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Reprint requests to Sylvain G. Grenier, PhD, Biomechanics, Ergonomics and Kinesiology Laboratory, School of Human Kinetics, Laurentian University, Sudbury, ON P3E 2C6, Canada.
Grenier SG, McGill SM. Quantification of lumbar stability by using 2 different abdominal activation strategies.
Objective
To determine whether the abdominal hollowing technique is more effective for lumbar spine stabilization than a full abdominal muscle cocontraction.
Design
Within-subject, repeated-measures analysis of variance was used to examine the effect of combining each of 4 loading conditions with either the hollow or brace condition on the dependent variables of stability and compression. A simulation was also conducted to assess the outcome of a person activating just the transversus abdominis during the hollow.
Setting
Laboratory.
Participants
Eight healthy men (age range, 20−33y).
Interventions
Electromyography and spine kinematics were recorded during an abdominal brace and a hollow while supporting either a bilateral or asymmetric weight in the hands.
Main Outcome Measures
Spine stability index and lumbar compression were calculated.
Results
In the simulation “ideal case,” the brace technique improved stability by 32%, with a 15% increase in lumbar compression. The transversus abdominis contributed .14% of stability to the brace pattern with a less than 0.1% decrease in compression.
Conclusions
Whatever the benefit underlying low-load transversus abdominis activation training, it is unlikely to be mechanical. There seems to be no mechanical rationale for using an abdominal hollow, or the transversus abdominis, to enhance stability. Bracing creates patterns that better enhance stability.
WITH A GOAL TO DECREASE low back pain (LBP), stabilizing exercises, along with modifications of other daily activities, have been shown to be effective in randomized clinical trials.
However, the source of this effect is uncertain. This study assessed the influence of different abdominal activation strategies on mechanical stability in the lumbar spine. Lumbar spine stability is an important issue, especially given its potential link to mechanisms of injury and associated clinical efforts directed toward enhancing stability in patients.
The way in which patients activate their abdominal muscles is central to the stability theme. For example, findings that transversus abdominis is recruited later, in some LBP sufferers, have led to speculation that it is related to an unhealthy or unstable spine.
The strategy to recruit the transversus abdominis, through the abdominal hollowing technique, has been proposed as an effective way to increase stability.
investigated the effect of bracing and hollowing on sacroiliac joint laxity in a nonfunctional task. They found that both improved stiffness but concluded that hollowing was better. However, the muscle resting levels differed between groups. Until now, most of the supporting evidence, for the transversus abdominis being an important contributor to stability, has been indirect and qualitative.
Motivation to focus on the transversus abdominis in the clinic has been provided by Hodges and Richardson
who documented delays of the transversus abdominis during rapid arm movements in some people who have a history of low back disorders, although this result has not been confirmed in recent reports.
that, although the direction of limb movement does not affect preactivation time, the magnitude of the arm movement perturbation does. Although this does not provide direct evidence linking the transversus abdominis to stability, it was suggested that this is evidence of motor control deficit in chronic back pain patients. Additional evidence of other types of motor control deficiencies in those with LBP include larger delays of onset in several torso muscles when the entire torso is moved quickly,
Previous history of LBP with work loss is related to lingering deficits in biomechanical, physiological, personal, psychosocial and motor control characteristics.
It appears that the transversus abdominis may be activated independently of other abdominal muscles at very low levels of challenge (1%−2% of maximum voluntary contraction [MVC]).
But at higher levels of activation, when people perform tasks requiring spine stability, the transversus abdominis has also been shown to be a synergist of internal oblique.
