Low back pain (LBP) is the second most frequent reason, after upper respiratory tract infections, for physician visits (25). Low back pain is the primary cause of activity limitation in both men and women (21) and accounts for one-third of all workers' compensation costs (1). Although 70–90% of back pain episodes subside within 2–3 months of onset, recurrence rates of 25–60% in the following 12 months have been reported (8,9). Of greatest concern are those individuals who become significantly disabled by chronic LBP. This cohort amounts to 6–10% of the LBP population, yet because of repeated treatments, long-term work absence and early retirement, accounts for up to 75–90% of the total cost (4,7). Recent clinical guidelines have established that exercise is a first-choice recommendation for the treatment of chronic LBP (2). However, uncertainty remains as to the best way to prescribe an LBP exercise rehabilitation program.
The uncertainty in exercise prescription for LBP rehabilitation can be attributed, in part, to the lack of information about what happens to muscle activity and lumbar movement during exercise in patients with chronic LBP. Most recommended exercises are based on observations in asymptomatic individuals. Differences in muscle recruitment and trunk stiffness have been found between individuals with and without LBP in nonexercise experimental contexts. Individuals with LBP have been found to have greater trunk muscle activity compared to asymptomatic individuals during trunk movements in the frontal, sagittal, and transverse planes (35). Increases in trunk muscle activity are associated with greater trunk stiffness (22,28). Recent evidence has found that individuals with LBP had greater trunk stiffness than pain-free individuals when subjected to external perturbations (17). Patients with LBP have also been shown to exhibit decreased whole-body balance and lumbar position sense compared to asymptomatic individuals (12,27,29). It is reasonable to believe that patients with LBP will exhibit greater muscle activity during core stability exercises compared to asymptomatic individuals. If increased activity is observed in patients with LBP, modification techniques such as abdominal bracing that are thought to increase muscle activity during exercise to provide additional stability may in fact have little additional benefit.
Described as a stiffening of the abdominal wall with neither a hollowing inward or pushing outward, abdominal bracing has been shown to enhance spine stability in experimental contexts where the trunk is exposed to sudden perturbations (37). Although the abdominal brace is recommended to be used during exercise when a patient with LBP exhibits signs of spinal instability (16,26), it is a common clinical observation for bracing to be prophylactically prescribed during all trunk exercises for individuals with and without LBP. Although research has evaluated exercises such as the side bridge and quadruped (aka bird–dog or superman) using bracing, there has been no comparison to performing the same exercises without bracing (19,20). Moreover, there is no evidence examining the application of bracing during exercise in patients with chronic LBP.
Therefore the purpose of this study was to investigate trunk muscle activity during several commonly performed rehabilitation exercises in individuals with and without chronic LBP. Low back pain patient outcomes were compared to those of a group of matched controls to determine if the acute exercise response elicited in LBP populations is unique. A secondary purpose was to investigate the extent to which abdominal bracing affects muscle activity during exercise in both groups. We hypothesized that during exercise, trunk muscle activity would be elevated in patients with LBP compared to in healthy controls and that abdominal bracing would increase trunk muscle activity in all individuals.
Experimental Approach to the Problem
Muscle activity (electromyography [EMG]) and lumbar spine range of motion (LROM) were recorded during performance of side bridge, quadruped, modified push-ups, partial squats, and standing shoulder flexion exercises, with and without application of the abdominal bracing technique. Exercise order was randomized among participants. All participants attended 2 sessions separated by 7 days. The first was a familiarization session in which participants were instructed in the exercises to be performed and taught the abdominal bracing technique. The second session was the testing session in which each exercise was performed 3 times, with and without abdominal bracing. Data analysis was undertaken to determine if muscle activity (normalized to maximal voluntary isometric contraction [MVIC]) or LROM differed between or within groups when performing the exercises with and without bracing.
