[
research report
]
DUSTIN R. GROOMS, MEd, ATC, CSCS1 • TERRY L. GRINDSTAFF, PT, PhD, ATC, SCS, CSCS2
THEODORE CROY, PT, PhD3 • JOSEPH M. HART, PhD, ATC4 • SUSAN A. SALIBA, PT, PhD, ATC, FNATA4
Clinimetric Analysis of Pressure
Biofeedback and Transversus Abdominis
Function in Individuals With Stabilization
Classification Low Back Pain
A
pproximately 30% of individuals in the United States
experience low back pain (LBP) over the course of a year, with
5% to 10% developing chronic symptoms.48 Individuals with
chronic LBP can benefit from exercise therapy interventions.21,29
A common initial exercise for individuals with chronic LBP is the
abdominal drawing-in maneuver (ADIM).30,33,34,37,42,52,56,66 This exercise
TTSTUDY DESIGN: Descriptive laboratory study.
TTOBJECTIVE: To determine if a proposed clinical
test (pressure biofeedback) could detect changes
in transversus abdominis (TrA) muscle thickness
during an abdominal drawing-in maneuver.
TTBACKGROUND: Pressure biofeedback may be
used to assess abdominal muscle function and
TrA activation during an abdominal drawing-in
maneuver but has not been validated.
TTMETHODS: Forty-nine individuals (18 men, 31
women) with low back pain who met stabilization
classification criteria underwent ultrasound imaging to quantify changes in TrA muscle thickness
while a pressure transducer was used to measure
pelvic and spine position during an abdominal
drawing-in maneuver. A paired t test was used
to compare differences in TrA activation ratios
between groups (able or unable to maintain pressure of 40  5 mmHg). The groups were further
dichotomized based on TrA activation ratio (high,
greater than 1.5; low, less than 1.5). Sensitivity,
specificity, and likelihood ratios were calculated.
TTRESULTS: There was not a significant difference
(P = .57) in TrA activation ratios (able to maintain
pressure, 1.59  0.28; unable to maintain pressure, 1.54  0.24) between groups. The pressure
biofeedback test had low sensitivity of 0.22 (95%
confidence interval [CI]: 0.10, 0.42) but moderate
specificity of 0.77 (95% CI: 0.58, 0.89), a positive
likelihood ratio of 0.94 (95% CI: 0.33, 2.68), and a
negative likelihood ratio of 1.02 (95% CI: 0.75, 1.38).
TTCONCLUSION: Successful completion on
pressure biofeedback does not indicate high TrA
activation. Unsuccessful completion on pressure
biofeedback may be more indicative of low TrA
activation, but the correlation and likelihood
coefficients indicate that the pressure test is
likely of minimal value to detect TrA activation.
This study was registered with ClinicalTrials.
gov (NCT01015846). J Orthop Sports Phys Ther
2013;43(3):184-193. Epub 16 November 2012.
doi:10.2519/jospt.2013.4397
TTKEY WORDS: lumbar stabilization, sonography,
trunk control
is thought to selectively activate the
transversus abdominis (TrA) muscle19,34,49,55,66-68 with minimal lumbar spine
motion.1,19,20,34,63 The TrA has been shown
to demonstrate decreased activation in
some individuals with LBP.2,22,25,30,42,44,54
The contraction of the TrA is thought to
contribute to dynamic stabilization of
the lumbar spine23,35,42,51,52,57 via improved
motor control38,40-42 and increasing intraabdominal pressure and tension of the
thoracolumbar fascia.10,13,14 Exercise interventions, which include TrA activation
training, have been shown to decrease
symptoms and improve self-reported
function in individuals with LBP.8,35,51,57,58
Symptom resolution is thought to be due
to improved ability to activate the TrA as
well as other local spinal stabilizers.55,57
Achieving competence with the ADIM
and restoring normal motor control
patterns of the TrA may be challenging.2,9,22,51,66 Determining correct performance of the ADIM often relies on
electromyography (EMG),40,43 ultrasound
imaging,31,66 muscle palpation,11,55 or pressure biofeedback.1,9,20,37,61,62,70 Surface
EMG of the abdominal musculature has
a number of limitations, which include
School of Health and Rehabilitation Sciences, The Ohio State University, Columbus, OH. 2Department of Physical Therapy, Creighton University, Omaha, NE. 3US Army Medical
Specialist Corps; US Army-Baylor University Doctoral Program in Physical Therapy, Fort Sam Houston, TX. 4Kinesiology Program, Department of Human Services, University of Virginia,
Charlottesville, VA. The protocol of this study was approved by the University of Virginia Human Subjects Institutional Review Board (number 14046). The opinions or assertions
contained herein are the private views of the authors and are not to be construed as official or as reflecting the views of the Department of the Army or the Department of Defense. The
authors certify that they have no affiliations with or financial involvement in any organization or entity with a direct financial interest in the subject matter or materials discussed in the
manuscript. Address correspondence to Dustin R. Grooms, The Ohio State University, School of Health and Rehabilitation Sciences, 453 West 10th Avenue, 228C Atwell Hall, Columbus,
