Isometric and isokinetic muscle strength measurements of the lumbar flexors and extensors with BioniX Sim3 Pro in patients with chronic low back pain: A pilot study

Abstract

BACKGROUND:

The World Health Organization states that low back pain is the leading cause for disability worldwide. Patients with chronic low back pain (CLBP) show important decreases in lumbar strength and can now be assessed by using the new isokinetic dynamometer BioniX Sim3 Pro which offers very detailed measurements.

OBJECTIVE:

To compare lumbar flexion and extension strength values, as well as extension-flexion ratio (EFR) based on isokinetic velocity and gender, for patients with CLBP on the BioniX Sim3 Pro.

METHODS:

A retrospective analysis was performed on data from 20 men and 22 women with CLBP. Maximum torque in flexion and extension was measured isometrically and isokinetically. Statistical analysis was performed on these parameters.

RESULTS:

EFR shows a downward trend with increasing speed of measurement (isometric to isokinetic 30 ${}^{\circ}$ /s and isokinetic 30 ${}^{\circ}$ /s to isokinetic 90 ${}^{\circ}$ /s, respectively): in men from 1.21 to 1.04 ( $p<$ 0.001) and from 1.04 to 0.93 ( $p=$ 0.207). In women, EFR decreased from 1.41 to 1.13 ( $p<$ 0.001) and from 1.13 to 1.00 ( $p=$ 0.144).

CONCLUSIONS:

Patients with CLBP have a decreased strength in lumbar extension and flexion compared to the age-corrected normal values of Bionix Sim3 Pro. With increasing speed of testing protocol, a decrease in EFR is observed in this population.

Keywords

Isokinetic low back pain bionix lumbar torque values

1. Background

1.1 Chronic low back pain

Chronic low back pain (CLBP) is a common condition with a point prevalence of 23%, being the leading cause for disability worldwide [1, 2, 3]. In the vast majority of cases, no clear cause of these pain symptoms can be found and so the term ‘nonspecific chronic low back pain’ will be used [3]. Nevertheless, abnormalities in the control of trunk muscles in patients with CLBP are noted. While performing daily tasks, the trunk muscles should be addressed to create stability around the lumbar spine. In CLBP, a delay in contraction is observed at the level of these stabilizing muscles [4]. Chronic pain could lead to a pathological reorganization of the motor cortex, making control of the trunk musculature less efficient [5, 6]. Physiotherapeutic techniques such as motor control training and sensory feedback training respond to this reorganization and by using the plasticity of the motor cortex these techniques can correct the pathological changes [7, 8, 9]. The lumbar multifidus muscles are the main stabilizer of the lumbar spine and recent magnetic resonance studies show an important association between CLBP and the presence of atrophy and fat infiltration at the level of these muscles. These effects persist even after the pain episode and could be the reason for recurrent complaints [10, 11].

CLBP is associated with decreased strength and endurance while performing lumbar extension and flexion [12, 13, 14, 15, 16]. Research was also conducted on the ratio between maximum torque in extension and maximum torque in flexion, during an isometric or isokinetic movement and expressed in Newton meters (Nm). This extension-flexion ratio was determined in different populations. Under normal circumstances, torque values in lumbar extension are greater than in lumbar flexion, leading to a normal extension-flexion ratio of approximately 1.3 (130%) in a healthy population [17, 18, 19, 20]. Athletes show an increased extension-flexion ratio that reaches normal values of about 1.5 [21]. An exception within the sport are the rowing sports where a lower extension-flexion ratio of approximately 1.1 occurs [19, 22]. Patients with chronic low back pain have a decreased extension-flexion ratio [12, 16, 23, 24].

Stabilizing exercises were also investigated in patients with CLBP. Such a rehabilitation program leads to a significant increase in strength in lumbar extension and correlated to an increasing extension-flexion ratio [25]. Posture exercises and resistance training also lead to strength building mainly of the lumbar extensors [26, 27]. These exercises are known to cause significant decreases in pain and an increase in muscle mass of the trunk muscles [28, 29]. In practice, rehabilitation of chronic back pain takes the form of a back school, where multidisciplinary treatment is offered by a team of doctors, physiotherapists, occupational therapists, dieticians and psychologists [3, 30]. CLBP is associated with an increased presence of depressive feelings [31]. It is therefore important to pay attention to these psychological factors in the treatment of CLBP [32].

