Comparison of muscle activation of the transversus abdominis and muscle strength between individuals with low back pain, herniated disc and healthy individuals: a cross-sectional study.

BACKGROUND: Low back pain and disc herniation are common problems in the world population, being characterized by discomfort in the region of the spine, resulting in functional capacity and quality of life reduced. Some of the causes of these conditions seem to be associated with the biomechanical imbalance of the muscles that act in the spine. There are methods to assess the level of activation and strength of the stabilizing muscles of the spine, such as the Pressure Biofeedback Unit (PBU). This study aims to compare the level of activation of the transverse abdomen muscle and back strength in healthy, low back pain and herniated disc individuals. METHODOS: a cross-sectional study was carried out with 30 men that were homogeneously distributed in three groups: healthy (HG), with low back pain (LBPG) and with herniated disc (HDG). The primary outcomes were the level of activation of the transversus abdominis, assessed by PBU, and back strength, assessed by dynamometry. Pain, exibility and disability were evaluated as secondary outcomes. RESULTS: The mean age of the participants in this study was 30.47 ± 9.74 years. Regarding the activation of the transversus abdominis, no differences were found between groups (p = 0.155). For strength, LBPG and HDG were different compared to HG (p = 0.028 and p = 0.045, respectively). Pain was different between the HG and both LBPG and HDG (all p < 0.001). Regarding exibility, no differences (p > 0.05) were found. The HDG had the highest disability score and was statistically different of the HG (p = 0.005), but with no difference from LBPG (p = 0.087). CONCLUSION: the activation of the tranversus abdominis is similar between healthy, non-specic back pain and herniated disc individuals; however, the latter presents a reduced level of strength and more disability.


Introduction
Low Back Pain (LBP) is a common condition in the world population [1]. It is one of the main causes of musculoskeletal diseases and demand for health care [2,3]. More than 80% of the population will experience this symptom at some time in their lives; of those, 95% will recover within a few months, while the others will develop chronic LBP [4].
LBP can be classi ed as speci c, those that have a de ned cause, and non-speci c, when the pathoanatomical cause cannot be determined [5]. The most common condition among people with speci c back pain is disc herniation; however, the majority is classi ed as non-speci c [5,6].
It has been demonstrated that individuals with LBP present musculoskeletal dysfunctions, such as delayed recruitment, insu cient muscle control, and reduced cross-sectional area, strength, endurance, and exibility [7,8,9,10]. The musculoskeletal system composing the core muscles related to the spine consists of global muscles (rectus abdominis, external oblique, anterior bers of the internal oblique, and the thoracic portion of the iliocostalis) and local stabilizing muscles (multi dus, psoas major, transversus abdominis, quadratus lumborum, diaphragm, internal oblique muscles, the lumbar part of the iliocostalis, and the longissimus muscles) [11]. The transversus abdominis seems to be the key stabilizing muscle, and its dysfunctions are associated with the development of LBP [9,12].
One of the methods used to assess these muscles is the pressure biofeedback unit (PBU). This is a noninvasive technique considered valid, reliable, low-cost, and easy to handle that uses the tension generated by the muscle contractions to measure their activity [13]. The measurement of strength is another effective way to evaluate the functional ability of the back muscles. The dynamometry is a valid and reliable instrument that can be used and provides objective strength values [14]. Although these devices are widely used to evaluate back pain, there are no studies comparing individuals with speci c and nonspeci c back pain.
Thus, this study aimed to compare the activation of the transversus abdominis and the back muscle strength between self-reported healthy individuals, individuals with non-speci c LBP, and individuals with disc herniation.