OH 43210. E-mail: [email protected] t Copyright ©2013 Journal of Orthopaedic & Sports Physical Therapy
1
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cross-talk of other muscles in close proximity, such as the internal oblique (IO)
and external oblique (EO).3,17,46 Indwelling needle EMG only samples a small
portion of the muscle and has a risk of
infection and puncturing the abdominal
cavity.3
Ultrasound imaging has been utilized
as a noninvasive method to estimate
muscle activity via measures of muscle
thickness.6,7,22,31,34,39,44,50,53,57,59,65,66 A recent
systematic review concluded that ultrasound imaging can be used confidently
to measure muscle activation during low
levels of isometric contraction of the TrA,
such as during the ADIM.47 This conclusion was based on 3 investigations that
found a correlation between EMG activity and muscle thickness.22,39,50 Hodges
et al39 found that only below 20% of
the maximal voluntary contraction,
the relationship between muscle thicknesses and EMG amplitude was linear.
However, McMeeken et al50 examined a
larger sample size and reported that the
relationship was linear at all levels when
holding an isometric contraction of the
TrA. A limitation of both the Hodges et
al39 and McMeeken et al50 investigations
is that they utilized a cohort of healthy
individuals. Only 1 study has examined
muscle thickness and EMG amplitude in
individuals with LBP.22 These investigators found agreement between EMG amplitude and ultrasound data but did not
complete a formal analysis.22 Regardless,
ultrasound imaging has been utilized in a
number of studies investigating abdominal muscle thickness change as an indicator of muscle activation between groups
of individuals with lumbar spine pathology,2,4,15,27,28,30,44,54,59,66 healthy individuals,4,6,7,31,36,44,54,64,67 and as an intervention
outcome.25,28,31,59 Activation of the TrA
may be quantified by examining the ratio
of muscle thickness during contraction to
muscle thickness at rest, which is known
as the TrA activation ratio (TrA contracted thickness/TrA relaxed thickness).39,66
Unfortunately, the cost and technical
proficiency associated with EMG and
ultrasound imaging techniques present
FIGURE 1. The pressure biofeedback unit.
barriers to widespread use in clinical
environments.
Alternative options to assess ADIM
performance include muscle palpation11,55
and pressure biofeedback.1,9,11,20,37,61,62,70
The ability to assess ADIM performance
via muscle palpation has been shown to
have moderate reliability11 but is dependent on the skill of the examiner, and its
validity has not been established. It is not
possible to directly palpate the TrA, limiting tactile feedback through the IO muscle.2,11,31 A pressure biofeedback unit, such
as the Stabilizer (Chattanooga Group,
Hixson, TN) (FIGURE 1), may provide a
cost-effective, readily available clinical
tool to objectively assess abdominal muscle function, including TrA activation,
during an ADIM.55,61 The device consists
of a pressure bladder that is placed between the lumbar spine and table, on
which the patient is in a supine position,
and a pressure gauge, similar to a sphygmomanometer, that detects movement of
the lumbar spine via changes in pressure
applied to the air-filled bladder.61
In theory, correct performance of an
ADIM will not result in pressure fluctuations, thus indicating a stable lumbar
spine. Individuals who are not able to appropriately utilize the TrA must rely on
superficial muscles, such as the rectus
abdominis and EO, to maintain spine stability, which is indicated by an increase in
pressure due to flattening of the lumbar
spine against the pressure bladder.1,9,37,61
Thus, the pressure fluctuations are used
to indirectly determine TrA muscle activity.1,9,18,37 However, it is unknown whether
this clinical test is truly associated with
an ability to detect an isolated TrA contraction during the ADIM.18
Previous studies have assessed the
ability of the pressure biofeedback unit to
measure abdominal muscle function with
the patient in a prone position.9,11,37,62 But
the test is also utilized in the supine position,1,20,55,61 with no published research
regarding its validity to assess TrA activation during the ADIM. A recent systematic review on the properties of the
pressure biofeedback unit concluded that
the most important question regarding
the unit’s use in clinical practice has yet
to be answered, that is, its ability to assess
the activity of the TrA.18 Therefore, the
purpose of this study was to determine if
a proposed clinical test (pressure biofeedback) could detect changes in TrA muscle
thickness during an ADIM. The supine
position allowed ultrasound imaging collection to be completed simultaneously
with the pressure biofeedback test. A
secondary purpose was to describe the diagnostic accuracy of the pressure biofeedback test utilizing sensitivity, specificity,
and likelihood ratios compared to the ultrasound measurement of TrA activation.