1.2 Measurements with devices

To measure the strength of the trunk muscles, the Biering-Sørensen test is especially recommended, because of the very good reliability (an inter-tester ICC of 0.94 and an AUC of 0.93 compared to isokinetic Biodex 2000 measurements) and the absence of a learning effect during several test moments [33, 34]. The patient is hereby fixed in the prone position on the examination table at the level of the pelvis and lower limbs, the upper body hanging off the examination table and held in a neutral position. Failure to maintain this posture (of isometric extension) for three minutes indicates weakening of the back extensors and thus an increased risk of CLBP. However, with the advent of robotics, objective measurements of torque and range of motion can be performed in the various axes of the lumbar spine (flexion, extension, lateral flexion and rotation). Isokinetic dynamometers are devices that can measure these back movements with good reliability, both in a healthy population and in a population with CLBP or weakened abdominal musculature, with an intrarater ICC ranging from 0.87 to 0.95 and a interrater ICC ranging from 0.95 to 0.98 [35, 36, 37, 38, 39, 40]. Isokinetic measurement means that the device allows the examined movement at a constant speed. If the patient pushes harder, this torque will be measured, but the device will not go faster. Thus, the patient experiences increased resistance if more torque is applied. Isotonic measurement means that the device will exert a constant resistance, which is set in advance. With increasing torque, the patient will only gain speed during the test. Isometric tests are static. The device measures the torque that is built up, without movement. Despite several studies testing the reliability of measurement results within one device, much less is known about the validity of this data between two different devices [41]. Body posture during the measurement is also crucial, as the extension-flexion ratio is much higher in a sitting position compared to standing [42, 43]. Normal values for healthy populations have been studied for each device for decades [44, 45]. A new isokinetic device in the clinical setting must therefore be calibrated sufficiently with its own research data. The BioniX Sim3 Pro is a new, innovative measuring device that allows isometric, isotonic and isokinetic measurement in the axes flexion/extension, lateral flexion and rotation. During the performance of isokinetic measurements in one axis, the (compensatory) torques in the other two axes can also be measured. It also measures the time needed to reach maximum torque, acceleration and deceleration, average work and more. This could make the BioniX Sim3 Pro a very useful dynamometer for clinical and scientific purposes. Although the device contains normal values for a healthy population, no information has yet been published on the strength measurements of patients with CLBP using the BioniX Sim3 Pro. Therefore, this study aims to collect data in patients with CLBP with this new dynamometer.

2. Methods

2.1 Population

This pilot study was carried out after approval of the Ethical Committee of AZ Jan Palfijn in Ghent (Belgium). A retrospective analysis was performed on the data of all patients who presented themselves between 15/10/2019 and 20/05/2020 for CLBP and had a three-dimensional back study performed on the BioniX Sim3 Pro. Assessment with the BioniX Sim3 Pro is an integral part of the preliminary investigations at the Physical Medicine and Rehabilitation Service at AZ Jan Palfijn in Ghent at the start of the rehabilitation program for patients with CLBP. This assessment was only prescribed by the physical physician, neurosurgeon or orthopedist specialized in back pathology and working in AZ Jan Palfijn with the aim of assessing trunk muscles in the context of starting the back rehabilitation program.

Inclusion criteria were: (1) Adult men and women, (2) Chronic low back pain for more than three months (non-specific, lumbar facet disease and lumbar disc disease), and (3) On specific referral by a back specialist in AZ Jan Palfijn (physical physician, neurosurgeon or orthopedist). Exclusion criteria were: (1) Pregnancy, (2) Acute increase in back pain, and (3) Fever, flu, or other acute symptoms that prevent optimal strength performance. Based on these criteria, 20 men and 22 women could be included in this pilot study.

2.2 Measuring instruments

All data was collected on the same day for each patient. With the BioniX Sim3 Pro, fixation is performed at the shoulders and pelvis (Fig. 1). It has been confirmed in the past that fixation at the anterior superior iliac spine leads to correct, reliable measurements of the back musculature in standing position [46]. Measurements of lumbar flexion and extension are performed in the order isometric, isokinetic 30 ${}^{\circ}$ /s, and isokinetic 90 ${}^{\circ}$ /s. The starting position for each measurement is from a standing, neutral position (0 ${}^{\circ}$ flexion). During isokinetic tests, multiple repetitions were always requested in order to reach the maximum torque. Figure 2 shows an example of the graphs of measurements during extension (left) and flexion (right) during an isokinetic test at 90 ${}^{\circ}$ /s of a test subject. Each line represents a repetition, each time indicated in a different shade of color. In case of inconsistency within the repetitions (diversity between the different lines on the graph), the patient was motivated to achieve reproducible torque values.

