Introduction

Nonspecific low back pain (NSLBP), characterized by a lack of a specific etiology, accounts for 85% of low back complaints [ 1 , 2 ]. Despite efforts to understand and manage this worldwide health problem recurrences are common and have high economic costs [ 1 ]. Therefore, it remains a crucial objective to study this population to improve LBP rehabilitation protocols. Appropriate global trunk and local muscle activation is essential for spinal stability during different daily activities such as quiet stance and movements, including those made in response to mechanical perturbations [ 3 ]. Increasing evidence that shows muscular alteration in people with LBP (e.g. amplitude, timing, and recruitment patterns) [ 4 - 6 ] creates an urgent need to develop new exercise tools targeted at muscular recruitment and spinal stability disorders. Therefore, there have been efforts to introduce exercises in a “task-specific” manner, matched to daily and occupational activities [ 7 ] such as using vibrating tools or traveling in different types of vehicles.

Recently, the flexi-bar has been proposed as an effective, portable, and inexpensive tool in LBP rehabilitation. It has been increasingly used to train strength, endurance, coordination, balance, proprioception, and joint stability [ 8 , 9 ], although its effects have not been well documented. Flexi-bar exercises involve oscillation of the bar by active elbow movements that need a stabilized trunk foundation for upper extremity movements [ 10 ]. This tool automatically triggers activity in local (multifidus, transverse abdominis, and internal oblique) and global core muscles (rectus abdominis and erector spine) to control the trunk against oscillatory forces created by the flexi-bar [ 11 , 12 ]. Recent studies have investigated factors that should be considered to guide clinicians’ prescription practices, e.g. different postures (standing, sitting, quadruped, and post pelvic tilt) and bar orientation (vertical or horizontal) in healthy people [ 10 - 13 ]. They suggested that posture and bar orientation can determine the muscles that are targeted in training with a flexi-bar. However, how motor control regulates this task on a rigid and foam surface remains poorly understood. According to earlier work, the central nervous system regulates muscular control at a more proximal level and increases the activity level on an unstable support surface [ 14 - 16 ]. Therefore, the mechanisms that control responses to perturbations may differ when the flexi-bar is used on different surfaces. Also, to the best of our knowledge, previous studies have investigated muscle responses mostly at the trunk level during flexi-bar use. However, LBP may contribute to altered strategies at distal or proximal sites in the low back. Therefore, evaluating lower extremity muscle activity alongside trunk activity may be helpful to gain better insights into the effects of the flexi-bar. Only a few studies showed pain, disability, and transverse abdominis thickness improvements after flexi-bar exercise programs in people with LBP [ 17 , 18 ]. However, there are no published studies evaluating trunk and lower extremity EMG activity in individuals with LBP during flexi-bar use. Such studies may provide information to better explain the biomechanical mechanisms of clinical improvements after flexi-bar use in this population. Also, detailed studies of muscle activation during flexi-bar use can help clinicians to choose the optimal exercise conditions for specific LBP impairments. Whether trunk and lower extremity muscle responses differ between healthy people and those with LBP, or whether the type of support surface modulates muscular control during flexi-bar use, have not been determined. Accordingly, this study aimed to evaluate and compare trunk and lower extremity muscle activities during flexi-bar use on a soft and rigid support surface in a sample of healthy people and people with LBP. We hypothesized that EMG activity of the trunk and lower extremity muscles would be modulated by flexi-bar use in ways that differed between the two groups and between the soft and rigid surfaces.

Material and Methods

Participants

Eighteen healthy men and 18 men with NSLBP participated in this cross-sectional study. Healthy right-handed volunteers with no history of LBP were recruited by announcements. Volunteers with a diagnosis of NSLBP were referred by physicians from local university hospitals. The inclusion and exclusion criteria of the participants are shown in Table 1. This study was approved by the local Ethics Committee of Shiraz University Medical Sciences under file number 14399.