However, none of these studies have showed a direct link to the transversus abdominis and the mechanism for efficacy is not known. For example, Vezina and Hubley-Kozey
have measured abdominal wall activation levels during abdominal hollowing exercises and reported that none of their subjects were able to activate only the transversus abdominis. Furthermore, Davidson and Hubley-Kozey
showed that, as the demand of an exercise progression increased, the abdominal muscles converged, such that all muscles were activating to the same percentage of maximum at the highest exercise level. A recent study
Trunk muscle stabilization training plus general exercise versus general exercise only: randomized controlled trial of patients with recurrent low back pain.
suggests that hollowing and attempts to specifically activate the transversus abdominis reduced the efficacy of the exercise. One could argue that there is no quantitative evidence identifying the transversus abdominis as an important stabilizer, although our own clinical efficacy study
Preliminary development of a clinical prediction rule for determining which patients with low back pain will respond to a stabilization exercise program.
has shown that stabilization exercises are effective for patients with LBP. Nevertheless, it is not possible to make legitimate claims as to the stabilizing role of any specific muscles, only that the general approach was effective.
Quantification of Stability
The definition of stability is a critical issue. Because clinicians generally do not have the technical capability to quantify stability, kinematic “indicators” of instability have evolved. In and of itself, this is not a problem, except that these indicators can be disguised with appropriate muscle recruitment. Stability, as assessed in this study, is defined as the ability of the spinal column to survive an applied perturbation (known as Euler column stability). If the work done (input energy or disturbance) is greater than the potential energy of the column (energy stored in disks, ligaments, muscles, tendons), equilibrium will not be regained. From a clinical perspective, there is a subtle, but important, difference between excessive motion and instability; excessive motion does not imply instability, only the potential for instability (for a detailed description, see Cholewicki and McGill
Fig 1Stability analysis. This schematic depicts the process leading up to the calculation of stability. Abbreviations: DM, distribution moment; dL/dt, derivative of length with respect to time; EMG, electromyography.
In the context of lumbar stability, the effect of the transversus abdominis on the spine is still inconclusive. Isolated contraction of the transversus abdominis beyond very low levels of activation has not been measured because all abdominal muscles become involved at more functional levels of activation (5%−20%).
An investigation of specific abdominal muscle activation strategies and individual muscles’ role in that strategy should provide insight to the clinical decision-making process. The hollowing technique has been developed as a transversus abdominis motor pattern retraining technique by Jull and Richardson
to address the motor disturbances. It appears that others have assumed that it should be used as a technique to enhance spine stability. This article evaluates 2 abdominal activation strategies, hollowing and bracing, which are techniques used clinically to improve lumbar spine stability. Given the clinical issue stated previously and our previous work
examining the stabilizing role of many torso muscles, we were motivated to quantify the mechanical impact of the 2 strategies on lumbar stability. It was therefore hypothesized that the abdominal hollowing strategy would be inferior to that of a bracing strategy for enhancing stability. Spine stiffness is always stabilizing (summarized in McGill
). The question motivating this study therefore is as follows: is the isolated transversus abdominis recruitment associated with the abdominal hollowing technique a more effective stabilizer than a full abdominal girdle cocontraction?
Methods
In the present study, the comprehensive lumbar spine model used to quantify stability
(ie, activating only the transversus abdominis and internal oblique), simulations were also conducted to artificially activate the muscles in an “ideal” way. This was done, together with real, in vivo data collection, with the understanding that subjects may have had imperfect technique. In this way, we were able to evaluate “perfect hollowing” and bracing, as well as the imperfect clinical reality.
Data Collection
The study proceeded with 2 components in parallel. The first was the in vivo data collection, and the second was the simulation. During the in vivo task, torso muscle electromyographic activity and spine kinematics were recorded from each subject. These data were then input to the biomechanic model to determine spine stability. In the simulation, a single subject’s data were modified to reflect the anatomic and electromyographic changes associated with an “ideal” hollow and brace. In this way, several combinations of muscle activation were tested to assess their effect on stability.
In vivo: subjects
Data were collected from 8 healthy (no back pain in the past year) men between the ages of 20 and 33. All subjects provided informed consent, and the study was approved by the university ethics committee. Subjects had a mean height ± standard deviation (SD) of 1.82±0.06m (range, 1.73−1.88m), a mean weight of 79.8±11.5kg (range, 60.5−93.6kg), and a mean age of 23.8±4.33 years (range, 20−33y). None had any experience with the transversus abdominis recruitment training.