Figure 1.

Study setup of BioniX Sim3 Pro (illustration with permission).

Table 1

Socio-demographic data

	Total ( $n=$ 42)		Men ( $n=$ 20)		Women ( $n=$ 22)		$p$ -value
Age (years)	39.9	$\pm$ 13.8	40.4	$\pm$ 15.7	39.5	$\pm$ 12.2	0.845
Height (cm)	169.9	$\pm$ 9.4	175.4	$\pm$ 7.8	164.9	$\pm$ 7.9	$<$ 0.001 ${}^{*}$
Weight (kg)	75.6	$\pm$ 14.5	77.4	$\pm$ 13.6	73.9	$\pm$ 15.3	0.441
BMI (kg/m ${}^{2}$ )	26.3	$\pm$ 5.1	25.2	$\pm$ 4.5	27.2	$\pm$ 5.6	0.213

Mean $\pm$ standard deviation. ${}^{*}$ Significantly different ( $p\leqslant$ 0.05).

Table 2

Maximum torque values

	Extension (Nm)	Flexion (Nm)	Extension-flexion ratio	$p$ -value
Men ( $n=$ 20)
Isometric	117.8 $\pm$ 67.3	109.2 $\pm$ 56.5	1.21 $\pm$ 0.74	0.472
Isokinetic 30 ${}^{\circ}$ /s	102.9 $\pm$ 48.6	97.7 $\pm$ 36.0	1.04 $\pm$ 0.26	0.367
Isokinetic 90 ${}^{\circ}$ /s	63.1 $\pm$ 43.5	65.3 $\pm$ 34.4	0.93 $\pm$ 0.29	0.640
Women ( $n=$ 22)
Isometric	63.5 $\pm$ 27.2	51.4 $\pm$ 25.6	1.41 $\pm$ 0.63	0.047 ${}^{*}$
Isokinetic 30 ${}^{\circ}$ /s	58.2 $\pm$ 26.5	53.5 $\pm$ 21.4	1.13 $\pm$ 0.36	0.257
Isokinetic 90 ${}^{\circ}$ /s	32.6 $\pm$ 23.9	34.3 $\pm$ 25.7	1.00 $\pm$ 0.22	0.248

Mean $\pm$ standard deviation. ${}^{*}$ Significantly different ( $p\leqslant$ 0.05).

Figure 2.

Example of the BioniX Sim3 Pro results.

For this study, the maximum torque values of flexion and extension were collected and the extension-flexion ratio was determined for each patient. The BioniX Sim3 Pro contains normal values for populations of different ages (under 20 years, 20–30 years, 30–40 years, 40–60 years and over 60 years). Unfortunately, there are currently no studies on the reproducibility of measurement result with the BioniX Sim3 Pro (interrater and intrarater reproducibility).

2.3 Statistical analysis

All statistical analyses were performed using IBM Statistical Package for the Social Sciences (SPSS) version 25 for Windows 8.1. Kolmogorov-Smirnov tests were performed to check the normality of all parameters. Afterwards, descriptive analyses were performed on the male and female populations. Unpaired $t$ -tests were performed on the data between the two populations. Subsequently, descriptive analyses were also performed for the results of the torque values of lumbar extension and flexion, as well as extension-flexion ratio. Linear regressions with multivariate analyses were applied to the results of the torque values. Models were created with gender, followed by the basic data (age, height and weight) separately and combined, to test for confounding. Statistical significance for all tests was accepted at $p\leqslant$ 0.05.

3. Results

The study population consists of 20 men and 22 women, older than 18 years and referred after a consultation with a back specialist (Orthopedics, Neurosurgery or Physical Medicine). All patients had CLBP with recurrent complaints for at least 3 months and were good candidates for multidisciplinary back rehabilitation. The study population includes patients with specific and nonspecific CLBP without red flags.

Basic data between men and women are described in Table 1. A significant difference in height was found in the study population between men and women ( $p<$ 0.001). The other parameters were not significantly different between the male and female population: age ( $p=$ 0.845), weight ( $p=$ 0.441) and BMI ( $p=$ 0.213).

With the exception of the isokinetic 90 ${}^{\circ}$ /s extension measurement in women, all strength measurements were normally distributed. All maximum torque values in Table 2 were formulated as mean and standard deviation for practical usability. The isokinetic 90 ${}^{\circ}$ /s extension values in women is 26.5 [10.9–52.7] Nm when expressed as median [percentile 25 – percentile 75].

