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Abstract The increased intra-abdo-
minal pressure (IAP) commonly ob-
served when the spine is loaded dur-
ing physical activities is hypothsized
to increase lumbar spine stability.The
mechanical stability of the lumbar
spine is an important consideration
in low back injury prevention and re-
habilitation strategies. This study ex-
amined the effects of raised IAP and
an abdominal belt on lumbar spine
stability. Two hypotheses were tes-
ted: (1) An increase in IAP leads to
increased lumbar spine stability, (2)
Wearing an abdominal belt increases
spine stability. Ten volunteers were
placed in a semi-seated position in a
jig that restricted hip motion leaving
the upper torso free to move in any
direction. The determination of lum-
bar spine stability was accomplished
by measuring the instantaneous trunk
stiffness in response to a sudden load
release. The quick release method
was applied in isometric trunk flex-
ion, extension, and lateral bending.
Activity of 12 major trunk muscles
was monitored with electromyogra-
phy and the IAP was measured with
an intra-gastric pressure transducer.
A two-factor repeated measures de-
sign was used (P < 0.05), in which
the spine stability was evaluated un-
der combinations of the following
two factors: belt or no belt and three
levels of IAP (0, 40, and 80% of
maximum). The belt and raised IAP
increased trunk stiffness in all direc-
tions, but the results in extension
lacked statistical significance. In fle-
xion, trunk stiffness increased by
21% and 42% due to 40% and 80%
IAP levels respectively; in lateral
bending, trunk stiffness increased by
16% and 30%. The belt added be-
tween 9% and 57% to the trunk stiff-
ness depending on the IAP level and
the direction of exertion. In all three
directions, the EMG activity of all
12 trunk muscles increased signifi-
cantly due to the elevated IAP. The
belt had no effect on the activity of
any of the muscles with the excep-
tion of the thoracic erector spinae in
extension and the lumbar erector
spinae in flexion, whose activities
decreased. The results indicate that
both wearing an abdominal belt and
raised IAP can each independently,
or in combination, increase lumbar
spine stability. However, the benefits
of the belt must be interpreted with
caution in the context of the de-
creased activation of a few trunk ex-
tensor muscles.
Key words Lumbar spine, stability ·
Lumbar spine, intra-abdominal
pressure · Lumbar spine, abdominal
belts · Lumbar spine,
electromyography
ORIGINAL ARTICLE
Eur Spine J (1999) 8:388–395
© Springer-Verlag 1999
Jacek Cholewicki
Krishna Juluru
Andrea Radebold
Manohar M. Panjabi
Stuart M. McGill
Lumbar spine stability can be augmented
with an abdominal belt
and/or increased intra-abdominal pressure
Received: 17 August 1998
Revised: 1 March 1999
Accepted: 18 March 1999
This study was supported by a grant from
Gaylord Rehabilitation Research Institute
J. Cholewicki (쾷) · K. Juluru ·
A. Radebold · M. M. Panjabi
Biomechanics Laboratory,
Department of Orthopaedics
and Rehabilitation,
Yale University School of Medicine,
P.O. Box 208071, New Haven,
CT 06520-8071, USA
e-mail: cholewicki@biomed.med.yale.edu,
Tel.: +1-203-737 2887,
Fax: +1-203-785 7069
S. M. McGill
Occupational Biomechanics Laboratories,
Department of Kinesiology,
Faculty of Applied Health Sciences,
University of Waterloo, Waterloo,
Ontario, Canada
Introduction
Chronic low back pain (LBP) creates a profound socio-
economic problem in today’s society [17, 18, 28, 51, 70].
Recent studies support the hypothesis that patients suffer-
ing from LBP of mechanical origin try to compensate for
their injuries with additional or different muscle recruit-
ment patterns, presumably to increase spine stability [4, 9,
14, 56, 61]. In healthy individuals, mechanical stability is
provided to the spine by trunk muscles and ligaments [5,
6, 11, 19, 20]. Injuries and chronic mechanical defects in
the osteoligamentous structures reduce spine stability
[53]. To maintain a normal level of stability, trunk mus-
cles must compensate by altering their normal activation
pattern [54, 55]. The question of whether wearing the ab-
dominal belt or the rise in the intra-abdominal pressure
(IAP) can increase the lumbar spine stability to protect it
from acute low back injury, still remains unanswered.