In vivo: kinematics
Spine kinematics were recorded with a 3Space Isotrak unit,
Polhemus, 40 Hercules Dr, PO Box 560, Colchester, VT 05446.
which measured lumbar flexion and extension, lateral bend, and axial twist at a sample rate of 60Hz. The 3Space electromagnetic field source was strapped in place over the sacrum, and a sensor was worn over the T12 vertebrae, isolating lumbar motion. The 3Space unit returns Euler angles. These were adjusted for anatomic relevance to the spine’s orthopedic axes. Flexion and extension corresponds to rotation about y, lateral bend to rotation about x, and twist to rotation about z.
Bortec Biomedical, 239 Springborough Wy, Calgary, AB T3H 5M8, Canada.
(12-bit analog-to-digital conversion; sampling rate, 1024Hz; frequency response, 10−1000Hz; common mode rejection ratio, 115dB at 60Hz; input impedance, ≈10GΩ). As per the previously validated electrode positions,
7 muscles on each side of the body were recorded for a total of 14 muscles, including the rectus abdominis (2cm lateral to the umbilicus), internal oblique (caudal to the anterior superior iliac spine and medial to the inguinal ligament), external oblique (15cm lateral to the umbilicus), latissimus dorsi over the muscle belly (15cm lateral to T9), thoracic erector spinae (5cm lateral to T9 over the muscle belly), lumbar erector spinae (3cm lateral to L3), and the multifidus (2cm lateral to L5, angled slightly with superior electrode more medial). The electromyographic signals were normalized to the amplitudes measured during the MVC procedure after rectification and low-pass filtering at 2.5Hz. In addition to preparing the electromyography for force estimation, this process also removed electrocardiographic interference.
MVCs, for normalization, were performed in 2 separate trials.
Electromyographic activity of the abdominal and low back musculature during the generation of isometric and dynamic axial trunk torque: implications for lumbar mechanics.
First, the abdominal flexors were contracted maximally in a seated position, leaning backward at 45°. Then, the extensors were contracted maximally in a Biering-Sorensen position.
In both cases, the experimenter provided sufficient resistance for a maximal isometric contraction. Once this was done, the hollowing and bracing strategies were explained and demonstrated.
In vivo: protocol
Subjects were asked to hollow their abdomen without sucking in their belly. They were told that they should be able to breathe normally. Assuming that the transversus abdominis was synergistic in its activation with the internal oblique,
the subjects practiced until they were able to easily achieve the required internal oblique activation target of 20% of MVC, as displayed on an oscilloscope. This compares with a range of approximately 12% of MVC of external oblique activation in hollowing and 32% of MVC of external oblique activation in bracing measured by Richardson et al.
A brace is different from the hollow in several ways; the brace involves activation of all abdominal muscles to a level that increases torso stiffness. This does not mean that no motion can occur, only that the motion is controlled. Furthermore, abdominal bracing causes the back extensors to become active, which further enhances spine stiffness. Initially, the electromyographic patterns were verified with oscilloscope, and observations of initial muscle contraction with ultrasound ensured that the activation patterns reproduced the technique reported in the literature.
The ultrasound probe was placed at the rectus border so that the 3 layers of the abdominal wall could be viewed. In the current study, there were considerable variations in the recruitment patterns used to achieve the hollowing. Hollowing trials were collected when subjects could show, on an oscilloscope, a decrease in external oblique and rectus abdominis activity, along with an increase in internal oblique activity, although this increase was often minimal. The testing was performed in an anatomically neutral standing posture, with a 10-kg load in either or both hands, depending on the condition. Over a period of 25 seconds, subjects were then asked to relax for 5 seconds, “hollow” the abdomen for 5 seconds, relax for 5, “brace” the abdomen for 5 seconds, and relax for the final 5 seconds. These trials were repeated, in random fashion, 3 times each with (1) no load in the hands (bilateral no lift), (2) with 10kg in each hand (bilateral lift), (3) 10kg in the right hand only, and finally with (4) 10kg in the left hand only. Different load conditions were used to target the effect of asymmetric activation, as well as to address the issue of increased intervertebral stiffness with higher compression loads. The kinematic and electromyographic data were then input to the custom stability model. Note that, for this part of the experiment, because transversus abdominis activity was not measured, it was assumed to be a synergist of the anterior and caudal portions of internal oblique. Thus, the internal oblique recording was used to drive the transversus abdominis in the model. Two studies have shown this to be a realistic assumption.