Procedure

All participants were seen on two days at the local biomechanics laboratory. On the first day, if all inclusion and exclusion criteria were met, they were given an explanation of the protocols and written informed consent was obtained from all participants. The demographic characteristics and pain intensity in the recurrence period of LBP according to a visual analogscale (VAS) were recorded (Table 2). All participants practiced the flexi-bar exercise until they were able to perform it correctly and efficiently with active rhythmic movements of their elbows and minimum movements of their trunk and upper arm. The rhythm was controlled by a metronome set at 5 Hz (300 bpm).

On the second day, their scores on the Persian Oswestry Disability Index (ODI) [ 19 ] and Fear Avoidance Beliefs Questionnaire (FABQ) [ 20 ] were recorded, and pain value on a VAS on the test day was obtained. A 5-min warm-up and cool-down were performed before and after the start and end of the procedure. The order of the 4 conditions based on tasks (quiet standing or flexi-bar) and the type of support surface (rigid or foam) was varied randomly (Table 3).

Trunk and right lower extremity muscle EMG activity and elbow and lumbar angle values were measured in all trials for 30 s with 2 synched DataLINK Biometrics Ltd DLK900 units (Biometrics Co, UK) and software version 8.60. The data were recorded with a sampling rate of 1000 Hz (13-bit resolution) and an amplification gain of 1000, a common-mode rejection ratio of 96 dB, and a bandwidth 20–450 Hz. Thirteen bipolar paired electrodes (Biometrics surface EMG SX230, 1-cm diameter Ag/Ag-Cl and 2-cm center-to-center distance electrodes) were attached for the left and right internal oblique (LIO, RIO), external oblique (LEO, REO), thoracic erector spinae (LTES, RTES), lumbar erector spinae (LLES, RLES), right rectus abdominus (RRA), rectus femoris (RRF), biceps femoris (RBF), tibialis anterior (RTA) and gastrocnemius (RGast) muscles with double-sided adhesive tape [ 11 , 12 , 21 ] (Table 4). Also, two ground references (Biometrics R506) were firmly attached to the ulnar styloid processes with wrist bands. Two goniometers (Biometrics SG150 and SG150B) were used for the right elbow joint and lumbar region to control elbow and lumbar movements during the flexi-bar exercises. In this way we tried to ensure correct, consistent performance in both groups.

To minimize skin resistance, the areas were shaved and cleaned. Before the experiment, 3 repetitions of 7 s maximum voluntary contraction (MVC) were performed to normalize EMG activity of each muscle based on previous studies [ 10 , 12 ] (Table 4).

During the tests, participants stood with their legs separated by their hip width, and wore an eye mask. Vision occlusion was used to increase the proprioceptive challenges in postural control [ 22 , 23 ]. In QS trials, as a control position, the participants remained still. In flexi-bar trials, a flexi-bar (Liveup, Nantong, China) 621.4 g in weight, 160 cm long, and with a gripping area in the center, was used at 5 Hz natural frequency of oscillation. All participants oscillated the flexi-bar by holding the center grip with a 2-hand symmetrical hold in vertical alignment, and sagittal plane oscillation. Each test was performed 3 times. A 2-min rest was allowed between each test.

Data processing

Raw data were processed in Matlab software (Matlab R2015b, USA). A second-order Butterworth bandpass filter at 20–450 Hz and a 50 Hz notch filter were used. Then the root mean square (RMS) values of EMG data were calculated over a 200-ms moving window. The average of the three repetitions of the MVC tests was calculated for each muscle. Finally, the EMG activity of all muscles was normalized as the percent of MVC. The mean angle values for the elbow joint and lumbar region were obtained with goniometers.

Statistical analysis

Statistical analyses were performed with SPSS v. 21 at a significance level of P<0.05. The Kolmogorov–Smirnov test was used to verify the normality of all data. Demographic characteristics and mean elbow and lumbar angle values were compared between groups with the Mann–Whitney or independent t-test. A repeated measures analysis (group: healthy vs. LBP as the between-group variable, task: QS vs. flexi-bar and surface: rigid vs. foam as within-group variables) was used to compare %MVC of the trunk and lower extremity muscle activity between groups and conditions. Bonferroni correction was used for all significant interaction results.