METHODS
Design
T
his investigation was a descriptive laboratory study of clinical tools
used to evaluate TrA activation during an ADIM. The dependent variable
was the TrA activation ratio, recorded simultaneously with ultrasound imaging.
The independent variable was the ability to maintain pressure within 5 mmHg
above and below 40 mmHg, measured
using a pressure biofeedback unit.1,20,61
Subjects
Forty-nine subjects 18 to 50 years of age
(18 men, 31 women) were recruited from
a university campus and the surrounding community to participate in this
study (TABLE 1). Subjects, not all of whom
were currently seeking medical care for
their current episode of LBP, responded
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[
TABLE 1
research report
Group Demographics
Able to Maintain
Pressure (n = 38)
Unable to Maintain
Pressure (n = 11)
Total (n = 49)
P Value
Age, y*
23.0  6.7
23.4  3.3
23.1  6.0
.87
Height, cm*
173.9  11.1
172.5  8.8
173.6  10.5
.70
Mass, kg*
75.7  16.1
71.4  6.1
74.7  14.5
.40
Body mass index, kg/m2*
24.8  3.1
24.0  1.5
24.6  2.8
.44
4.1  1.6
4.0  2.7
4.1  1.9
.90
Modified Oswestry*
12.8  5.3
13.0  3.7
12.9  5.0
.97
FABQ*
14.6  5.4
16.2  5.7
14.9  5.8
.40
Symptom duration, mo*
40.1  43.4
35.2  32.6
39.0  41.0
.73
Pain with prolonged sitting†§
32 (84%)
10 (90%)
42 (86%)
.61
Pain with prolonged standing†§
26 (68%)
9 (82%)
35 (71%)
.35
Pain with walking†
13 (34%)
1 (9%)
14 (29%)
.09
Pain with running†
18 (47%)
3 (27%)
21 (43%)
.29
Pain in the morning (rising)†
21 (55%)
8 (73%)
29 (59%)
.23
Pain in the evening (lying)†
19 (50%)
7 (64%)
26 (53%)
.34
Pain scale*
Abbreviation: FABQ, Fear-Avoidance Beliefs Questionnaire.
*Values are mean  SD.
†
Values are frequency (%).
‡
Prolonged sitting and standing were participant-defined, without a specific frame established. But
examples were provided, such as working at a desk, watching a movie, or standing in line.
to research advertisements that briefly
outlined some of the inclusion criteria.
The inclusion criteria consisted of physical examination and history findings
consistent with stabilization classification LBP or the best fit for stabilization
(FIGURE 2).16,24,32 Because the stabilization classification criteria have not been
validated, other evidence-based clinical
examination (best-fit) criteria were used
to broaden the selection of individuals
who may benefit from stabilization exercise, and have been described in detail
in other manuscripts.16,24 Forty-seven individuals met stabilization classification
criteria, whereas only 2 subjects qualified after meeting best-fit criteria. Exclusion criteria were significant neurological
symptoms distal to the hip, spinal surgery,
pregnancy, known cancer, or tumor. Subjects responded to advertisements and
were prescreened over the phone for potential inclusion conflicts. This study was
approved by the University of Virginia
Institutional Review Board, and informed
consent was obtained from each subject.
After obtaining informed consent, all
participants completed a health history
form, the modified Oswestry Low Back
Pain Disability Questionnaire to assess
how LBP affects activities of daily living,
and the Fear-Avoidance Beliefs Questionnaire to assess the influence of psychological factors, such as fear and avoidance, on
individuals with LBP, and underwent a
physical examination consisting of screening for the inclusion and exclusion criteria
to determine study eligibility.