Spine stability was calculated by computing the potential energy of the spinal system. For the purpose of summing potential energy, the muscle and tendons were considered as linear springs, whereas the intervertebral disks were considered as torsional springs. The elastic potential energy was then summed in each rotational degree of freedom, and the work done by the external load was then subtracted from this. The second derivative of the potential energy matrix, if greater than or equal to 0, indicated a stable system.
Specific components of the model are described as follows.
Passive contribution to potential energy
The skeleton of the model consisted of 5 lumbar vertebrae between a rigid pelvis and sacrum and a rigid ribcage. The vertebrae were linked by torsional springs representing the vertebral disks, which generated passive stiffness and allowed rotation but no translation (about 3 orthogonal axes). Three-dimensional lumbar motion, measured with the 3Space Isotrak was proportionally distributed among all vertebrae.
In addition to the restorative moment of the passive tissues, the muscles also contributed a restorative moment to balance the external load (fig 2). The muscle force and stiffness estimates were obtained from a distribution-moment muscle model that used electromyographic and muscle length as input.
The muscle forces were then applied through 118 muscle fascicles to the skeletal components such that the moment they created balanced the moment generated by the external load. Stability calculations used all of these variables as input, together with the muscle stiffnesses calculated by the distribution-moment model.
Fig 2A schematic representation of a simplified spine motion segment to show the concept of linear (muscles, ligaments, tendons) and torsional (intervertebral disks) springs working to sustain an applied load.
Anatomic improvements were made to better represent the application of transversus abdominis forces on the spine. Specifically, the fascial attachment of the transversus abdominis on the lumbar vertebrae was represented with 10 fascicles bilaterally on the 5 segments (2 originating on the posterior tip of the lumbar spinous processes and the other 2 originating on the transverse process of the lumbar vertebrae). To capture the line of action of the fascial attachments on the posterior spinal processes, the 10 fascicles converged on a point 60cm directly lateral of L5. This arrangement also closely approximated the experimental findings of Tesh et al
that the compression cosine of the lateral transversus abdominis force was 39% of its resultant magnitude. Because transversus abdominis activity was not measured, the stiffness of the transversus abdominis was calculated by using internal oblique activity magnitude because these have been shown to be synergistic. The length of the transversus abdominis fibers was calculated based on the real hoop-like architecture of the muscle. This architecture was intended to simply evaluate the effects of the muscles recognizing their attachment to the spine. There were no assumptions made regarding the role of intra-abdominal pressure to stiffen the abdominal wall.
Simulations
The objective of the simulation trials was to artificially adjust abdominal muscle activity levels to imitate “ideal” hollowing and bracing strategies because subjects could not completely isolate, or preferentially activate, any single abdominal muscle to the extent required.
A total of 1 each of 5 types of simulations was performed.
1
Simulation of the hollowing strategy: the transversus abdominis and internal oblique electromyographic signals were adjusted to an activity level at 20% of MVC, whereas the rectus abdominis and external oblique were adjusted to 2% of MVC. The activity levels in the back extensor muscles were simply those measured.
2
Simulation of the bracing strategy: all abdominal electromyographic signals were replaced by activity at 20% of MVC. The activity levels in the back extensor muscles were simply those measured.
3
Simulation of the bracing strategy: the stabilizing effect of the transversus abdominis was evaluated, and isolated, with a sensitivity test in which transversus abdominis activation was zeroed during a bracing simulation (bracing with no transversus abdominis).
4
Simulation of the bracing strategy: the procedure in simulation 3 was repeated by zeroing each abdominal muscle pair, similar to the muscle “knockout” approach of Cholewicki and VanVliet.