The early hypotheses that the IAP relieves part of the
compressive loads borne by the lumbar spine [1, 27, 49]
have been accepted by some [12, 22, 34, 35, 65, 66] and
refuted by others [2, 29–31, 42, 50, 52]. The current con-
sensus seems to be that the compressive forces, arising
from the contraction of abdominal wall musculature to
generate the IAP, offset the beneficial action of the hydro-
static forces thought to alleviate spinal compression via
IAP. The increased IAP commonly observed when the
spine is loaded during physical activities is hypothesized
to increase lumbar spine stability [41–44, 63]. For ex-
ample, patients with nonspecific low back pain exhibit
higher IAP during lifting than normal controls [16, 23].
The IAP also increases in response to a sudden trunk load-
ing in healthy individuals [10]. The above cited studies in-
directly point towards the association of IAP with the me-
chanical stability of the lumbar spine. However, direct ex-
periments are needed to test this hypothesis.
Abdominal belts have been shown to help individuals
in generating higher IAP levels during load-handling ac-
tivities [22, 34, 35, 43]. There exists anecdotal evidence
that people “feel safer” wearing abdominal belts when ex-
erting large forces [38]. This is especially true for weight
lifters and power lifters, who use belts apparently for no
obvious benefit other than to increase their IAP during
lifting [22, 34, 35]. While a few studies reported marginal
improvement in lifting capacity with the use of abdominal
belts [59, 62], the overwhelming evidence suggests that
belts have no effect on muscle strength [37, 39, 58], fa-
tigue [8, 39], or low back injury incidence [48, 57, 69].
Although one of the epidemiological studies claimed that
belt wearing reduced the number of low back injuries
[32], several methodological flaws make such interpreta-
tion of the results questionable. The most serious concern
was that the belt wearing policy was not the sole er-
gonomic intervention implemented at the time of this
study. At the present time, it appears that abdominal belts
are widely prescribed in industry and rehabilitation with-
out a convincing scientific justification of their benefits
[3, 13, 24, 45, 47, 68]. Often reported subjective feelings
of increased support may stem from abdominal belts pas-
sively increasing trunk stiffness and/or reducing its range
of motion [21, 36, 46, 60, 64]. However, the direct evi-
dence that belts modulate spine stability is still lacking.
The purpose of the present study was to examine the
effect of IAP and wearing an abdominal belt on lumbar
spine stability by measuring trunk stiffness with a quick
release method in trunk flexion, extension, and lateral
bending. Two hypotheses were tested:
1. An increase in IAP leads to increased lumbar spine sta-
bility, and
2. Wearing an abdominal belt helps to increase spine sta-
bility. Activity of major trunk muscles was monitored
with electromyography to add to the interpretation of
results.
Materials and methods
This was a two-factor experimental design in which spine stability,
a dependent variable, was evaluated under a combination of two
independent variables: wearing or not wearing the abdominal belt
and the level of intra-abdominal pressure (IAP). The determination
of lumbar spine stability was accomplished by measuring the in-
stantaneous trunk stiffness in response to a sudden load release that
subjects were resisting (quick release method). Electromyographic
signals (EMG) from major trunk muscles were recorded before
and after the release to add to the interpretation of results.
Ten subjects with no previous history of low back pain (aver-
age age 28, SD 4 years; height 177, SD 7 cm; weight 78, SD 14 kg)
were placed in a semi-seated position in a jig that restricted hip
motion leaving the upper torso free to move in any direction (Fig.
1). In the quick release method, the subjects exerted isometric
trunk extension, flexion, and lateral bending to the left, at 35% of
their maximum, resisting a cable attached to a chest harness at ap-
proximately the T9 level. The cable was held with an electromag-
net, which was suddenly released by the researcher (without warn-
ing the subject) when the required force level was achieved. The
resulting trunk motion was measured at 100 Hz with an inductive
sensor (Flock of Birds, Ascension Technologies, VT) placed on
the back at the T9 level.
The EMG signals were recorded from 12 muscles (left and right
rectus abdominis, external and internal oblique, latissimus dorsi,
thoracic and lumbar erector spinae) according to a previously es-
tablished protocol [5, 6]. The signals were band-pass preamplified
between 20 and 500 Hz, amplified, and converted to digital data at
1600 Hz. It was assumed that a muscle activation pattern estab-
lished prior to a sudden trunk perturbation determines the spine
stability and, in turn, the kinematics of the trunk response to that
perturbation. Accordingly, 200 ms of EMG data, recorded imme-
diately before the magnet release, were digitally rectified and aver-
aged. The baseline EMG values, recorded when the subjects were ly-
ing completely relaxed, were subtracted from the quick release EMG.
These baseline signals contained mostly electrode and amplifier noise.