Reliability
Prior to enrolling subjects, 2 examiners
underwent a 15-hour supervised ultrasound imaging training program provided by an experienced physical therapist,
who had 10 years of experience in direct
patient care, had undergone advanced
training in rehabilitative ultrasound imaging 3 years prior to this study, and was
board certified in orthopaedic physical
therapy. After training, a pilot study that
consisted of 10 participants was used to
establish intrarater and interrater reliability for the ultrasound imaging measurement technique. Two images of the
]
lateral abdominals were taken, 1 during
rest and 1 during a single ADIM contraction. This sequence was repeated 3 times,
for a total of 6 images per rater. TrA muscle thickness measurements (rest and
contracted) had excellent intrarater reliability (intraclass correlation coefficient
[ICC3,3]>0.98; 95% confidence interval
[CI]: 0.90, 0.99) and excellent interrater reliability (ICC2,3>0.99; 95% CI: 0.96,
0.99). These reliability estimates have
been previously reported28,59 and were
consistent with previous reports of this
measurement technique.36,64,66,67
Instruments
Ultrasound Exam Ultrasound images
were obtained with the LOGIQ Book XP
(GE Healthcare, Waukesha, WI), with
an 8-MHz linear transducer. The images
were captured on the involved side of
pain and on the right side in the event of
central or bilateral pain. The TrA was visualized in the supine hook-lying position
(subject lying supine, with feet flat on
table and hips flexed to visually approximated 45° and knees to 90°), with the
ultrasound transducer placed along the
lateral abdominal wall on the midaxillary
line superior to the iliac crest.36,44,55,66,67
The image-capturing process of the anterolateral abdominal wall was standardized to begin along the midaxillary line
midway between the iliac crest and the
inferior angle of the rib cage.36,44,55,65-67
The transducer was oriented transversely
perpendicular to the abdominal musculature, with a slight tilt toward the pubis, to
align with the fibers of the TrA.36,65,66 The
transducer position was then adjusted to
optimize the image, so that the lateral insertion of the TrA on the thoracolumbar
fascia was approximately 2 cm from the
edge of the image.22,30,36
Pressure Biofeedback Unit Pressure biofeedback was tested with the Stabilizer
(Chattanooga Group) attached in line to a
pressure transducer (TSD104A; BIOPAC
Systems, Inc, Goleta, CA), interfaced with
an MP150 data acquisition and analysis
system (BIOPAC Systems, Inc). Data
were sampled at 200 Hz. The pressure
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biofeedback unit detected movement of
the lumbar spine through an air-filled
reservoir. In the supine testing position,
the unit was placed under the lumbar
spine and inflated to 40 mmHg.
Testing Procedures
Subjects were positioned in a supine
hook-lying position for all measurements
and practice trials. Subjects were given
standard instructions (“Take a breath in
and, as you exhale, gently draw your navel in toward your spine”) to perform the
ADIM.34,47,67 These instructions have been
used previously and are common in the
assessment of TrA contractibility. 34,47,67
The cue to draw in “gently” is used to limit overaggressive force generation or any
need to modify pelvic or spinal position,
as the TrA contraction does not require
pelvic or spinal position change, which
would produce compensatory muscle-activation patterns of the rectus abdominis,
IO, EO, or erector spinae musculature,
indicating possible loss of TrA motor control.34,38,55 Although other, more standardized methods to quantify TrA volitional
contractibility exist, they require the use
of dynamometers, extremity movements,
and EMG technology that make simultaneous data collection during the supine
ADIM with the pressure unit difficult.37
The use of ultrasound allows direct simultaneous data collection along with
the pressure unit. The more sophisticated
methods described by Hodges et al37 to
assess TrA activation cannot be done simultaneously with the clinical measure,
because they are 2 very different motor
control tasks (supine isometric contraction versus isometric flexion/extension of
the extremities37).
In this study, subjects were asked to
maintain the ADIM contraction for 10
seconds. Participants were allowed 5
practice repetitions with verbal and tactile feedback from the examiner to correct errors such as breath holding, pelvic
tilt, or bracing.44,67 The subjects were not
allowed to see the pressure gauge or the
ultrasound images at any time. Following 5 practice trials, subjects rested for
Do they meet at least 3 of the
following?
• Average SLR >90°
• Positive prone instability test
• Positive aberrant movements
• Age <40 y
Yes
Included
Yes
Included
No
Excluded
No
Do they meet at least 6 of the
following?