A right and left external oblique set to 0% of MVC.
B
A right and left internal oblique set to 0% of MVC.
C
A right and left rectus abdominis set to 0% of MVC.
In addition to these trials, across conditions, a sensitivity analysis was also conducted to assess the effect of the transversus abdominis alone on stability. First, only the transversus abdominis was set to 0% of MVC. This was compared with the condition in which all abdominal muscles were left untouched. Then, all abdominals except for the transversus abdominis were set to 0, whereas the transversus abdominis was set to 20% of MVC and then to 100% of MVC so that the transversus abdominis was the only active muscle. In addition to modifying the muscle activity of selected muscles in all simulations, the moment arm of rectus abdominis (and consequently the attachments of internal and external oblique) was shortened by 5cm, for the hollowing simulations, to mimic the “drawing in” of the abdomen (Fig 3, Fig 4). These artificially modified data were then input to the spine stability model as described earlier and in figure 1.
Fig 3Because the moment is calculated as the product of force and perpendicular distance, moving the muscle attachment (A) closer to the joint (J) (as occurs during hollowing) shortens the moment arm of rectus abdominis. This has a large effect on the resulting potential energy, and, consequently, stability as well. Stability of a column with guy wires (or a spine with muscles) is a function of geometry (the location of the support attachments), stiffness, the “balance” in stiffness in all directions, and column imperfections, such as curvature. Hollowing reduces the potential energy and stability by narrowing the geometric base.
Fig 4In the hollowing simulation, the moment arm of rectus abdominis was reduced by 5cm (“B” in left panel), when compared with the bracing condition (right panel), to account for the change in anatomic geometry. “A” defines the relaxed condition.
The dependent variables from the in vivo trials were spine compression and the stability index, which is a general indicator of the stability of the equilibrium state of the spine.
a within-subject, repeated-measures analysis of variance was performed to examine the effect of combining each of 4 loading conditions with the hollow or brace condition on the dependent variables of stability and compression. In this study then, the independent variable of “muscle activity pattern” had 2 conditions, hollowing and bracing, whereas the second independent variable of “load type” had 4 conditions: (1) load in both hands, (2) no load, (3) right-hand load, and (4) left-hand load. The Tukey post hoc honestly significant difference test was used to investigate differences.
Simulation analysis
Simulation trials were compared to the reference bracing trial, with the “muscle knockout” model, as well as with the simulated hollowing condition by using a paired t test. The change in (dependent) compression and stability index between bracing and hollowing trials was expressed as a percentage change.
(1)
where BRC is bracing and HLW is hollowing.
Results
Bracing stability was always greater than hollowing stability, and asymmetric loads always produced greater stability than symmetric loads. In in vivo trials, stability differed significantly for hollowing and bracing conditions (bracing > hollowing, P=.001) and between the loading conditions (P=.009) (table 1). Figure 5 shows a hollow-brace composite for 1 subject showing this difference. Compression values were not different, either between hollowing and bracing (P=.54) or between loading conditions (P=.095). One of the subject’s results were excluded because the abdominal recruitment patterns were not consistent with what had been requested for the trials (ie, hollowing of the abdomen was not achieved with a 20% of MVC internal oblique activation). Post hoc testing revealed that spine stability, in the no-load condition, was less than both right- and left-side loads but not less than the bilateral load condition.
Table 1Mean Stability and Compression Value From In Vivo Data Trials
Trial
Mean Stability Index (Nm/rad)
Mean Compression (N) at L4-5
Hollowing: no load
474.6±85.4
1866.5±451.4
Hollowing: 2-hand load
495.6±70.8
1929.2±360.0
Hollowing: left-hand load
517.7±55.8
2003.0±264.6
Hollowing: right-hand load
533.4±57.7
2041.7±286.6
Bracing: no load
511.3±39.5
1911.0±189.0
Bracing: 2-hand load
527.3±59.8
1989.1±247.9
Bracing: left-hand load
555.0±70.1
2060.4±223.1
Bracing: right-hand load
546.1±59.7
2008.6±266.1
NOTE. Values are mean ± SD. In the case of stability, there were differences between muscle activity patterns (bracing > hollowing, P<.001) and between the loading conditions (P=.009). The compression values had no differences, either between loads or between activation types.