Finally, the data were normalized to the EMG activity recorded dur-
ing the maximum voluntary contractions (MVC). With the exception
of lateral bending trials, left and right EMG values were averaged.
The IAP was measured with a transducer (Micro-tip MPC500,
Millar Instruments, TX) inserted into the stomach via the naso-
esophageal pathway. The subjects were instructed to increase their
IAP with the Valsalva maneuver and to hold it at the indicated
389
390
level while slowly increasing the trunk isometric force. The IAP
and the desired IAP target level lines were displayed on an oscillo-
scope for visual feedback to the subjects. Once the trunk isometric
force had reached its target, without warning, the researcher initi-
ated the data collection by releasing the electromagnet. All data
were collected in the about trigger mode for 1 s before and 2 s af-
ter the release.
Values for trunk stiffness were obtained from the trunk motion
data in accordance with a standard quick release protocol [25, 26,
33, 67, 72]. The trunk was modeled as a second-order system with
viscoelastic properties oscillating freely after the release of a mo-
ment that subjects were resisting. Amplitude and frequency of
such oscillations measured immediately after the release, but be-
fore voluntary muscle intervention took place, were determined by
trunk inertia (I), damping coefficient (B) and stiffness coefficient
(K) established prior to the release:
(1)
where θ is the trunk angle, mg is trunk weight, and L is the height
measured from the L5-S1 joint to the center of trunk mass, as-
sumed to be at T9 level. Trunk mass and moment of inertia were
calculated from the subject’s weight and height [71]. Coefficients
B, K and one additional integration constant C were obtained with
a curve fitting algorithm where the objective was to gain the best
match between the modeled and measured trunk rotation trajecto-
ries. This procedure was applied to the double integrated Equation 1
[67], because integration is numerically a more robust operation
than differentiation:
(2)
A preliminary study indicated that the minimum length of a data
record needed to identify parameters in the Equation 2 accurately
was equivalent to at least one-quarter of the wavelength. There-
fore, angular trunk motion data, taken from the time of magnet re-
lease to the point of maximum trunk deflection, was used for a
curve fit (Fig.2).
In each quick release direction, three trials were performed at
each of the three IAP levels (0, 40, and 80% of individual’s maxi-
mum). To reduce the testing time, only the 0% and 80% IAP trials
were repeated while wearing a standard 10-cm-wide nylon belt
(model SANB4, Altus Athletic, Altus, OK). A force of 180 N,
measured with a spring scale, was used to tighten the belt while the
subjects were actively attempting to minimize their waist circum-
ference. This force was used to standardize the belt tightness to a
moderate, comfortable level. All trials were performed in a ran-
domized order.
Trunk stiffness coefficients were averaged between three trials
and normalized for each subject to the stiffness value obtained
with no belt and no IAP. The effect of belt and IAP level, (inde-
pendent variables), on trunk stiffness, (a dependent variable), was
tested with two-factor, repeated measures ANOVA (2 × 3 unbal-
anced design, P < 0.05).
Results
On average, Equation 2 fitted the trunk angular deflection
data with Root mean square (RMS) error of 0.47° (SD =
0.56°) (Fig.2). Although the subjects were instructed to
maintain their IAP level at a given target, some variation
was present. In addition, it was not possible to have no
IAP when generating isometric trunk exertions. There-
fore, the average (SD) IAP values measured at the time of
magnet release were 14.1 (5.8), 42.8 (8.0), and 71.3 (9.3)
% of maximum for the trials labeled as 0, 40, and 80% re-
spectively.
I B dt K dt Ct mgL dtθθ θ θ + + + = sin
∫∫∫∫∫
22 2
I B K mgL
˙˙ ˙
θθθ θ + + = sin
Fig.1 A subject positioned in a hip motion restraining apparatus.
The intra-abdominal pressure (IAP) was built up with Valsalva
maneuver up to the target level, while subjects resisted the cable
load of 35% of their maximum. Stability was assessed upon the
electromagnet release of the cable load
Fig.2 An example of the trunk rotation responses to sudden un-
loading. The three curves represent trials under the following con-
ditions: (0% IAP) a subject was instructed not to increase his intra-
abdominal pressure, (80% IAP) a subject was instructed to reach a
target set at 80% IAP, (Belt) a subject was wearing an abdominal
belt and was instructed not to increase his intra-abdominal pres-
sure. Markers indicate the raw data and the lines indicate a curve
fit according to the Equation 2. K = Normalized trunk stiffness
Both the belt and IAP increased trunk stiffness in all
directions, but the results in extension lacked statistical
significance and will not be emphasized further (Fig.3A).