Methods
This is a cross-sectional study that was carried out in the city of Natal (Rio Grande do Norte -Brazil) from July to November 2016. The convenience sample was composed of male individuals aging between 25 to 60 years and intentionally divided into three groups: Healthy Group (HG), Low Back Pain Group (LBPG), and Herniated Disc Group (HDG). Those individuals with non-speci c LBP for at least three months were included in the LBPG. For the HDG, the individuals should have a clinical diagnosis of lumbar disc herniation and magnetic resonance image with up to six months of validity after diagnosis. For the HG, those who did not have herniated discs or low back pain in the past six months were included. The individuals who have undergone any physical therapy treatment in the last three months or did not perform the evaluation procedures correctly were excluded from the study.
An assessment form prepared by the researchers and containing individual, clinical, and and occupational data regarding weekly working hours was used to evaluate the participants. The pain was evaluated using the visual analog scale (VAS) [15], while exibility was assessed using the Schober test [16], and disability through the Roland-Morris questionnaire [17]. A 300kgf dorsal dynamometer (KRATOS®) was used to assess the back muscle strength [14] and the activation of the transversus abdominis was assessed using the PBU [13].
To measure the back muscle strength, the participants were positioned with feet fully supported on the dynamometer platform, and knees extended. The trunk should be exed at 120º, the cervical spine aligned to the trunk, and the dynamometer arms at knee-length. While holding the dynamometer arm with elbows extended and no shoulder movement, the participants were asked to perform three maximal voluntary isometric contractions (MVIC) for trunk extension, during 5 seconds, with a 60-s interval between each repetition (Fig. 1) and the highest value was used for data analysis.

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The PBU was performed using a sphygmomanometer with a pressure range between 0 and 300 mmHg.
For this, the participants were positioned prone on a plinth with the sphygmomanometer below the lower abdomen (at the level of the umbilicus), arms kept alongside the body, feet placed over the plinth, and head rotated to the right (Fig. 2). The sphygmomanometer was in ated to 70 mmHg, and the participants were instructed to move the abdominal wall uptwards and inwards without moving the spine and pelvis. This position was maintained for 10 seconds. An excellent contraction was considered if a variation of 4-6 mmHg was observed [18]. A variation above or below this pressure corresponded to an insu cient contraction.
All statistical analyses were performed using the Statistical Package for the Social Sciences (SPSS) software, version 22.0 (IBM Corp., USA). Data are shown as mean ± standard deviation, and data normality was performed using the Shapiro-Wilk test. One-way ANOVA with Tukey's posthoc test was performed to determine the homogeneity of the initial values and compare the differences between groups (HG, LBPG, and HDG) for the following variables: pain, exibility, disability, and strength. Chi-Square test was used to analyze categorical variables. A signi cance level of 5% (two-tailed) was considered.

Results
Thirty individuals, ten in each group, were selected and evaluated. There were no sample losses. The sample characterization and homogeneity are shown in Table 1.  Values of mean and standard deviation (SD) of all variables and groups and ANOVA comparison results between groups.
Regarding the activation of the transversus abdominis, 60% of the HG participants had excellent activation, while 30% and 20% were excellent in the LBPG and HDG, respectively. Besides that, no signi cant differences were found between groups (p = 0.155) ( Table 3).