Fig 5A composite of 2 trials of in vivo subject data (a 2-hand load trial chosen at random) shows that bracing had greater stability than an attempted hollowing. The ideal hollowing was not achieved. Ideal hollowing was tested in the simulations. Note the initial overactivation when trying to establish the hollow.
The simulation data supported the in vivo data, showing that the hollowing was not as effective as bracing for increasing stability in the lumbar spine. In fact, bracing improved stability over hollowing by 32%, with only a 15% increase in compression. Figure 6 shows the difference in stability, whereas figure 7 shows the difference in compression for 1 in vivo subject. In table 2, 1 subject’s data were used for the simulation. The unmodified hollow data were compared, for that subject, with the brace data. The various simulations were also compared with the full brace trial. When removing only the transversus abdominis from the bracing pattern (see table 2, Simulation bracing: no transversus abdominis), stability decreased by .14% with a less than 0.1% decrease in compression. Furthermore, the importance of the transversus abdominis relative to other abdominal muscles in affecting stability was very small when assessed by a “muscle-knockout” approach
Fig 6Lumbar stability increased by a significant margin (32%) under the simulated bracing condition. The hollow was conducted with 20% activation in both the lateral oblique and the transversus abdominis and 2% activation in both rectus abdominis and external oblique. This was considered feasible after observing our subjects. Most of the stability was obtained through the internal oblique. Abbreviations: BRC, bracing; HLW, hollowing; Stb, Stability.
Fig 7Although the external load component contributes to lumbar compression equally among conditions, compression due to muscle activation differed between the simulation bracing and hollowing condition by as much as 500N. Note that the hollowing here included both the internal oblique and transversus abdominis as in figure 6. Abbreviation: Cmp, compression.
Table 2One Subject’s Data Were Taken and Modified for the Simulation
Compared With Bracing
Stability % Change
Compression % Change
In vivo: no load hollowing
7.2
2.3
In vivo: 2-hand load hollowing
6.0
3.0
In vivo: left-hand load hollowing
6.7
2.8
In vivo: right-hand load hollowing
2.3
−1.7
Simulation hollowing
32.5
15.3
Simulation bracing: no TA
.14
.00
Simulation bracing: no EO
16.5
11.0
Simulation bracing: no IO
32.5
12.7
Simulation bracing: no RA
12.6
10.6
NOTE. The unmodified hollow data were compared, for that subject, to the brace data. The various simulations were also compared with the full brace trial. The percentage increase in stability of bracing over hollow is clear, especially for the ideal simulation of bracing. Each condition was compared with pure bracing trial. Note that in the subject data, compression increased under right-hand load hollowing conditions, although only by 1.7%.
Fig 8Stability index in simulation trial. A single, 2-hand load trial was processed, with 4 sensitivity tests, to isolate the role of different muscles. The bottom trace is actually 2 conditions using the transversus abdominis as the only active abdominal muscle (at 100% and 20% of MVC). Whether the transversus abdominis is active at 20% or 100% of MVC had little effect and those 2 plots are superimposed. The top trace is actually 2 conditions: (1) with all abdominal muscles active in a brace pattern (at whatever level the subject had activated them) and (2) with only the transversus abdominis removed. Only 1 line is visible because removing the transversus abdominis has almost no effect, and its plot overlays the full activation plot. The variability in the stability plot, including what stability is present, results from variable activity in the extensor (and other modeled) muscles. Abbreviations: abs, abdominals; TrA, transversus abdominis.