The maximum torque values are all significantly different between men and women (Table 2): isometric extension ( $p=$ 0.004) and flexion ( $p=$ 0.001), isokinetic 30 ${}^{\circ}$ /s extension ( $p=$ 0.001) and flexion ( $p<$ 0.001) and isokinetic 90 ${}^{\circ}$ /s extension ( $p=$ 0.007) and flexion ( $p=$ 0.002). No significant difference was observed in extension-flexion ratio between males and females: isometric ( $p=$ 0.352), isokinetic 30 ${}^{\circ}$ /s ( $p=$ 0.352), and isokinetic 90 ${}^{\circ}$ /s ( $p=$ 0.380).

A control for possible confounders was performed in the study of differences in maximum torque values between men and women. Age, height and weight were taken into account. Height is a confounding factor for all flexion strength results as well as for isokinetic 90 ${}^{\circ}$ /s extension ( $p\leqslant$ 0.05), when added in a linear regression model in conjunction with gender. The confounding factor height shows a positive correlation with all maximum torque values. Height is correlated with isometric extension (rs $=$ 0.403, $p=$ 0.011) and flexion (rs $=$ 0.640, $p<$ 0.001), isokinetic 30 ${}^{\circ}$ /s extension (rs $=$ 0.434, $p=$ 0.004) and flexion (rs $=$ 0.664, $p<$ 0.001) and isokinetic 90 ${}^{\circ}$ /s extension (rs $=$ 0.419, $p=$ 0.006) and flexion (rs $=$ 0.471, $p=$ 0.002). Age and weight are not a confounder for the differences in strength measurements between men and women.

The small (non-significant) difference in extensionflexion ratio between men and women is also due to height ( $p=$ 0.033) and not gender itself ( $p=$ 0.623). The relatively larger body length in men therefore leads to an advantage in strength of lumbar flexors and thus a relatively smaller extension-flexion ratio.

Both extension and flexion torque values show a decreasing trend with increasing speed of performed test on the BioniX Sim3 Pro. The extension-flexion ratio also shows a decreasing trend, noting the relatively greater decrease in power of the extensors with increasing speed (in the order isometric, isokinetic 30 ${}^{\circ}$ /s and isokinetic 90 ${}^{\circ}$ /s). A paired-samples $t$ -test was performed on EFR from isometric to isokinetic 30 ${}^{\circ}$ /s and from isokinetic 30 ${}^{\circ}$ /s to isokinetic 90 ${}^{\circ}$ /s. In men, EFR decreased from 1.21 to 1.04 ( $p<$ 0.001) and from 1.04 to 0.93 ( $p=$ 0.207). In women, EFR decreased from 1.41 to 1.13 ( $p<$ 0.001) and from 1.13 to 1.00 ( $p=$ 0.144).

4. Discussion

4.1 Interpretation of the results

The torque values and ratio values in men and women show consistent trends across isometric and isokinetic measurements, without unnatural outliers. These conclusions are also very similar to previous research [44, 45]. Higher strength measurements of trunk musculature are observed in men versus women. This is consistent with findings in the literature [47]. Height does play a role as a confounder here. It would therefore be interesting to conduct further research into the possibility of expressing torque values in relation to height. Both male sex and greater height are associated with greater lumbar flexor strength (isometric and isokinetic). In addition, this study shows that extension torque is greater than flexion torque during isometric and slow isokinetic test, although the 90 ${}^{\circ}$ /s measurement shows greater flexion torque than extension torque. It has already been established in the past that the increasing speed of isokinetic tests leads to a slightly greater decrease in extension torque, resulting in a decrease in EFR; admittedly on an older device (Cybex II), also measured in a standing position [44, 45]. A possible explanation could be that with increasing speed relatively less torque can be generated by (a part of) the lumbar extensors. Some studies cannot reproduce this decrease in EFR [48]. As mentioned in the introduction, an extension-flexion ratio of 1.3 is considered as normal value; isometrically measured in standing position with pelvic fixation. However, the results of extension-flexion ratio in this population with CLBP during the isometric measurements are comparable to the extension-flexion ratio in healthy populations in some studies. Research with a larger population seems appropriate here to determine whether isokinetic measurements are therefore more suitable for determining the extension-flexion ratio between persons with and without CLBP. Analysis of confounding factors shows an association with height, due to an association between height and strength of the lumbar flexors in this population. However, further research with larger populations is needed to confirm these associations.