• Younger age
• Aberrant motions present
• Greater SLR ROM
• Hypermobility with spring
testing
• Increasing episode frequency
• 3 or more prior episodes
• Positive posterior pelvic pain
provocation test
• ASLR
• Modified Trendelenburg
• Pain with palpation of long
dorsal SI ligament or pubic
symphysis
FIGURE 2. Screening procedure. Younger age was defined as less than 50 years. Greater SLR ROM was defined as
greater than 70°. Abbreviations: ASLR, active straight leg raise; ROM, range of motion; SI, sacroiliac; SLR, straight
leg raise.
2 minutes. Next, the resting ultrasound
measures were obtained, followed by
contracted measures during an ADIM.
The image was captured between 2 and
4 seconds after the onset of the ADIM
contraction. This allowed the participant
to contract and exhale and the image to
stabilize sufficiently. Three images were
captured at rest and during contraction,
and all were recorded during normal
breathing at the end of expiration.
Data Processing
An investigator blinded to the pressure
readings measured all ultrasound images. After images were acquired, they
were exported to another computer, and
total lateral abdominal thickness, EO,
IO, and TrA were measured using ImageJ Version 1.41o (National Institutes of
Health, Bethesda, MD). Total lateral abdominal thickness was measured by the
distance between the superficial portion
of the deep fascial border of the TrA and
the deep portion of the superior fascial
border of the EO.66 The individual thicknesses of the EO, IO, and TrA were then
measured using the distance between the
superior edge of the deep hyperechoic
fascial line and the inferior edge of the
superior fascial line of each respective
muscle. Analysis was completed on the
average of 3 measures for each muscle
image, across 3 separate images. The
measures were all taken at the center of
the image, approximately 5.1 cm from the
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[
research report
]
Pressure, mmHg
Unable to maintain pressure
Able to maintain pressure
ADIM
Rest
ADIM
45
40
FIGURE 3. Patient position for the abdominal
drawing-in maneuver, demonstrating the clinical
use of the pressure biofeedback unit. For this
investigation, pressure was recorded digitally.
35
0
10
20
30
Time, s
muscular TrA insertion visualized at the
rightmost section of the screen. The TrA
activation ratio was calculated from the
TrA muscle thickness at rest to the contracted thickness (TrA contracted thickness/TrA relaxed thickness).39,66
Based on the TrA activation ratio, subjects were classified into groups (high and
low) delineated by a TrA activation ratio
of 1.5. We chose 1.5 because TrA activation values less than 1.5 are typical for
individuals with LBP who meet stabilization classification criteria,44 as well as
in nonspecific LBP.15,54,67 Values found in
healthy individuals are typically greater
than 1.5, ranging from 1.5 to greater than
2.0.4,15,27,44,67
An investigator blinded to the ultrasound image results measured pressure
readings. Pressure data were collected
continuously during each ADIM (10
seconds) and ultrasound imaging measurements. The mean pressure value was
calculated utilizing the average pressure
of the 2 seconds immediately prior to and
after (4-second window) image capture
(FIGURE 3). Clinically55,61 and in previous
reports,1,20 performance of the ADIM
with minimal changes in pressure (less
than 5 mmHg) has been classified as
“able to maintain pressure” and a successful ADIM, whereas performance of the
ADIM with a mean variance of greater
than 5 mmHg during the test epoch was
classified as “unable to maintain pressure” and an unsuccessful ADIM.1,20,55,61
Though there is a paucity of literature
FIGURE 4. Example of BIOPAC MP150 output to illustrate the pressure data. The mean pressure value was
calculated utilizing the average pressure of the 2 seconds immediately prior to and after image capture. The
image-capture points are indicated by the arrows, and the 2 seconds prior to and after by the blue lines around
the arrows in this example plot. The tick marks are at 2-second intervals. Above the first ADIM, the pressure
fluctuations are high and go above 45 mmHg for sufficient time for the average pressure across the 4 seconds
to be above the 5-mmHg gradient. In the second ADIM, the pressure remains between 45 mmHg and 40 mmHg.