There were no interactions between the belt and IAP ef-
fects in any of the three directions. In flexion, trunk stiff-
ness increased by 21% and 42% due to 40% and 80% IAP
levels respectively (Fig.3B); in lateral bending, trunk
stiffness increased by 16% and 30% (Fig.3C). With no
IAP, the belt increased trunk stiffness by 29% and 9% in
flexion and lateral bending respectively. At 80% of maxi-
mum IAP, the belt added 41% to the trunk stiffness in
flexion and 57% in lateral bending. The effects of both fac-
tors on trunk stiffness were additive. Thus, the combined
effects of wearing an abdominal belt and the increased IAP
level to 80% of maximum provided 83% and 86% more
trunk stiffness in flexion and lateral bending respectively.
In all three directions, the EMG activity of all 12 trunk
muscles increased significantly due to the increased IAP
(Figs.4–6). The belt had no effect on the activity of any of
the muscles, with the exception of the thoracic erector
spinae in extension and the lumbar erector spinae in flex-
ion. Activity of those muscles decreased significantly
only at 80% IAP level due to wearing the belt (Figs.4, 5).
Discussion
Effects of the increased intra-abdominal pressure (IAP)
and the wearing of an abdominal belt on lumbar spine sta-
391
Fig.3A–C Normalized trunk stiffness obtained from a quick re-
lease method for various combinations of intra-abdominal pressure
and belt conditions. A Quick release in trunk extension, B flexion,
and C lateral bending to the left
Fig.4 Intra-abdominal pressure (% maximum) and belt effects on
muscle activity (% maximum voluntary contraction, MVC) aver-
aged 200 ms prior to the load release in trunk extension trials (RA
rectus abdominis, EO external oblique, IO internal oblique, LD
latissimus dorsi, TE thoracic erector spinae, LE lumbar erector
spinae)
Fig.5 Intra-abdominal pressure (% maximum) and belt effects on
muscle activity (% MVC) averaged 200 ms prior to the load re-
lease in trunk flexion trials
A B C
bility were studied by measuring the kinematics of trunk
response to a sudden load release, according to the so-
called quick release protocol. The more stable the struc-
ture prior to a perturbation (load release in this case), the
smaller is its deflection amplitude and the higher is its
oscillation frequency in response to that perturbation
(Fig.2). To quantify these trunk displacements, Equation
2 was used. Although Equation 2 represents a simplified
model, it fit the experimental data very well (average
RMS error < 0.5°).
In a quick release method, the stability of a multi de-
gree-of-freedom system is characterized by the aggregate
mechanical impedance parameters: inertia, damping, and
stiffness coefficients. The latter (stiffness) determines the
stability of the static equilibrium. It was this stiffness that
was used in the present study to quantify spine stability at
the time of load release. All three coefficients computed
from the trunk kinematics were dependent on the formu-
lation of the model and the assumed height, mass, and in-
ertia parameters (Equation 2). However, because the same
model was used in all conditions tested for each individ-
ual, and because only relative values of stiffness were an-
alyzed, the reliability of the results was assured.
The results indicated that both wearing an abdominal
belt and increased IAP can each independently, or in com-
bination, increase trunk stiffness, and therefore, increase
lumbar spine stability under sudden loading/unloading
conditions. However, the activation patterns of trunk mus-
cles suggest that the mechanisms of the spine stabilization
are different for those two factors. It is likely that the in-
crease in spine stability due to IAP was gained from the
concomitant increase in muscle coactivation needed to
generate a high IAP. This observation is consistent with
the IAP mechanism described by Cholewicki et al. [7].
These authors demonstrated with a physical and biome-
chanical model that the contraction of abdominal muscles
necessary to create IAP would stiffen the lumbar spine area.
Stabilizing the lumbar spine with the belt alone is most
likely a passive mechanism stemming from the interaction
of the wide and stiff belt placed between the ribcage and
pelvis. In contrast to the increased IAP effect, there was
virtually no change in muscle activity whether the belt
was worn or not. The exception of the thoracic and lum-
bar erector spine muscles, whose activation was signifi-
cantly smaller when the belt was worn in trunk extension
and flexion respectively, indicates further that a passive,
and not an active, mechanism was responsible for the in-
creased spine stability. These results are consistent with
McGill et al. [46], who reported an increase in passive
trunk stiffness in lateral bending and axial rotation due to
wearing an abdominal belt.