Is the abdominal hollowing technique and its specific transversus abdominis recruitment pattern a more effective stabilizer than a full abdominal girdle cocontraction? These data suggest that it is not. The bracing strategy provided greater stability than hollowing in both the simulation and in vivo data. Furthermore, for our subjects, the ability to activate just the transversus abdominis at functional levels was extremely challenging, if not impossible, as evidenced by all other abdominals’ electromyographic activity not being silent. This suggested that the attempt to hollow would, in effect, result in some degree of bracing. The simulations showed that the transversus abdominis had virtually no effect on stability. Although Hodges et al
have shown that the transversus abdominis does contribute stiffness to the spine in a porcine model, our sensitivity tests (see fig 8) show that its relative contribution is very small when compared with the other abdominal muscles. Assessment of this relative performance was absent from their experiment. Performing a hollowing pattern by using just the transversus abdominis greatly reduced stability, as opposed to all other abdominal muscles being active (fig 9). In addition, the compression-stability ratio favored bracing. The fact that the bracing strategy was more effective at stabilizing than the hollowing strategy should not be interpreted as evidence to diminish abdominal hollowing as a tool for training, or retraining the recruitment of the transversus abdominis from a motor control perspective, because this muscle does form a component of the abdominal girdle. However, many therapists and coaches, in our experience, recommend “drawing in” of the abdomen in an effort to increase stability. Based on our results, this appears to be misdirected. In fact, 2 studies have recently concluded that focusing on general activity
Trunk muscle stabilization training plus general exercise versus general exercise only: randomized controlled trial of patients with recurrent low back pain.
is more beneficial for both pain reduction and functionality.
Fig 9Stability index with abdominal muscles removed from simulation. This histogram shows the stability, for actual subjects, performing the brace and an attempted hollow (shown at the far right). Then, the right and left abdominal muscle pairs were removed from the analysis, each in turn, and the stability was recalculated. Although the removal of the transversus abdominis made little difference to stability, internal oblique was the most important in affecting stability, followed by external oblique and finally by rectus abdominis. Abbreviations: Brc, normal brace; BrEO, brace with external oblique; BrIO, brace with internal oblique; BrRA, brace with rectus abdominis; BrTA, brace with transversus abdominis; Hlw, normal hollow; HwEO, hollow with external oblique; HwIO, hollow with internal oblique; HwRA, hollow with rectus abdominis; HwTA, hollow with transversus abdominis.
Several limitations should be addressed to guide interpretation of these results. The transversus abdominis in the model was driven by the internal oblique activation profile because it has been shown to share a synergistic activation for neutral static postures, such as those tested in this study.
Responses of intra-abdominal pressure and abdominal muscle activity during dynamic trunk loading in man [published erratum in: Eur J Appl Physiol 1993;67:97].
The way in which the transversus abdominis was modeled was an attempt to capture its force, moment, and stabilizing effects on the spine. The architecture of the transversus abdominis suggests a limited capability to influence lumbar stability on its own. In contrast, the oblique vertical “wrap” of the internal oblique, for example, has a much higher potential to directly stabilize and compress the spinal column. Also, although a transversus abdominis muscle was modeled, the interaction between the transversus abdominis and intra-abdominal pressure was not accounted for. Intra-abdominal pressure is linked to abdominal wall activity, which stiffens and stabilizes the lumbar spine. However, at least 1 study
suggests that a hollowing strategy is unlikely to generate a greater intra-abdominal pressure than full bracing. This being the case, if pressure does affect stability, even more stability would result when bracing as compared with hollowing. The work of Tesh et al
contributed some insight as to the modeling of the transversus abdominis, its link with abdominal pressure, and resistance to lateral bending moments in full flexion. The force generated by our transversus abdominis equivalent was on the order of 50N for a 20% MVC. They also report that intra-abdominal pressure may contribute as much as 40% of the restorative moment in lateral bending. Our own work shows that intra-abdominal pressure contributes significant stiffness, especially in the neutral posture.
We speculate that there may be a binding of the abdominal muscle layers, when contracted together, that creates more stiffness than the sum of the parts. If this is the case, then the transversus abdominis would be an important member of the “abdominal team” but no more important than any of the other muscles.