Two groups were recruited (Table 1): a group with chronic nonspecific LBP group (n = 10) and an age, height, and weight matched group of healthy controls (n = 10). Although some participants were familiar with exercises tested here, none reported regular exercise participation that would indicate a high level of proficiency with the movements. Inclusion for the LBP group required pain in the lumbar or lumbosacral region which has restricted functioning or required professional consultation, with a minimum Oswestry disability index score of 15%. The pain may be of idiopathic or musculoskeletal causes and of gradual onset. The pain must have been present for at least the previous 12 weeks (mean ± SE, 3.0 ± 1.1 years). Subjects were continually monitored throughout the study for change in pain status using a verbal visual analog pain scale. Inclusion for the control group required that subjects must not have experienced LBP within the last 3 months nor have experienced a period of LBP lasting >2 weeks within the last 2 years. Exclusion criteria for both groups included reported mechanical or structural abnormalities of the spine as diagnosed by x-ray or magnetic resonance imaging scans, reported neuromuscular disorder or joint disease, signs of nerve root compression, signs of clinical instability (positive prone instability test, aberrant movement patterns, positive straight leg raise test), inflammatory disease, cancer disease, acute worsening of pain symptoms, recent stabilization exercise performance or therapeutic treatment (within last 6 weeks), and pregnancy (current or within the last 2 years). Informed written consent was received from all participants before their entry into the study. The local ethics committee approved all procedures used in this study, which were conducted in accordance with the Declaration of Helsinki.
Surface EMG was employed to record muscle activity of the rectus abdominis (RA), external oblique (EO), and erector spinae (ES) muscles. Pairs of Ag/AgCl Blue sensor electrodes (Ambu Ltd., Ballerus, Denmark) with an electrode diameter of 14 mm were used to record bilateral muscle activity. The EMG signals were amplified with a BioVision (BioVision Technologies, Wehrheim, Germany) amplifier system (common mode rejection ratio of 110 db @ 50 Hz, input impedance >100 MΩ) and collected with a Power Lab (Sydney, Australia) 16-bit analog to digital acquisition system and Chart V 5.5 software (ADI Instruments, Sydney, Australia). The EMG signals were bandpass filtered from 20 to 500 Hz and sampled at 1,000 Hz. After collection, EMG signals were rectified and smoothed using a root mean square (RMS) calculation with a 50-millisecond analysis window.
Before application of the electrodes, the skin was carefully prepared to reduce electrical impedance to below 5 kΩ (measured with an analog multimeter) by shaving excess hair and clearing the loose hair with a paper towel, abrading with fine sandpaper, and cleaning with an isopropyl alcohol swab. After allowing the skin to dry, the electrode pairs were placed with a center-to-center distance of 2 cm over the trunk muscle sites. All electrode pairs were aligned parallel to the direction of the muscle fibers and placed on both sides of the body.
The following site placements were used for the recording electrodes; RA electrodes were placed 3 cm superior to the umbilicus and 2 cm lateral to the midline. The EO electrodes were placed in an oblique arrangement above the anterior superior iliac spine and lateral to the umbilicus. The ES electrodes were placed 3 cm lateral to the level of the L4/L5 spinous process. A reference electrode was placed over the lateral epicondyle of the humerus.
The average RMS of the EMG signal from a 3-second isometric phase for each exercise was normalized to MVICs performed for each muscle (see below). The mean of the 3 trials performed for each exercise task was used for data analysis. For the unilateral exercise tasks (quadruped, side bridge) the EMG data were analyzed according to the ipsilateral (same side as moved leg for quadruped, same side as leg on the ground for side bridge) and contralateral muscles. For the bilateral exercises (push-up, squat, shoulder flexion), the EMG data between right and left sides were averaged for analysis.
The MVICs were performed for each muscle before testing using previously outlined protocols for each muscle (19,20). Briefly, the RA MVIC was performed by a resisted sit-up with the subject lying supine in a partial trunk flexion position with maximal isometric resistance applied by the experimenter through the shoulders. Maximal EO activation was measured in a separate trial via a resisted cross sit-up position with the subject lying supine and moving the shoulder toward the contralateral knee with maximal isometric resistance applied to the ipsilateral shoulder (3). The ES MVIC was performed using a resisted trunk extension movement in the Biering–Sorenson position. Two MVICs were recorded for each muscle, after 3–5 submaximal familiarization contractions performed for each movement. The MVICs were performed over a 5-second interval involving a gradual build up to maximal muscle activity. At least 2-minute rest was provided between each MVIC (10). Based on the maximum value from RMS of the processed EMG signal, the reference MVIC was obtained as the highest activity from the 2 trials (13).