Abbreviation: ADIM, abdominal drawing-in maneuver.
to support a cutoff of 5 mmHg as an “inability to maintain pressure” during the
supine ADIM, this was chosen because
it is considered a clinical cutoff point for
failure to maintain spinal neutral. Allison
et al1 showed that changing spinal position to increase pressure above 5 mmHg
induced compensations and increased
activation from the rectus abdominis and
EO, and Drysdale et al20 established that
the ADIM with a maintained pressure of
40 mmHg had low (10% of maximum
voluntary contraction or less) rectus abdominis and EO activation. Thus, it has
been suggested that when the ADIM is
performed correctly (with TrA activation
and not compensating musculature) the
pressure should not increase.1,20,60,70 However, current evidence does not directly
address the presumption that the absence
of detectable compensation strategies
via the pressure biofeedback unit guarantees that the TrA has been activated as
expected.18
Unlike the clinical test, we chose to
digitize the output (FIGURE 4) for better
evaluation of the pressure changes and to
accurately determine any change in pressure during the course of the test.
Statistical Analysis
A sample-size estimate of 48 participants
was calculated as being necessary to detect a 0.3 difference in TrA contraction ratio with a standard deviation of 0.36.28,67
Our sample size of 49 powered the ability
to detect a correlation between pressure
and TrA activation as low as 0.38.
Measures of abdominal muscle thickness (EO, IO, and TrA) at rest and during the ADIM are reported descriptively
(TABLE 2). An independent t test was
utilized to compare differences in demographics and TrA activation ratio
between groups, based on performance
during the pressure biofeedback test
(ability to maintain pressure, inability to
maintain pressure). Spearman and Pearson correlations were utilized to assess
the association between TrA activation
and changes in pressure. A receiver-operating-characteristic (ROC) curve based
on the collected data was also created. All
statistical analyses were performed with
SPSS Version 18 (SPSS Inc, Chicago, IL).
Probability for statistical significance was
determined a priori at P<.05. Additionally, a 2-by-2 contingency table (FIGURE
5) was created based on dichotomized
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TABLE 2
Lateral Abdominal Muscle Thickness*
Able to Maintain Pressure (n = 38)
Unable to Maintain Pressure (n = 11)
EO relaxed, cm
0.69  0.27
0.63  0.18
EO contracted, cm
0.71  0.26
0.70  0.21
IO relaxed, cm
1.05  0.33
1.02  0.17
IO contracted, cm
1.18  0.39
1.21  0.23
TrA relaxed, cm
0.40  0.11
0.39  0.05
TrA contracted, cm
0.62  0.13
0.60  0.11
TrA contraction ratio
1.59  0.28
1.54  0.24
Abbreviations: EO, external oblique; IO, internal oblique; TrA, transversus abdominis.
*Values are mean  SD.
0.89), with low positive (0.46; 95% CI:
0.25, 0.82) and negative (0.53; 95% CI:
0.36, 0.69) predictive values when attempting to determine TrA activation
utilizing pressure biofeedback. The positive likelihood ratio was 0.94 (95% CI:
0.33, 2.68), and the negative likelihood
ratio was 1.02 (95% CI: 0.75, 1.38). No
correlation was found between pressure
and TrA activation (Pearson r = –0.09, P
= .5; 95% CI: –0.36, 0.18; Spearman r =
0.07, P = .58; 95% CI: –0.20, 0.35). The
area under the ROC curve was 0.41 (95%
CI: 0.25, 0.58).
DISCUSSION
Unable to maintain pressure
True positive (n = 5)
False positive (n = 6)
False negative (n = 18)
True negative (n = 20)
T
60
55
50
45
Able to maintain pressure
Pressure Biofeedback, mmHg
65
40
35
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
High TrA activation
Low TrA activation
TrA Activation Ratio
FIGURE 5. Scatter plot of transversus abdominis activation and pressure.
pressure biofeedback exam results (unable or able to maintain pressure) and
ultrasound imaging TrA activation ratio
(high, greater than 1.5; low, less than 1.5).
Sensitivity, specificity, positive and negative predictive values, and likelihood ratios were also calculated.
RESULTS
T
here were no significant differences (P>.05) between any of the
group demographics (TABLE 1). When
participants were dichotomized based on
performance during the pressure biofeedback test, there was no significant difference (P = .57) in TrA activation ratios
(able to maintain pressure, 1.59  0.28
mmHg; unable to maintain pressure, 1.54
 0.24 mmHg). Forty-seven percent of
the individuals in this study with LBP
had TrA activation ratio values that were
considered suboptimal (below 1.5).44
The pressure biofeedback test had low
sensitivity (0.22; 95% CI: 0.10, 0.42) and
moderate specificity (0.77; 95% CI: 0.58,
he use of ultrasound as a criterion for muscle activation in this
investigation is considered an accurate assessment of muscle function based
on its correlation with EMG amplitude
and use in previous investigations. The
ADIM instructions were to encourage
a low-level isometric contraction of the
TrA without engaging other musculature.