While it is clear that a belt itself contributes to lumbar
spine stability, as does the voluntary increase in IAP, its
benefits must be interpreted with caution. The fact that the
activity of some muscles decreased when the belt was
worn may indicate the reduction in overall muscle coacti-
vation causing reduction in active spine stabilization by
the muscles. Perhaps subjects perceived added stiffness
derived from the belt, and they therefore decreased mus-
cle coactivation. If this is the case, long-term abdominal
belt usage may lead to regression of the active spine sta-
bilizing system. This hypothesis would be consistent with
the outcome of a large study dealing with the incidence of
low back injury and long-term belt usage among airline
workers [57]. They found that abdominal belt use did not
reduce overall incidence of back injuries. However, when
the belts were removed after several months, frequency of
low back injuries increased. If our subjects had had time
to get used to wearing the belt, we might not have seen
any increase in the spine stability. The belt effect might
have been negated completely by decreased muscle coac-
tivation. Future studies should address the effects of long-
term belt usage on spine stability and on the stabilizing
function of trunk musculature.
Although the effects of IAP and a belt on spine stabil-
ity were studied here only in an upright posture, the re-
392
Fig.6A, B Intra-abdominal pressure (% maximum) and belt ef-
fects on muscle activity (% MVC) averaged 200 ms prior to the
load release in trunk lateral bending to the left trials. A Right side
muscles, B left side muscles (Subscripts R and L indicate the side)
A
B
393
sults could be extrapolated to other tasks and kinds of
lumbar supports. If the increase in IAP is possible in a
given posture, then the increase in spine stability will fol-
low by the virtue of abdominal muscle coactivation. If the
belt passively stiffens the trunk in an upright posture, it is
likely that this effect will be even more pronounced when
the spine moves away from the neutral posture, or when a
wider and stiffer belt is used. However, the spine is most
vulnerable to loss of stability when it is in a neutral pos-
ture [5, 6]. While only the trunk-unloading mode was stud-
ied,
it was used to estimate the instantaneous trunk stiff-
ness and consequently the stability of the lumbar spine in
the state in which it was at the time of load release. If the
spine becomes more stable, it will exhibit greater resis-
tance to perturbation, irrespective of whether this pertur-
bation occurs in a spine loading or unloading mode. Fur-
thermore, the quick release tasks, used in this study to
es
timate spine stability, may simulate spine unloading
events
occurring during sudden slips and trips that often
result in an accidental low back injury while handling
loads [40].
The findings of the present study are relevant to the
design of low back injury prevention and rehabilitation
strategies. Increased spine stability may provide greater
protection against injury following unexpected or sudden
loading. Therefore, the increased IAP and/or muscle coac-
tivation observed in low back pain patients and prescrip-
tion of abdominal belts may serve to compensate the ini-
tial injury to the spine and to restore or increase its stabil-
ity [54, 55]. Lumbar supports and orthotics are among the
commonly prescribed modalities for prevention and treat-
ment of low back pain. There exists a concern, however,
that a long-term usage of lumbar supports may lead to
trunk muscle weakness [15] and increased risk of injury
when the wearing of lumbar supports is discontinued [57].
In some specific cases, abdominal belts may be beneficial
in helping injured workers return earlier to work [69]. Im-
proved understanding of the mechanism by which IAP
and abdominal belts increase lumbar spine stability will
help to define better both the target population and the
length of time for the treatment with lumbar supports to
be the most effective.
The variability among the individual trunk stiffness
values along with variability in muscle activation patterns
suggests some interaction among active spine stabilizing
strategies. Cholewicki et al. [7] identified two possible
mechanisms by which trunk muscles can stabilize the
lumbar spine. One was antagonistic muscle coactivation
and the second was activation of only the abdominal mus-
culature and generation of IAP. Using a physical model,
they demonstrated that both of these mechanisms might
function separately or in combination, leading to different
critical load (stability) values. To verify this hypothesis,
calculations of spine stability should be performed with a
detailed mathematical model [5] using the EMG values
obtained in this study as input. If the muscle activation
patterns alone can predict the spine stability, then the IAP
and abdominal belt mechanisms for stabilizing the lumbar
spine hypothesized here and by Cholewicki et al. [7] will
be supported. Future studies may also provide an explana-
tion for the lack of a more pronounced increase in spine
stability due to the IAP and belt in trunk extension.
Conclusions
1. Both wearing an abdominal belt and raising IAP can
each independently, or in combination, increase lum-
bar spine stability.
2. Increase in spine stability due to high IAP is likely
gained from the concomitant increase in muscle coac-
tivation needed to generate this IAP. In contrast, the
stabilizing effect of the belt alone appears to be a pas-
sive mechanism.
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