A new device for isometric, isokinetic and isotonic measurements was used for this study. Previous research confirms the good reliability of results with these dynamometers [36, 37]. Unfortunately, no research has yet been published on the accuracy or normal values of Bionix Sim3 Pro. The company did offer normal values upon delivery of the device. However, the details of this population have not been described, so these data should be used with caution. There is no insight on reliability studies performed with this device, nor do we have information on the minimally detectable change or minimal clinically important difference. The normal values of a male population (30–60 years) for maximum torque in extension and flexion are isometric 236 Nm and 182 Nm, isokinetic 30 ${}^{\circ}$ /s 225 Nm and 174 Nm and isokinetic 90 ${}^{\circ}$ /s 197 Nm and 146 Nm, respectively. The extension-flexion ratio here is therefore approximately 1.3 for all measurements. The normal values of a female population (30–60 years) for maximum torque in extension and flexion are isometric 148 Nm and 91 Nm, isokinetic 30 ${}^{\circ}$ /s 202 Nm and 157 Nm and isokinetic 90 ${}^{\circ}$ /s 178 Nm and 131 Nm, respectively. The extension-flexion here is therefore approximately 1.6 for the isometric test and 1.3 for the isokinetic tests. The maximum torque values in this study are much lower than the normal values provided by the company.

All our measurements were made with the same device and always by trained personnel with the same protocol and working method. Everyone involved in the measurements with BioniX Sim3 Pro had received training on the use of this device. On the other hand, this study is in no way affiliated with or sponsored by third parties.

4.2 Study limitations

This monocentric pilot study was conducted in a population of 42 patients with chronic CLBP providing some preliminary findings that may help in future research. A larger population here would provide more certainty about the usefulness of the results. Also, no control population was included to compare the torque values. A comparison with a healthy population with similar baseline data would have been more optimal, since normal values may have been influenced by factors such as investigator, device settings, pain and psyche.

Confounding review from baseline data was performed in this study. It could be of added value if pain, kinesiophobia and other psychological factors were also determined to exclude confounding from this, since this could influence the interpretation of the torque values [49, 50].

Examination with a BioniX Sim3 Pro is always performed in a standing position. Sitting posture, however, reduces possible compensatory mechanisms by the hip flexors, which may lead to a better assessment of the strength of the lumbar flexors and thus of the trunk muscles [51]. To minimize the effect of the pelvis, sufficient attention was paid to fixing the pelvis in the area of spina iliaca anterior superior and sacrum [46].

When patients need to become familiar with a new measuring device, it is noted that an initial measurement is less reliable than the subsequent measurements [40]. Our results are based on patients who have never had a measurement of trunk musculature with an isokinetic device. All patients were allowed to test the movement gently before taking the actual test. There were always several repetitions of each movement, in order to reach the maximum torque. The authors note a very good coherence in the graphs within the individual measurement results with BioniX Sim3 Pro (Fig. 2). Research is still needed to further correlate these results with the actual movements performed by the trunk muscles.

All patients who had registered for multidisciplinary rehabilitation concerning CLBP in hospital AZ Jan Palfijn (second-line care) were eligible for this study. This always happened upon referral from a back specialist and in the presence of a good indication for back rehabilitation. Therefore, there was little selection bias.

5. Conclusion

For the new measuring device BioniX Sim3 Pro, no data have yet been published about patients with CLBP. This pilot study provides preliminary values on isometric and isokinetic torque values in patients with CLBP. The torque in lumbar extension and flexion in this population with CLBP is much lower than the normal values of the device for a population of the same age without CLBP. Both extension and flexion torque values show a decreasing trend with increasing speed of performed test on the BioniX Sim3 Pro. Therefore, the speed of the performed isokinetic test should always be noted when assessing measurements lumbar torque values. Height is strongly correlated with maximum torque values, explaining the relatively high torque values in extension and especially flexion torque in men. This explains a slightly lower extension-flexion ratio in men versus woman. Further research should be done with BioniX Sim3 Pro in a bigger population to provide strong, additional data on maximal torque values in healthy candidates and patients with CLBP, as well as calculating the reliability and validity of this novel device. Calculation of minimally detectable change or minimal clinically important difference would also make the BioniX Sim3 Pro a more valuable tool in clinical practice.

Footnotes

Conflict of interest

The authors have no conflicts of interest to report. There was no financial support for the implementation or publication of this study.

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