Single axis inclinometers were employed to measure LROM in degrees using a technique developed by Loebl (23). The inclinometer signals were amplified ±5 V with BioVision amplifiers and collected with a Power Lab A/D acquisition system and Chart V 5.5 software (ADI Instruments). The signal was sampled at 1,000 Hz and had a 15-Hz low-pass filter applied to it. The spinous process of S1 and T12 were palpated, and the inclinometers were affixed to these reference points using double-sided tape. To prevent any movement artifact, caused by skin movement, the inclinometers were attached while the subjects were standing in a slightly flexed position. To derive LROM during the exercise, the relative motion at the lower inclinometer was subtracted from the relative motion at the upper inclinometer. This has been found to be a reliable and valid method of measuring LROM (31). Maximal LROM was calculated during each exercise trial, and the mean of the 3 trials was used for data analysis.
During the familiarization session, subjects were taught abdominal bracing in a supine position with the lumbar spine in a neutral posture. Abdominal bracing was described as a tensing of the abdominal and low back musculature, without drawing in or flaring out the muscles (36,37). Subjects were required to breathe naturally during performance of the contraction. Subjects were required to bilaterally palpate inward using their thumbs onto each targeted muscle at the level of the umbilicus. They were instructed to lightly press inward and then produce a level of tension that would resist that movement the next time they palpated the muscle. This technique was performed around the abdominal and lumbar musculature in isolation, then synchronously for the correct bracing contraction. The EMG biofeedback was provided to ensure contraction of the abdominal and low back muscles was elicited in the brace. The brace was closely monitored by a research assistant to ensure that no movement or flaring out of the abdominal wall occurred. After attempting a maximal brace contraction to establish a ceiling intensity level (rated as a 10), the participant was instructed to perform the brace contraction at an intensity level of approximately 2–3 out of 10 (34). Three trials were recorded in isolation for reliability analysis. Participants were required to initiate this level of brace (2–3 out of 10) and maintain this contraction, during each exercise task while maintaining normal breathing.
Description of Exercises
Visual depictions of all experiments are presented in Figure 1.
The quadruped exercise was performed with the subjects on their hands and knees, with the shoulders, hips, and knees at 90° (20). Subjects were instructed to lift a leg and the contralateral arm so that the hip and knee were fully extended and the shoulder flexed. This position was defined as the isometric phase for EMG analysis
The side bridge exercise was performed with subjects lying on their side with legs extended. The ipsilateral leg to the side being tested was placed in front of the contralateral leg. Subjects were instructed to raise their hips to maintain a straight line over their whole body length, and to support themselves on one elbow and their feet. The uninvolved arm was placed across the chest with the hand resting on the involved shoulder. The raised position was where the 3-second isometric hold was performed.
The starting point of the push-up exercise was with the hands placed beneath the shoulder joints, the arms extended, spine in normal posture and knees shoulder width apart resting on the ground. The bottom position was defined as the point at which the subject had flexed their elbows to 90° with the trunk lowered toward the surface, without the chest touching the floor surface. The bottom position was defined as the isometric phase.
The starting position was in an upright position. Subjects were instructed to keep the arms elevated to 90° of shoulder flexion throughout the task. For each subject, an adjustable beam was placed at the bottom of the squat to indicate where 70° of knee flexion was reached. This was defined as the bottom range of the movement and was where the isometric hold of 3 seconds was performed.
