexion and extension, and abdominal and extensor static endurance. However, although the ICC analysis indicated good reproducibility, the Bland and Altman analysis revealed quite wide range of measurement error.

Wider LOA in the Bland-Altman analysis for the interrater repeatability compared with intrarater repeatability may be explained with a different time interval between measurements. The interrater measurements were made within 1 hour, whereas the intrarater measurements were carried out in the course of 1 week. This was due to practical reasons and makes comparison of intrarater and interrater values a little more difficult. In the trunk extension test-retest measurements, all but one subject performed worse in the latter measurement. This can be explained by a systematic error of performing the measurements with an insufficient time interval in between; the subjects may not have had enough time to recover from the first test. However, in all other trunk strength tests, the average values of the latter measurement were better and variation between tests was randomly scattered around zero. It is possible, although unlikely, that differences exist due to different placement of the resting pads. A Finnish study (22) reported that a more caudal placing of the pelvic support increased measured torque in extension and in flexion. According our experience, tightness of the sternal resting pad is also a significant factor for differences between measurements. Because the study aimed to evaluate repeatability between 2 independent measurements, the height and the tightness of the pads were not recorded, so the settings of the pads were independent of the other measurements. In follow-up studies, repeatability could be improved by repeating measurements with the same settings of the pads.

When assessing reproducibility of the body sway measurements, the ICC values ranged from 0.39 to 0.74 in the intrarater reliability analysis indicating rather poor reproducibility. In the interrater reproducibility tests, ICC indicated better reliability between the 2 testers ranging from 0.61 to 0.85. Poorer ICC values compared with trunk muscle testing could be explained by smaller variation between subjects as it has been shown that larger variation between results of different subjects leads to higher ICC values (20). Values of the Bland-Altman analysis were similar to previous results (13), which also revealed quite wide range for LOA.

The values of the isometric trunk muscle strength in our cohort of young adults were generally higher compared with those presented by others (26). On the other hand, the trunk rotation values were lower than the values presented by Stoll et al. (30). The deviations can be largely explained by the different methods to measure strength in these studies. Moreover, the subjects in the study by Sinaki et al. (26) were slightly younger and the subjects in the study by Era et al. (9) older than the subjects in this study. The previous studies were also made with relatively small groups of subjects of a certain age. Our present study was conducted with a remarkably large cohort of young healthy adults, which makes the results generalizable also for the future studies.

There are no reference values for inclinometric body sway measurements. However, our results revealed significantly smaller body sway path length in young adults than those reported in elderly persons (13). Our results are consistent with earlier results (10,23), which indicate that body sway increases with aging. The men in our data had higher postural sway when standing than women, which might be explained by the fact that men were taller than women. Thus, the estimated center of gravity (the calculation height) was located higher and moved a longer distance in men than in women.

Intraclass correlation coefficient as a sole indicator showed an excellent inter- and intraobserver reproducibility in trunk muscle testing where biological variation between the subjects was high. However, ICC or any other correlation measures do not reveal absolute differences between measurements (3,20). In our data, the LOA indicated a wide range of measurement error, especially in interrater values. For example, the mean value for repeated interrater trunk flexion measurements was 68.2 kg, mean difference was −5.6, SD of the difference was 11.9, and LOA ranged from −29.4 to 18.2. Hopkins (11) demonstrated a simple way to calculate typical error of the measurements by dividing SD of the differences by √2. In this case, a typical error would be 8.4 as an absolute value and 11.9% if expressed as a percentage of its respective mean. Critical error of the measurements can be calculated by multiplying typical error by 2.77 as proposed by Hopkins. In this case, critical error would be 33.0%, meaning that only changes bigger than this would indicate significant change in performance. Our results are consistent with Madsen (17), who found critical differences to be more than 20% in the isokinetic trunk muscle testing. However, our sample sizes for both intra- and interrater tests were quite small, which may widen the 95% LOA (20). Our subjects had no previous experience performing maximal isometric contraction of the trunk muscles, which may further explain some of the variation. Because of the large population in our study (N = 874), multiple sessions were not possible. For future studies, we recommend subjects to have 1 or 2 training sessions before testing.

Although ICC analysis indicated good reproducibility, the Bland and Altman analysis revealed quite wide range of measurement error. Critical difference between repeated measurements is quite large, which limits the value of these tests when assessing individuals or small groups. However, reproducibility of isometric trunk muscle testing is comparable to other methods currently in use for assessing performance capacity. Inclinometric testing device offers a portable and low-cost method to assess body sway, the device provides real-time information on the absolute movements of the body. However, there are limitations in the reproducibility of the measurements, which should be noted when using this device.

Practical Applications

In this study, we have collected a valuable normative data for trunk muscle strength and inclinometric body sway in young adults. The data can be used as reference data in future studies. The normative values can be used as a tool when assessing effects or progression of numerous different pathological conditions. As isometric trunk muscle testing is fairly reproducible, it can be used when testing large populations in future studies. Due to weaker reproducibility, the use of the inclinometric body sway testing is questionable in future studies; due to a slight systematic improvement between measurements, the subjects should be allowed to practice the measurement procedure beforehand.


This study was supported by grants from the Academy of Finland (Dr Karppinen; 200868), Deaconess Institute of Oulu and Ministry of Education, Finland. We certify that no party having a direct interest in the results of the research supporting this article has or will confer a benefit on us or on any organization with which we are associated, and the results of the present study do not constitute endorsement of the product by the authors or the National Strength and Conditioning Association.


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lumbar strength; postural balance; reliability of results

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