Results

There was no statistically significant difference in demographic characteristics between groups (Table 2). No statistically significant difference was found between groups in lumbar and elbow joint angular movements (Table 2). This indicated that both groups performed the exercises similarly.

The results showed significant main effects for tasks in all muscles except RRF (Table 5). The activity levels of all muscles were significantly greater during the flexi-bar than the QS condition (Figure 1). However, this effect was dependent on the support surface for RTA and RGast activity (significant task×surface interaction) (Figure 2). The results of post-hoc analysis showed that RTA and RGast activity was significantly greater in the flexi-bar than the QS condition on both the foam and rigid surface (Figure 2). In addition, significant main effects for support surface were found for RRF, RTA, and RGast activity. The results showed significantly greater muscle activity on the foam than on the rigid surface (Table 5, Figure 1). However, this effect was task-dependent for RTA and RGast (significant task×surface interaction) (Table 5, Figure 2). The results of post-hoc analysis showed that RTA and RGast activity was significantly greater on the foam than on the rigid surface in both the flexi-bar and QS condition (Figure 2).

Figure 1. The results of comparing the main effects of group, task and surface in trunk and lower extremity muscles. Values of all axes in each graph are %MVC (maximum voluntary contraction). (RRA: Right rectus abdominis, REO and LEO: Right and left external oblique, RIO and LIO: Right and left internal oblique, RTES and LTES: Right and left thoracic erector spine, RLES and LLES: Right and left lumbar erector spine, RRF: Right rectus femoris, RTA: Right tibialis anterior, RBF: Right biceps femoris, RGAST: Right gastrocnemius). *Significant differences

Figure 2. The results of post hoc analysis of surface×task interaction (Wilcoxon/paired-T tests) in the right tibialis anterior and gastrocnemius. Values in parentheses are standard deviations (SD). *Significant differences. QS: Quiet stance, RTA: Right tibialis anterior, RGast: Right gastrocnemius

Discussion

To the best of the authors’ knowledge, this study is the first to characterize and compare trunk and lower extremity muscle activity between healthy individuals and those with LBP while they used a flexi-bar, and the differences in activity according to support surface.

No significant differences were found between the LBP and healthy groups. This result might be due to only mild pain intensity on the test day, and to the fact that none of the tests during the procedure caused an increase in pain in any of the participants. The ODI score in the LBP group indicated a minimal degree of disability [ 24 ]. Therefore, a possible explanation for our result may be related to the participants’ mild clinical pain and disability. Because similar studies are scarce, it was not possible to compare the present results with earlier findings. The unpublished study by Herasi et al. (2015) was the only one that compared trunk muscle activity during flexi-bar use in healthy people and those with LBP. In consonance with the present results, they found no significant differences between groups [ 25 ]. Previous studies reported identical postural stability in healthy people and those with LBP in simple test conditions such as QS [ 16 , 26 ]. It appears that the simple tasks used in the present study were not challenging enough to reveal between-group differences in muscle responses. On the other hand, other tasks tested in the present study included perturbations on an unstable support surface with sight deprivation, which is a complex task. Marras et al. (2004) found that high-stress conditions may create a greater need for control in both healthy individuals and those with LBP [ 27 ]. This may account for the smaller between-group differences. The findings of the present study indicated that the flexi-bar may elicit the same strategies in people with LBP as are used by healthy individuals. This, in turn, suggests that it may be safe for clinicians to advise flexi-bar use in the population with NSLBP.

This study showed that almost all evaluated muscle activities increased to control posture during flexi-bar use. The flexi-bar perturbation resulted in postural challenges by applying an added external load to the body, triggering resistance and increased muscle activity. Bervis et al. suggested that the dynamic property of the applied load (a load with continuous changes in force) is another characteristic that could significantly increase challenges to postural stability in addition to the changes in load [ 28 ]. Previous studies mostly evaluated muscles at the trunk level, and in consonance with our results, showed a significant increase in rectus abdominis, external oblique, internal oblique, and erector spine activities during flexi-bar use compared to a non-flexi-bar condition [ 10 ]. The present study indicated that using the flexi-bar caused a wide range of responses in the trunk, hip, and ankle. The flexi-bar is a training tool that can effectively transfer vibration to the whole body from the upper extremities, thereby enhancing muscle activity at different levels [ 29 ]. Based on the results, it appears that the flexi-bar perturbation resulted in increased muscle activity not only at the trunk level but also in other joints in the kinetic chain. Therefore, the flexi-bar may be suggested as an economical tool in terms of time and cost that can simultaneously train many different muscles that are important in postural control.

