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Approximately 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

is thought to selectively activate the transversus abdominis (TrA) mus-cle19,34,49,55,66-68 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 intra-abdominal pressure and tension of the thoracolumbar fascia.10,13,14 Exercise in-terventions, 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 challeng-ing.2,9,22,51,66 Determining correct per-formance of the ADIM often relies on electromyography (EMG),40,43 ultrasound imaging,31,66 muscle palpation,11,55 or pres-sure biofeedback.1,9,20,37,61,62,70 Surface EMG of the abdominal musculature has a number of limitations, which include

TT STUDY DESIGN: Descriptive laboratory study.

TT OBJECTIVE: To determine if a proposed clinical test (pressure biofeedback) could detect changes in transversus abdominis (TrA) muscle thickness during an abdominal drawing-in maneuver.

TT BACKGROUND: Pressure biofeedback may be used to assess abdominal muscle function and TrA activation during an abdominal drawing-in maneuver but has not been validated.

TT METHODS: Forty-nine individuals (18 men, 31 women) with low back pain who met stabilization classification criteria underwent ultrasound imag-ing 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 pres-sure 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.

TT RESULTS: There was not a significant difference (P = .57) in TrA activation ratios (able to maintain pressure, 1.59 0.28; unable to maintain pres-sure, 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).

TT CONCLUSION: 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

TT KEY WORDS: lumbar stabilization, sonography, trunk control

1School 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: Grooms.69@osu.edu T Copyright ©2013 Journal of Orthopaedic & Sports Physical Therapy

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

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cross-talk of other muscles in close prox-imity, such as the internal oblique (IO) and external oblique (EO).3,17,46 Indwell-ing 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 ultra-sound 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 conclu-sion was based on 3 investigations that found a correlation between EMG activ-ity and muscle thickness.22,39,50 Hodges et al39 found that only below 20% of the maximal voluntary contraction, the relationship between muscle thick-nesses 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 investiga-tors found agreement between EMG am-plitude and ultrasound data but did not complete a formal analysis.22 Regardless, ultrasound imaging has been utilized in a number of studies investigating abdomi-nal muscle thickness change as an indica-tor of muscle activation between groups of individuals with lumbar spine pathol-ogy,2,4,15,27,28,30,44,54,59,66 healthy individu-als,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 contract-ed thickness/TrA relaxed thickness).39,66 Unfortunately, the cost and technical proficiency associated with EMG and ultrasound imaging techniques present

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 depen-dent on the skill of the examiner, and its validity has not been established. It is not possible to directly palpate the TrA, limit-ing tactile feedback through the IO mus-cle.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 mus-cle function, including TrA activation, during an ADIM.55,61 The device consists of a pressure bladder that is placed be-tween the lumbar spine and table, on which the patient is in a supine position, and a pressure gauge, similar to a sphyg-momanometer, 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 fluc-tuations, thus indicating a stable lumbar spine. Individuals who are not able to ap-propriately utilize the TrA must rely on superficial muscles, such as the rectus abdominis and EO, to maintain spine sta-bility, 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 activ-ity.1,9,18,37 However, it is unknown whether

this clinical test is truly associated with an ability to detect an isolated TrA con-traction 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 po-sition,1,20,55,61 with no published research regarding its validity to assess TrA acti-vation during the ADIM. A recent sys-tematic 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 biofeed-back) could detect changes in TrA muscle thickness during an ADIM. The supine position allowed ultrasound imaging col-lection to be completed simultaneously with the pressure biofeedback test. A secondary purpose was to describe the di-agnostic accuracy of the pressure biofeed-back test utilizing sensitivity, specificity, and likelihood ratios compared to the ul-trasound measurement of TrA activation.

METHODS

Design

This investigation was a descrip-tive laboratory study of clinical tools used to evaluate TrA activation dur-

ing an ADIM. The dependent variable was the TrA activation ratio, recorded si-multaneously with ultrasound imaging. The independent variable was the abil-ity to maintain pressure within 5 mmHg above and below 40 mmHg, measured using a pressure biofeedback unit.1,20,61

SubjectsForty-nine subjects 18 to 50 years of age (18 men, 31 women) were recruited from a university campus and the surround-ing 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

FIGURE 1. The pressure biofeedback unit.