In an upright standing position, using a pronated grip, subjects held dumbbells that equated to 60% of shoulder flexion 1 repetition maximum (see below). The individual was instructed to flex their shoulder to approximately 90°, without consciously moving their trunk. The position at 90° shoulder flexion was where the 3-second isometric hold was maintained. Maximal isometric force was calculated at the familiarization session 1 week earlier using a force tensiometer (range ± 1,000 N, Power Lab analog to digital acquisition system, 100-Hz sampling frequency, ADI Instruments) with the subject's shoulder flexed to 90°. Two maximal efforts were recorded for the shoulder flexion movement for each limb. The effort was performed over a 5-second interval involving a gradual build up to maximal muscle activity. At least 2 minutes rest was provided between each maximal effort. The reference maximal force (N) was defined as the highest force output measured for each limb from the 2 trials, which was then averaged between the limbs for 1 reference value. The 60% trial load was calculated based on the average maximal force of both limbs so that the same absolute load was held in both hands. There were no between-limb differences in force output.
The intraclass correlation coefficient (r value) was calculated for between-trial reliability of muscle activity measured during bracing performed in isolation, exercise performance without bracing, and when bracing was applied during exercise. Muscle activity (% MVIC) and LROM (°) were analyzed using a multivariate analysis of variance, with between subject factors of group (LBP or control) and technique (normal or brace). The level of significance for all analyses was p ≤ 0.05. Data are presented as mean ± SE.
No subjects had a worsening of symptoms during the study. This indicates that the exercise tasks did not aggravate the subjects' condition or confound results. Muscle activity data are presented in Tables 2 and 3. Lumbar range of motion data are presented in Table 4.
Technique Performance and Reliability
Muscle activity during the isolated brace technique was 6.9 ± 2.5% for RA, 12.0 ± 7.9% for EO, and 3.1 ± 1.7% for ES. Between-trial reliability of muscle activity measured during bracing in isolation was r = 0.96, muscle activity during normal exercise ranged from r = 0.86 to 0.93, and muscle activity during the exercises with bracing ranged from r = 0.80 to 0.90.
Significant group × technique effects were observed for contra and ipsilateral ES (p < 0.05; Table 2). The ES activity was lower for the LBP group compared to for the control group during the quadruped without bracing. The ES activity for the LBP group was significantly increased during the braced condition (49 and 59% respective increases for ipsi and contra ES). Significant technique effects were observed for contralateral RA and EO (p < 0.05), with a 53% increase in activity for both muscles during the braced condition. No group or technique effects were observed for LROM.
Significant main effects of group were observed for ipsilateral RA and EO (p < 0.001; Table 2), with greater muscle activity in the LBP compared to the control group. A group x technique interaction was observed for ipsilateral RA (p < 0.05), with increased activity in the LBP group when the brace was applied. A main technique effect was observed for contralateral EO (p < 0.001), with an average increase of 80% during the braced condition. No group or technique effects were observed for LROM.
A significant technique effect was observed in both groups for EO (p = 0.01; Table 3). Average EO muscle activity for all participants increased 58% during the braced condition. No group or technique effects were observed for LROM.
A significant group × technique effect was observed for EO (p = 0.032; Table 3), with increased activity for the LBP group during the braced compared to the control group. A significant technique effect was observed for RA (p = 0.03), with increased activity (65% increase) during bracing. No group or technique effects were observed for LROM.
A significant group × technique effect was observed for EO (p < 0.01), with increased activity for the LBP group during the braced condition. No group or technique effects were observed for LROM.
This study investigated trunk muscle activity during several commonly performed rehabilitation exercises in individuals with and without chronic LBP. In contrast to our hypothesis, trunk muscle activity was not consistently higher during exercise in LBP patients compared to healthy controls. Indeed, muscle activity for LBP patients was, respectively, lower and higher for the quadruped and side-bridge exercises compared to for healthy controls. Some support was found for our second hypothesis that abdominal bracing would increase trunk muscle activity during exercise for all individuals. Muscle activity was not increased in a balanced manner across all the trunk muscles during every exercise when bracing was applied. Furthermore, it is interesting to note that only the individuals with LBP were able to increase muscle activity with bracing during the squat and shoulder flexion exercises.