It is interesting to note that flexi-bar use increased proximal muscle activity much more than ankle muscle activity (Figure 1). Therefore it may favor the use of muscle control in different segments, and elicit greater participation of proximal control. Earlier evidence showed that people with LBP use rigid postural control strategies instead of a flexible multi-segmental strategy [ 16 ]. The multi-segmental strategy uses coordinated postural corrections at several joints [ 30 ]. Individuals with LBP who do not use a multi-segmental strategy make less use of proximal lumbopelvic-hip control [ 16 ]. Therefore, flexi-bar use may be a potent postural control exercise that can train the motor control system to use a multi-segmental strategy with greater participation of proximal control. In this connection, earlier research demonstrated that continuous perturbation could favor the conversion of the ankle postural control strategy to a multi-segmental strategy [ 31 ].

This study showed that using a flexi-bar effectively recruited lower extremity activity in addition to trunk muscle activity. This finding may also be useful in lower extremity musculoskeletal problems. In addition, recent evidence suggested that adding core training to lower extremity rehabilitation may be helpful [ 32 ]. Therefore, flexi-bar exercising that targets both core and lower extremity muscle activity may be an effective approach in the treatment of lower extremity musculoskeletal problems and should be considered in future studies.

In previous studies, altered neuromuscular control and decreased use of abdominal and back muscles were found in people with LBP; this pattern of activation may lead to spinal instability and consequently to injury [ 5 , 33 ]. Because all crucial local and global core stabilizer muscles involuntarily resist external forces applied by the flexi-bar, this tool can be used as a core muscle regimen for NSLBP rehabilitation [ 34 ]. Also, the approximately equal level of trunk muscle activity on the right and left sides found in the present study indicated that coordinated exercising has the most beneficial effects on spinal stability [ 12 ]. An important consideration is that perturbations during flexi-bar use caused cyclic balance disturbances and consequently rapid oscillatory activity within milliseconds, especially in trunk muscles, to maintain stability [ 11 , 34 ]. The oscillatory nature of this type of exercise may enable it to target neuromuscular and proprioceptive deficiencies in people with LBP [ 15 , 35 ]. Future studies are needed to investigate this.

In the present study, the use of a foam support surface resulted in higher muscle activity in the RRF, RTA, and RGast compared to the stable surface. However, no changes were found in abdominal and back muscle activities. The significantly greater ankle movement on the foam surface (data not shown) may have resulted in greater use of the leg muscle stretch reflexes, and evidence has shown that these reflexes are still effective on foam surfaces [ 36 ]. Accordingly, the greater ankle muscle activity on the foam surface was predictable. This co-contraction may cause joint stiffness to stabilize the center of mass. Notably, in all test conditions vision was occluded, and this condition was suggested to be primarily linked with the ankle strategy [ 37 ]. However, the foam surface distorts sensory information and decreases the proprioceptive weight of the ankle [ 16 ]. Therefore, as the results indicate, the foam condition required proximal muscle control rather than the use of the surrounding joints and the ankle strategy [ 16 ]. It appears that muscle activation at the hip level without any significant increase in abdominal and back muscle activities was sufficient to control balance on the unstable support surface in the present study. Previous studies showed the hip strategy to be the primary resort [ 38 ], and also compensatory leg and trunk muscle activity in moving support conditions [ 39 , 40 ]. Some of the discrepancies with the present findings may be explained by differences in experimental conditions across studies. Nonetheless, the information from this part of the present results may be useful in planning interventions to target specific deficiencies such as impaired hip control in persons with LBP [ 41 ]. In this connection Figure 1 shows a lower non-significant RRF activity in people with LBP. The present results thus suggest that using a foam surface may help to train hip muscle activity in people with LBP.