Both Hodges et al39 and McMeeken et al50
independently found a direct correlation
between ultrasound and EMG output
during a low-intensity TrA contraction
(below 20%), and the latter investigation to higher activation levels. Hodges
et al39 normalized to a 50% contraction
level, but the pressure unit is utilized
to differentiate between contracted and
noncontracted states, as opposed to
discriminating between percentages of
contraction. The McMeeken et al50 investigation more closely aligns with our assessment criteria of resting-to-contracted
measures, patient position, and task instruction, and had a high correlation between EMG amplitude and ultrasound.
This study challenges the notion that
pressure biofeedback can clinically assess
TrA contraction during the ADIM.1,18,20,61
If an individual is able to complete the
ADIM and maintain pressure within the
acceptable clinical range (40  5 mmHg),
he or she is considered to have completed
an adequate ADIM and successfully contracted the TrA.1,20,36,55,61 Conversely, the
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[
inability to maintain pressure indicates
abdominal muscle dysfunction. Based
on the findings of this study, regardless of TrA activation level, the majority
of individuals (78%) with stabilization
classification LBP were able to maintain a pressure within clinically acceptable ranges (40  5 mmHg). However,
the ability to maintain pressure did not
indicate that an ADIM was performed
with high TrA activation when measured
by ultrasound imaging. The lack of consistency among the measures is demonstrated by the low sensitivity (0.22),
meaning that if an individual was able to
maintain adequate pressure, it was not
indicative of TrA activation. It is possible
that substitution or compensations may
be used to maintain pressure, regardless
of the function of the TrA. Conversely, individuals who were not able to maintain
adequate pressure during the ADIM were
more likely (specificity, 0.77) to have low
TrA activation, possibly identifying individuals with poor local muscle activation.
However, this finding must be interpreted cautiously, as the positive (0.94) and
negative (1.02) likelihood ratios were
both close to 1.0, indicating that the use
of pressure biofeedback to determine TrA
activation may be of little clinical value
(little change in posttest probability).
Pressure biofeedback can be utilized
during rehabilitation to give the patient visual real-time feedback to facilitate a neutral spine position during exercise.45,55,61
The device may be valuable when used
in this manner.1,9,20,26,55,61 However, as an
assessment tool, the ability to maintain
pressure during the ADIM does not imply
good function of the TrA. Conversely, if an
individual is unable to maintain pressure,
the clinician should consider ruling in TrA
dysfunction and the inclusion of interventions to address it.
The finding that those who meet the
stabilization classification have decreased
TrA activation44 was only true in 47% of
the participants in this study. Maintaining a neutral spine position during the
supine ADIM might have been too easy
an activity for the pressure biofeedback
research report
unit to detect high versus low TrA activation.66 Performing assessments that are
more challenging, such as the abdominal muscle test26 or prone ADIM,37 may
be required to demonstrate differences
in pressure biofeedback findings and
TrA activation, because deficits associated with functional lumbar instability
are more likely to occur during dynamic
activities.5 Individuals in this study also
had a mild level of self-reported disability (30%) on the Oswestry Low Back Pain
Disability Questionnaire and low levels of fear-avoidance beliefs (14 points).
The results of this study indicate that,
despite low levels of disability and fearavoidance beliefs, these individuals still
demonstrated decreased TrA activation,
potentially contributing to the high recurrence rate of LBP in this population
(stabilization classification).24,32
Selecting a TrA activation cutoff for
the ability to contract the TrA effectively
during the ADIM was difficult, due to the
variance in reported values.4,27,36,44,54,66,67
One study reported that LBP patients
could achieve a TrA activation of above
2.0 with minimal training.66 Others
have found TrA activations of healthy
individuals to be around 1.4.36,54 This
low activation is likely due to the differences in measurement technique, image
analysis, and familiarization trials and
feedback. Recently, it has been reported
that healthy participants can perform
the ADIM with 1.7367 and 2.044 activation levels, and the majority of the literature reports levels above 1.5.4,15,27,44,67
Our study methods for practice trials
and ADIM performance closely resemble the methods reporting 1.73 and 2.0
activation levels for healthy individuals.