It is not clear from this study whether the different muscle recruitment patterns observed in LBP patients are a cause or effect of their symptoms. The different muscle activity observed in the LBP patients may be related to alterations in afferent input into the central nervous system (CNS) subsequent to the onset of symptoms to regulate trunk and hip stiffness for maintenance of posture and balance (15). Johansson and Sojka proposed that muscle pain produces disturbances in proprioception, stiffness regulation, and motor control by altering stretch sensitivity and the discharge of spindle afferents via gamma fusimotor neurons (18). Experimental evidence for this has been provided in felines and humans (11,17,24,33). Neuroanatomic and neurophysiologic studies show that spinal ligaments and intervertebral disks are endowed with mechanoreceptors and nociceptive afferents to signal joint loads, motion, and the presence of inflammation (5,14,30). Ligamentomuscular reflexes have subsequently been established between the spinal ligaments and disks, and surrounding muscles, which act to directly or indirectly modify the load imposed on the passive spinal elements through active contraction (32).
The lack of worsening of symptoms in the LBP group and similar LROM observed between groups suggests that all the exercises investigated in this study are of use in rehabilitating LBP patients. The differences in muscle activity observed in the LBP patients may indicate neural recruitment strategies for each exercise that modify the load imposed on the spinal tissues and maintain similar lumbar movement in comparison to how a healthy individual performs the same exercise. Whether or not the different muscle recruitment patterns in LBP patients require correction with bracing when the exercise task is being performed with appropriate technique is unclear.
It has been suggested that a benefit of bracing during exercise is that this technique can stiffen the spine making the exercise tolerable for patients with LBP with spine instability (16). The patients measured in this study did not have clinically assessed spinal instability, and represent the majority of patients with LBP observed in practice who are assessed as having nonspecific or mechanical etiology. The results of this study demonstrated that bracing does tend to increase muscle activity during exercise for both groups, with greater increases in muscle activity for patients with observed during all exercises apart from the push-up when the brace was applied. Although increased activity was observed, this was not in a balanced or consistent pattern to provide a clear conclusion for what application of bracing may change if applied to all exercise. The inability to produce the abdominal brace in a consistent or balanced manner has previously been identified (6). There are 2 issues that require consideration for recommendation of abdominal bracing during prescription of these exercises. In the normal ‘unbraced’ condition, there were no differences in LROM between groups and no symptom exacerbation in the LBP group. Therefore, there does not seem to be a need to make the exercise tolerable for these patients who do not have assessed clinical instability, regardless of the observed changes in activity when bracing was applied. Considering that bracing is associated with increased spinal compression loading, it would be reasonable to suggest that an individual who can perform a low back rehabilitation exercise with appropriate technique and no symptom provocation does not need to perform the bracing contraction. The widespread use of abdominal bracing in clinical practice, whether it be for patients with LBP or healthy individuals, may not be justified unless symptoms of spinal instability are identified.
Body-weight–only resistance exercises are routinely prescribed by coaches and practitioners for various populations. This mode of prescription is commonly used in core stability training. In populations where core stability training is especially common, such as those with chronic LBP, there is a lack of information about the types of core stability exercises prescribed. The results of this study provide information about how patients with chronic nonspecific LBP perform core stability exercises in comparison to healthy matched controls. Furthermore, the abdominal bracing technique was investigated as it is a commonly observed clinical supplement to core stability exercise. Muscle activity in patients with chronic LBP was different from healthy controls during the quadruped and side-bridge exercises, but a consistent pattern of difference was not observed. These changes may reflect a nervous system strategy to modify the spinal load while maintaining similar technique to healthy controls. Abdominal bracing increased trunk muscle activity in an unbalanced manner across the different exercises. Considering that no exercise exacerbated symptoms in the LBP group, these patients did not exhibit signs of clinical instability, and LROM was not different between groups, the addition of bracing may not be required during exercise in patients with chronic nonspecific or mechanical LBP. As described in previous recommendations, bracing may make an exercise tolerable for a patient with LBP with spinal instability but may not be required for generic application to all core stability based exercise programs.
The author acknowledge the Tertiary Education Commission and the University of Auckland Exercise Rehabilitation Clinic for scholarship funding provided for this research. The authors have no professional relationships with any company or manufacturer who may benefit from the results of this study. The results of this study do not constitute endorsement of the product by the authors or the National Strength and Conditioning Association.
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