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to research advertisements that briefly outlined some of the inclusion criteria. The inclusion criteria consisted of physi-cal examination and history findings consistent with stabilization classifica-tion LBP or the best fit for stabilization (FIGURE 2).16,24,32 Because the stabiliza-tion 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 ex-ercise, and have been described in detail in other manuscripts.16,24 Forty-seven in-dividuals met stabilization classification criteria, whereas only 2 subjects quali-fied after meeting best-fit criteria. Exclu-sion criteria were significant neurological symptoms distal to the hip, spinal surgery, pregnancy, known cancer, or tumor. Sub-jects responded to advertisements and were prescreened over the phone for po-tential 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 Question-naire to assess the influence of psychologi-cal factors, such as fear and avoidance, on individuals with LBP, and underwent a physical examination consisting of screen-ing for the inclusion and exclusion criteria to determine study eligibility.

ReliabilityPrior to enrolling subjects, 2 examiners underwent a 15-hour supervised ultra-sound imaging training program provid-ed by an experienced physical therapist, who had 10 years of experience in direct patient care, had undergone advanced training in rehabilitative ultrasound im-aging 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 reli-ability for the ultrasound imaging mea-surement technique. Two images of the

lateral abdominals were taken, 1 during rest and 1 during a single ADIM contrac-tion. This sequence was repeated 3 times, for a total of 6 images per rater. TrA mus-cle thickness measurements (rest and contracted) had excellent intrarater reli-ability (intraclass correlation coefficient [ICC3,3]>0.98; 95% confidence interval [CI]: 0.90, 0.99) and excellent interra-ter 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

InstrumentsUltrasound 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 vi-sualized in the supine hook-lying position (subject lying supine, with feet flat on table and hips flexed to visually approxi-mated 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 an-terolateral abdominal wall was standard-ized 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 muscula-ture, 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 in-sertion of the TrA on the thoracolumbar fascia was approximately 2 cm from the edge of the image.22,30,36

Pressure Biofeedback Unit Pressure bio-feedback 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

TABLE 1 Group Demographics

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.

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

Pain scale* 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

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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 ProceduresSubjects 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 na-vel 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 lim-it 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-ac-tivation patterns of the rectus abdominis, IO, EO, or erector spinae musculature, indicating possible loss of TrA motor con-trol.34,38,55 Although other, more standard-ized methods to quantify TrA volitional contractibility exist, they require the use of dynamometers, extremity movements, and EMG technology that make simulta-neous data collection during the supine ADIM with the pressure unit difficult.37 The use of ultrasound allows direct si-multaneous data collection along with the pressure unit. The more sophisticated methods described by Hodges et al37 to assess TrA activation cannot be done si-multaneously with the clinical measure, because they are 2 very different motor control tasks (supine isometric contrac-tion 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 tac-tile feedback from the examiner to cor-rect 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. Follow-ing 5 practice trials, subjects rested for

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 ProcessingAn investigator blinded to the pressure readings measured all ultrasound im-ages. After images were acquired, they were exported to another computer, and total lateral abdominal thickness, EO,

IO, and TrA were measured using Im-ageJ Version 1.41o (National Institutes of Health, Bethesda, MD). Total lateral ab-dominal 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 thick-nesses 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

Do they meet at least 3 of the following?• Average SLR >90°• Positive prone instability test• Positive aberrant movements• Age <40 y

Yes

No

Included

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

Yes Included

No Excluded

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.

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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 con-tracted thickness (TrA contracted thick-ness/TrA relaxed thickness).39,66

Based on the TrA activation ratio, sub-jects were classified into groups (high and low) delineated by a TrA activation ratio of 1.5. We chose 1.5 because TrA activa-tion values less than 1.5 are typical for individuals with LBP who meet stabili-zation 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 ultra-sound image results measured pressure readings. Pressure data were collected continuously during each ADIM (10 seconds) and ultrasound imaging mea-surements. 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 success-ful 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 pres-sure” and an unsuccessful ADIM.1,20,55,61

Though there is a paucity of literature

to support a cutoff of 5 mmHg as an “in-ability 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 posi-tion 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 ab-dominis 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 How-ever, current evidence does not directly address the presumption that the absence of detectable compensation strategies via the pressure biofeedback unit guar-antees 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 pres-sure during the course of the test.