This discrepancy is possibly due to the
variability of methods and study designs
used in the ultrasound literature12 and
to the heterogeneity of any sampling
of individuals with LBP.24,44 It has been
suggested that the TrA may function differently in subgroups of patients with
LBP,44 hence the range of reported values. We selected 1.5 to allow for some of
this discrepancy and to be consistent with
]
the data that have been reported for the
stabilization subgroup of LBP.44 The 1.5
threshold would include individuals who
were capable of visible thickening of the
TrA after a consistent practice and training trial. But the score was lower than
typically reported for healthy individuals
and thus could potentially identify the
pathologic population. The range of TrA
activation values found in our study does
strengthen the finding that the pressure
test has limited ability to detect activation or lack of activation across a wide
range of performance.
Our inability to detect a correlation
between the pressure readings and TrA
activation ratio was likely the result of
the dichotomous data generated from
the pressure biofeedback test. The pressure biofeedback unit in supine testing is
thought to simply differentiate between
those who can and cannot activate their
TrA, and not to give quantifiable measures of how much they are activating
their TrA (not a linear relationship).
Because maintenance of pressure while
completing the ADIM is thought to
equate with TrA activation and maintaining a neutral spine position, any increase
or decrease in pressure during the ADIM
would signify a failed or poor ability to
hold a TrA contraction or a compensatory
activity of the global muscles.1,18,20,61
A ROC curve was created to determine if a more appropriate cutoff value
for TrA activation should be utilized for
this population of LBP individuals. The
area under the ROC curve was 0.41, indicating that there was no value to discriminate between TrA activation states
during the ADIM with that analysis. In
our study, only a few individuals were unable to successfully perform the pressure
biofeedback test, despite being an LBP
sample; the low pain and Oswestry scores
might have been a limiting factor.
A limitation of this study was that we
utilized a group of individuals with LBP
who met specific inclusion criteria. As
previously stated, these individuals were
not necessarily seeking care for their current episode of LBP but did have a mild
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level of self-reported disability (30%)
on the Oswestry Low Back Pain Disability Questionnaire and low levels of
fear-avoidance beliefs (14 points). Future
studies may consider utilizing a population with greater levels of self-reported
disability or a comparison group, such
as individuals with LBP who meet other
classification criteria.24 The pressure biofeedback test has been used on a broader
population of individuals with LBP,9,37,69
but has been utilized to detect motor control or muscle activation deficits,1,9,20,37,55,61
such as in those who meet stabilization
classification criteria.24,32 Thus, we attempted to validate the device in a select
population that would be most likely to
utilize the pressure biofeedback test.
Future Research Recommendations
Future research should examine the ability of a pressure biofeedback unit to assess TrA activation in other positions.
The pressure biofeedback assessment of
the ADIM may be better associated with
TrA activation in the prone than in the
supine position. The prone assessment
measures success based on displacement
of the abdomen, indicated by a decrease
in pressure, as opposed to maintaining
the lumbar spine in a static position.61
The EMG comparison with the prone
pressure biofeedback assessment37 was
based on timing of TrA activation and
not measured concurrently. Additional
assessment may be performed using the
abdominal muscle test with 4 levels of
progressive difficulty.26 The abdominal
muscle test has been shown to correlate
with increased muscle recruitment at
each level for the rectus abdominis, IO,
and EO.26 However, the TrA has not been
assessed during the abdominal muscle
test. It is possible that testing that presents a greater challenge to individuals
with LBP may allow for greater differentiation between groups (high and low TrA
activation).
Future research is also needed to establish normative data for the TrA activation ratio in patients with LBP. The
current clinical recommendation, relying
on visualized thickening of the TrA2,31 and
not an objective cutoff, is a limitation for
both screening and providing feedback
on performance.
5.
CONCLUSION
6.
O
7.
ur results question the clinical use of pressure biofeedback for
the purpose of assessing TrA activation during a supine ADIM. Identification of an inability to maintain pressure
during an ADIM may be of limited clinical value, but the ability to maintain pressure provides no information regarding
TrA activation. Clinicians and researchers who rely on pressure biofeedback to
assess TrA function during an ADIM
performed in supine should recognize
the limitations associated with pressure
biofeedback as an assessment tool for local muscle function. t
8.
9.
10.
11.
KEY POINTS
FINDINGS: The pressure biofeedback unit
has poor diagnostic accuracy to detect
TrA activation during an ADIM.
IMPLICATIONS: Clinicians should reconsider using the pressure biofeedback
unit as a proxy assessment for deep core
abdominal function.
CAUTION: These findings may not be generalized to individuals who do not meet
stabilization classification criteria for
LBP.
12.
13.
14.
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