In this study, the stability response was measured only with fully anticipated loads in isometric, neutral postures, for symmetric and asymmetric loads. It is possible that for a sudden load, the combination of a pressurization response and transversus abdominis activation might maintain sufficient stability until the remainder of the torso muscles are recruited. However, we remain skeptical of this possibility because the reported 30-ms delay for transversus abdominis onset is just a fraction of the 130-ms electromechanical delay inherent in the force production for trunk muscle.
In other words, a loss of stability is not likely to occur within the window between 30ms, the time at which the transversus abdominis turns on, and 130ms, the time at which it begins to produce force. It is therefore suggested that, although transversus abdominis onset delay
may be statistically significant, it is not mechanically significant. In any case, recent work done with cocontraction of the abdominals and its effect on stability, supports our findings.
A general cocontraction of the abdominal wall, balanced against antagonists and the load, is most effective in attaining and maintaining lumbar stability. There were considerable variations in the recruitment patterns, shown by subjects, used to achieve a hollowing. This is normal and is an advantage of our modeling approach, which is sensitive to the individual ways people activate muscle. The simulation was able to reproduce the ideal. Furthermore, this study was performed on young healthy males with no LBP. Injury, LBP, or population variability may result in different muscle recruitment patterns and stability profiles. Again, this possible weakness in the in vivo data was addressed via the simulations in which muscle activation was under complete control of the experimenters to elucidate the differences between ideal techniques and in vivo reality.
The simulations were important, for interpretation of this study, in 2 ways. First, the difficulty that participants had in achieving a hollowing recruitment pattern necessitated some way of testing the “ideal” hollowing pattern. Second, the simulations confirmed that, even if an ideal hollowing pattern was achieved, it would still fall short of the stability provided by bracing. This is the case regardless of the fact that in this study the transversus abdominis was not measured; the simulations assumed that it was optimally active. Recent work by Howarth et al
showed that the magnitude of the stability index is proportional to the level of risk of becoming unstable (of having the stability index go below 0). Nonetheless, it is still a relative measure so that it is not possible to compare the magnitude of the index between people. It should only be used comparatively within a subject, and even then, only within a testing session. For this reason, the participants acted as their own controls. Finally, this work examined Euler column stability; it is possible that the abdominal muscles work differently to buttress shear or pelvic instability. The mechanical effect on lumbar stability, of either the transversus abdominis or of hollowing, has not, to our knowledge, been reported in the literature before this. It must be clearly stated that this is a mechanical analysis of the role of the transversus abdominis and other abdominal muscles. Therefore, whatever role transversus abdominis isolation training has in rehabilitation, it cannot be explained by a mechanically based stability principle. If spinal stiffness is the ultimate goal in a training program, then bracing of the abdominal muscles is a clear winner over hollowing. In reality, a single focus on either strategy may not be optimal (or even possible) for functional tasks that show a great diversity in load and velocity.
Conclusions
This biomechanically based assessment suggests that bracing of the abdominal muscles provides greater lumbar spine stability than hollowing. According to our simulations, the potential of the transversus abdominis to enhance stability, on its own, appears to be very limited. The inability to isolate the transversus abdominis in a functional context may be a moot point because in healthy men bracing increases spine stability with minimal increase in spine compression loads. Muscles other than the transversus abdominis contribute relatively more to avoiding an unstable spine. Whatever the benefit underlying low-load transversus abdominis activation training, it is unlikely to be mechanical. There seems to be no mechanical rationale for using stabilization exercises to enhance a hollow for stabilization purposes; rather a brace creates patterns that better enhance stability.
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aPolhemus, 40 Hercules Dr, PO Box 560, Colchester, VT 05446.
bGraphic Controls Corp, Medical Products Div, PO Box 1274, Buffalo, NY 14240.
cBortec Biomedical, 239 Springborough Wy, Calgary, AB T3H 5M8, Canada.
Previous history of LBP with work loss is related to lingering deficits in biomechanical, physiological, personal, psychosocial and motor control characteristics.
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Supported by the Natural Sciences and Engineering Research Council of Canada (grant no. RGPIN36516-98).
No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the author(s) or upon any organization with which the author(s) is/are associated.
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