Statistical AnalysisA sample-size estimate of 48 participants was calculated as being necessary to de-tect a 0.3 difference in TrA contraction ra-tio 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 thick-ness (EO, IO, and TrA) at rest and dur-ing the ADIM are reported descriptively (TABLE 2). An independent t test was utilized to compare differences in de-mographics and TrA activation ratio between groups, based on performance during the pressure biofeedback test (ability to maintain pressure, inability to maintain pressure). Spearman and Pear-son correlations were utilized to assess the association between TrA activation and changes in pressure. A receiver-op-erating-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. Addition-ally, a 2-by-2 contingency table (FIGURE

5) was created based on dichotomized

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

40

45

Pres

sure

, mm

Hg

Time, s

ADIM Rest

Unable to maintain pressure Able to maintain pressure

ADIM

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.

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pressure biofeedback exam results (un-able 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 nega-tive predictive values, and likelihood ra-tios were also calculated.

RESULTS

There were no significant dif-ferences (P>.05) between any of the group demographics (TABLE 1). When

participants were dichotomized based on performance during the pressure biofeed-back test, there was no significant differ-ence (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,

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 at-tempting to determine TrA activation utilizing pressure biofeedback. The posi-tive 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

The use of ultrasound as a crite-rion for muscle activation in this investigation is considered an accu-

rate 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 investiga-tion 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 inves-tigation more closely aligns with our as-sessment criteria of resting-to-contracted measures, patient position, and task in-struction, and had a high correlation be-tween 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 con-tracted the TrA.1,20,36,55,61 Conversely, the

TABLE 2 Lateral Abdominal Muscle Thickness*

Abbreviations: EO, external oblique; IO, internal oblique; TrA, transversus abdominis.*Values are mean SD.

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

Pres

sure

Bio

feed

back

, mm

Hg

Able

to m

aint

ain

pres

sure

Unab

le to

mai

ntai

n pr

essu

re

35

1.0

True positive (n = 5) False positive (n = 6)

True negative (n = 20)False negative (n = 18)

1.2 1.4 1.6 1.8 2.0 2.2 2.4

40

45

50

55

60

65

Low TrA activation

TrA Activation RatioHigh TrA activation

FIGURE 5. Scatter plot of transversus abdominis activation and pressure.

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[ research report ]inability to maintain pressure indicates abdominal muscle dysfunction. Based on the findings of this study, regard-less of TrA activation level, the majority of individuals (78%) with stabilization classification LBP were able to main-tain a pressure within clinically accept-able 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 con-sistency among the measures is dem-onstrated 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, in-dividuals who were not able to maintain adequate pressure during the ADIM were more likely (specificity, 0.77) to have low TrA activation, possibly identifying indi-viduals with poor local muscle activation. However, this finding must be interpret-ed 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 vi-sual real-time feedback to facilitate a neu-tral 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 interven-tions 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. Maintain-ing a neutral spine position during the supine ADIM might have been too easy an activity for the pressure biofeedback

unit to detect high versus low TrA activa-tion.66 Performing assessments that are more challenging, such as the abdomi-nal muscle test26 or prone ADIM,37 may be required to demonstrate differences in pressure biofeedback findings and TrA activation, because deficits associ-ated 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 disabil-ity (30%) on the Oswestry Low Back Pain Disability Questionnaire and low lev-els of fear-avoidance beliefs (14 points). The results of this study indicate that, despite low levels of disability and fear-avoidance beliefs, these individuals still demonstrated decreased TrA activation, potentially contributing to the high re-currence 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 differ-ences 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 activa-tion levels, and the majority of the lit-erature reports levels above 1.5.4,15,27,44,67 Our study methods for practice trials and ADIM performance closely resem-ble 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 dif-ferently in subgroups of patients with LBP,44 hence the range of reported val-ues. 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 train-ing 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 activa-tion 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 pres-sure biofeedback unit in supine testing is thought to simply differentiate between those who can and cannot activate their TrA, and not to give quantifiable mea-sures 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 maintain-ing 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 deter-mine 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, in-dicating that there was no value to dis-criminate between TrA activation states during the ADIM with that analysis. In our study, only a few individuals were un-able 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 cur-rent episode of LBP but did have a mild

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level of self-reported disability (30%) on the Oswestry Low Back Pain Dis-ability Questionnaire and low levels of fear-avoidance beliefs (14 points). Future studies may consider utilizing a popula-tion with greater levels of self-reported disability or a comparison group, such as individuals with LBP who meet other classification criteria.24 The pressure bio-feedback test has been used on a broader population of individuals with LBP,9,37,69 but has been utilized to detect motor con-trol or muscle activation deficits,1,9,20,37,55,61 such as in those who meet stabilization classification criteria.24,32 Thus, we at-tempted to validate the device in a select population that would be most likely to utilize the pressure biofeedback test.