With regard to the differences between using the flexi-bar on a foam or rigid surface, the present results showed that exercising on a foam surface recruited all muscle activities at the same level as on a rigid surface, except for RRF, RTA, and RGast. Flexi-bar exercising on a foam surface placed greater demands on RRF, RTA and, RGast activity. It is worth noting that compared to the QS condition, the flexi-bar condition did not improve RRF activity, whereas the foam surface was associated with greater RRF muscle activity. Therefore, a combination of flexi-bar use on a foam surface may be a potential way to additively enhance hip muscle activation to specifically target hip control deficiency in people with LBP [ 41 ].

Both unstable support surfaces and flexi-bar use provide perturbing environments. The foam surface was associated with greater muscle activity only in the ankle and RRF. However, the flexi-bar led to a wider range of responses through the trunk, hip, and ankle to control posture. These findings may indicate that vibration caused by the flexi-bar was a greater challenge that required the participation of more proximal segments for postural control (i.e., trunk and hip level in addition to ankle level) than the foam surface condition (i.e., hip level in addition to ankle level). Therefore, the level of postural muscle control may depend on environmental conditions.

The lower extremity EMG recordings were obtained for one side only because of the limitation in the number of EMG channels. The results of this study are limited to healthy young men and young men with LBP and only mild disability. This study evaluated the effects of flexi-bar oscillation only in the sagittal plane. Future studies should also investigate the effects of flexi-bar use in other planes of oscillation. In view of the potential effects of flexi-bar use and support surface changes based on the present results, future studies should also investigate the effects of long-term flexi-bar exercising in different conditions in a clinical trial study design. This design may help to clarify whether flexi-bar exercising on foam would be more beneficial for clinical outcomes in LBP rehabilitation or not.

Conclusion

The results of this study provide important considerations about the potential effects of oscillatory forces caused by the flexi-bar and different support surfaces on trunk and lower extremity muscle activity in both healthy people and those with LBP, and can consequently guide clinicians in devising LBP rehabilitation programs. In addition, the results indicate that flexi-bar use can effectively increase muscle activation in multiple segments (trunk, hip, and ankle muscles) that are crucial for postural stability. Performing this postural control task on a soft surface seemed to target more proximal rather than ankle control. Therefore, using a flexi-bar may be helpful in LBP rehabilitation, and exercising on a foam surface may favor additive hip muscle activity in people with LBP. This information may help clinicians to use the most appropriate exercise depending on individual therapeutic goals.

Acknowledgments

The authors wish to thank the Research Consultation Center at Shiraz University of Medical Sciences for statistical advice, and all volunteers for their participation. Also, we thank K. Shashok (AuthorAID in the Eastern Mediterranean) for improving the use of English and Dr. Behdad Tahayori for his technical assistance. This research was supported by funding from Shiraz University of Medical Sciences, Vice Chancellery for Research Affairs (Gant number: 14399-06-01-1396).

Authors’ Contribution

S. Bervis, A. Motealleh, M. Parnianpour and S. Kahrizi conceived the idea. Design of the methodology and setting preparation was done by S. Bervis, A. Motealleh, M. Parnianpour, S. Kahrizi, Y. Amirmoezzi and SM. Shokouhyan. S. Bervis has gathered the data. Methodology has been written by S. Bervis, Y. Amirmoezzi and SM. Shokouhyan. Results and data analysis was carried out by Y. Amirmoezzi and SM. Shokouhyan. S. Bervis has written original draft of the manuscript by the supervision of A. Motealleh, M. Parnianpour and S. Kahrizi in explanation of the study results. Investigating the related literature was done by A. Motealleh, M. Parnianpour and S. Kahrizi. The research work was proofread and supervised by A. Motealleh, M. Parnianpour and S. Kahrizi. All the authors read, reviewed, edited, and approved the final version of the manuscript.

Ethical Approval

The Ethics Committee of Shiraz University of Medical Sciences approved the protocol of the study (Ethic cod: IR.SUMS.REHAB.REC.1396.97).

Informed consent

All the participants have written the informed consent in the project.

Conflict of Interest

None

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