Discussion
The purpose of this study was to compare the activation of the transversus abdominis and the back muscle strength between healthy individuals, individuals with non-speci c low back pain, and individuals with disc herniation.
The mean age was signi cantly different between groups, and the HDG had a higher value when compared with the HG and LBPG. Thus, age can be considered an important factor since older individuals are more prone to developing degenerative changes in the spine. This notion corroborates with Dammers [19] and Taylor et al. [20], which showed that spinal degeneration increases with age, and starts in both the caudal region (caused by the loss of proteoglycans) and in the upper and lower discs of the lumbosacral region, due to its proximity to the sacrum. Moreover, disc herniation is uncommon in the rst decades of life, being more prevalent in the subsequent four decades as a result of spinal overloading [21].
The body mass index and the working hours evaluated in this study were homogeneous for all participants. These factors are associated with spinal problems [22] due to the overload imposed by the time of work, prolonged static position, lifting of intermittent loads [23], and the own body structure in the case of overweight and obesity, resulting in musculoskeletal imbalances [24].
Although the HDG presented the greater instability to maintain a sustained contraction of the transversus abdominis, no signi cant differences were observed between the three groups studied. The instability observed in the HG can also be highlighted since its presence is probably involved in the complexity of low back pain [25].
Previous studies have shown that inadequate activation of the transversus abdominis is associated with back pain [26,27,28], and its ineffective contraction may lead to hypotrophy and reduced ring rate [29,30]. Also, those individuals with a previous history of asymptomatic back pain and ares present de cits in the transversus abdominis activation [31,32].
The reliability and reproducibility of the PBU to measure the transversus abdominis activity in both healthy [33,34] and individuals with non-speci c LBP [35] has been evaluated in previous studies and showed good results. However, more recently, the same research group conducted another study to verify the concurrent validity between PBU and surface electromyography (EMG) in patients with chronic LBP and found a low speci city and sensitivity of the PBU to evaluate the transversus abdominis activation [36]. The reasons for this result were probably related to the depth of the muscle, the EMG crosstalk, and the different purposes of these tools since EMG evaluates the electrical muscle activity and PBU assesses the abdominal displacement caused by abdominal muscle contraction.
Regarding the back strength during spine extension, a signi cant difference between the LBPG and HG was found, but with no differences between LBPG and HDG. This fact was probably in uenced by the pain level caused by the LBP. The fear of pain during maximum effort is an important factor taken into account by the patients. Therefore, those without pain felt safer and reached high strength values (119.2 ± 21.2 kgf), close to the reference values (114,0 ± 25,4 kgf) described by Eichinger et al. [14].
With regards to exibility, there was no difference between groups, showing that this variable is not associated with low back pain or herniated disc, and corroborating with Graup et al. [21], who described no association between lumbar exibility and pain in this region. However, pain is a predisposing factor for decreased lumbar exibility, as stated by Toscano [37], who showed that the range of motion was associated with acute or chronic LBP relief. As expected, back pain was not different between healthy individuals and those with speci c or non-speci c LBP.
Disability has been a widely used criterion to evaluate patients with LBP, and the Rolland Morris questionnaire is a valid and well-accepted instrument for this outcome [38]. In the present study, it was observed that the HDG showed more disability than the LBPG and HG. Although disability has a direct relationship with pain, and both the LBPG and HDG presented signi cant pain levels, its conception in chronic conditions is multifactorial and does not present a linear and homogeneous behaviour [39]. This may explain why no differences were found in the LBPG. Also, according to Porchet et al. [40], the herniated disc severity is related to disability; thus, it can be concluded that individuals with disc herniation present more disability despite having pain levels similar to those with LBP.
The present study has some limitations. The cross-sectional design did not allow a direct causal inference, and the reduced sample may have minimized the interaction effects between groups. Moreover, both the PBU and the Schober test still present con icting results, thus requiring further studies to con rm their validity. Further studies with larger samples and long-term follow-ups are needed to investigate the real effectiveness of these instruments.

Conclusion
We can conclude that the activation of the tranversus abdominis is similar between healthy, non-speci c back pain and herniated disc individuals; however, the latter presents a reduced level of strength and more disability.

Declarations
Ethics approval and consent to participate

Availability of data and materials
The data for this research are available in the database attached in the related les section.

Con ict of interests
The authors declare that they have no con ict of interests.

Funding
The postgraduate program in Rehabilitation Sciences at the Federal University of Rio Grande do Norte supported the researchers and nanced the costs of translation and nal writing of the manuscript. This study was nanced in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior -Brazil (CAPES) -Finance Code 001.
Authors' contributions CGS was responsible for the study design and data collection. Performed data analysis and nal writing.
YCM conducted the data collection and the writing of the manuscript.
SJCA performed statistical analysis and participated in the writing of the manuscript.
KSAC organized the data and methodological aspects for submission of the manuscript.
LBM reviewed the manuscript and the translation criteria.
Relationship between severity of lumbar disc disease and disability scores in sciatica patients. Neurosurgery, 50(6), 1253-1260. Figure 1 Assessment of the back muscle strength. Muscle activation test for transversus abdominis using a Pressure Biofeedback Unit.