Future Research RecommendationsFuture research should examine the abil-ity of a pressure biofeedback unit to as-sess 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 pres-ents a greater challenge to individuals with LBP may allow for greater differen-tiation between groups (high and low TrA activation).

Future research is also needed to es-tablish normative data for the TrA ac-tivation 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.

CONCLUSION

Our results question the clini-cal use of pressure biofeedback for the purpose of assessing TrA acti-

vation during a supine ADIM. Identifica-tion of an inability to maintain pressure during an ADIM may be of limited clini-cal value, but the ability to maintain pres-sure provides no information regarding TrA activation. Clinicians and research-ers 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 lo-cal muscle function. t

KEY POINTSFINDINGS: The pressure biofeedback unit has poor diagnostic accuracy to detect TrA activation during an ADIM.IMPLICATIONS: Clinicians should recon-sider using the pressure biofeedback unit as a proxy assessment for deep core abdominal function.CAUTION: These findings may not be gen-eralized to individuals who do not meet stabilization classification criteria for LBP.

REFERENCES

1. Allison GT, Kendle K, Roll S, Schupelius J, Scott Q, Panizza J. The role of the diaphragm during abdominal hollowing exercises. Aust J Physio-ther. 1998;44:95-102.

2. Anderson Worth SG, Henry SM, Bunn JY. Real-time ultrasound feedback and abdominal hol-lowing exercises for people with low back pain. N Z J Physiother. 2007;35:4-11.

3. Basmajian JV, De Luca CJ. Muscles Alive: Their Functions Revealed by Electromyography. 5th ed. Baltimore, MD: Williams & Wilkins; 1985.

4. Beazell JR, Grindstaff TL, Hart JM, Magrum EM, Cullaty M, Shen FH. Changes in lateral abdominal muscle thickness during an abdomi-nal drawing-in maneuver in individuals with and without low back pain. Res Sports Med.

2011;19:271-282. http://dx.doi.org/10.1080/15438627.2011.608053

5. Beazell JR, Mullins M, Grindstaff TL. Lumbar instability: an evolving and challenging concept. J Man Manip Ther. 2010;18:9-14. http://dx.doi.org/10.1179/106698110X12595770849443

6. Bunce SM, Hough AD, Moore AP. Measurement of abdominal muscle thickness using M-mode ultrasound imaging during functional activities. Man Ther. 2004;9:41-44.

7. Bunce SM, Moore AP, Hough AD. M-mode ul-trasound: a reliable measure of transversus ab-dominis thickness? Clin Biomech (Bristol, Avon). 2002;17:315-317.

8. Cairns MC, Foster NE, Wright C. Randomized controlled trial of specific spinal stabilization exercises and conventional physiotherapy for recurrent low back pain. Spine (Phila Pa 1976). 2006;31:E670-E681. http://dx.doi.org/10.1097/01.brs.0000232787.71938.5d

9. Cairns MC, Harrison K, Wright C. Pressure biofeedback: a useful tool in the quantification of abdominal muscular dysfunction? Physio-therapy. 2000;86:127-138.

10. Cholewicki J, Juluru K, McGill SM. Intra-abdom-inal pressure mechanism for stabilizing the lumbar spine. J Biomech. 1999;32:13-17.

11. Costa LO, Da Cunha Menezes Costa L, Cançado RL, De Melo Oliveira W, Ferreira PH. Intra-tester reliability of two clinical tests of transversus ab-dominis muscle recruitment. Physiother Res Int. 2006;11:48-50. http://dx.doi.org/10.1002/pri.39

12. Costa LO, Maher CG, Latimer J, Smeets RJ. Reproducibility of rehabilitative ultrasound imaging for the measurement of abdominal muscle activity: a systematic review. Phys Ther. 2009;89:756-769. http://dx.doi.org/10.2522/ptj.20080331

13. Cresswell AG, Grundström H, Thorstensson A. Observations on intra-abdominal pressure and patterns of abdominal intra-muscular activity in man. Acta Physiol Scand. 1992;144:409-418. http://dx.doi.org/10.1111/j.1748-1716.1992.tb09314.x

14. Cresswell AG, Oddsson L, Thorstensson A. The influence of sudden perturbations on trunk muscle activity and intra-abdominal pressure while standing. Exp Brain Res. 1994;98:336-341. http://dx.doi.org/10.1007/BF00228421

15. Critchley DJ, Coutts FJ. Abdominal muscle function in chronic low back pain patients: mea-surement with real-time ultrasound scanning. Physiotherapy. 2002;88:322-332.

16. Delitto A, Erhard RE, Bowling RW. A treatment-based classification approach to low back syndrome: identifying and staging patients for conservative treatment. Phys Ther. 1995;75:470-485; discussion 485-489.

17. De Luca CJ. The use of surface electromy-ography in biomechanics. J Appl Biomech. 1997;13:135-163.

18. de Paula Lima PO, de Oliveira RR, Costa LO, Laurentino GE. Measurement properties of the pressure biofeedback unit in the evaluation of

43-03 Grooms.indd 191 2/20/2013 3:21:27 PM

192  |  march 2013  |  volume 43  |  number 3  |  journal of orthopaedic & sports physical therapy

[ research report ]transversus abdominis muscle activity: a sys-tematic review. Physiotherapy. 2011;97:100-106. http://dx.doi.org/10.1016/j.physio.2010.08.004

19. De Troyer A, Estenne M, Ninane V, Van Gansbeke D, Gorini M. Transversus abdominis muscle function in humans. J Appl Physiol. 1990;68:1010-1016.

20. Drysdale CL, Earl JE, Hertel J. Surface electro-myographic activity of the abdominal muscles during pelvic-tilt and abdominal-hollowing exer-cises. J Athl Train. 2004;39:32-36.

21. Ferreira ML, Smeets RJ, Kamper SJ, Ferreira PH, Machado LA. Can we explain heterogeneity among randomized clinical trials of exercise for chronic back pain? A meta-regression analysis of randomized controlled trials. Phys Ther. 2010;90:1383-1403. http://dx.doi.org/10.2522/ptj.20090332

22. Ferreira PH, Ferreira ML, Hodges PW. Changes in recruitment of the abdominal muscles in people with low back pain: ultrasound measure-ment of muscle activity. Spine (Phila Pa 1976). 2004;29:2560-2566.

23. Ferreira PH, Ferreira ML, Maher CG, Herbert RD, Refshauge K. Specific stabilisation exercise for spinal and pelvic pain: a systematic review. Aust J Physiother. 2006;52:79-88.

24. Fritz JM, Cleland JA, Childs JD. Subgrouping patients with low back pain: evolution of a clas-sification approach to physical therapy. J Orthop Sports Phys Ther. 2007;37:290-302. http://dx.doi.org/10.2519/jospt.2007.2498

25. Gill NW, Teyhen DS, Lee IE. Improved contrac-tion of the transversus abdominis immediately following spinal manipulation: a case study using real-time ultrasound imaging. Man Ther. 2007;12:280-285. http://dx.doi.org/10.1016/j.math.2006.06.014

26. Gilleard WL, Brown JM. An electromyographic validation of an abdominal muscle test. Arch Phys Med Rehabil. 1994;75:1002-1007.

27. Gorbet N, Selkow NM, Hart JM, Saliba S. No dif-ference in transverse abdominis activation ratio between healthy and asymptomatic low back pain patients during therapeutic exercise. Reha-bil Res Pract. 2010;2010:459738. http://dx.doi.org/10.1155/2010/459738

28. Guthrie RJ, Grindstaff TL, Croy T, Ingersoll CD, Saliba SA. The effect of traditional bridging or suspension-exercise bridging on lateral abdomi-nal thickness in individuals with low back pain. J Sport Rehabil. 2012;21:151-160.

29. Hayden JA, van Tulder MW, Malmivaara AV, Koes BW. Meta-analysis: exercise therapy for nonspecific low back pain. Ann Intern Med. 2005;142:765-775.

30. Henry SM, Teyhen DS. Ultrasound imaging as a feedback tool in the rehabilitation of trunk muscle dysfunction for people with low back pain. J Orthop Sports Phys Ther. 2007;37:627-634. http://dx.doi.org/10.2519/jospt.2007.2555

31. Henry SM, Westervelt KC. The use of real-time ultrasound feedback in teaching abdominal hol-lowing exercises to healthy subjects. J Orthop

Sports Phys Ther. 2005;35:338-345. http://dx.doi.org/10.2519/jospt.2005.1757

32. Hicks GE, Fritz JM, Delitto A, McGill SM. Prelimi-nary development of a clinical prediction rule for determining which patients with low back pain will respond to a stabilization exercise program. Arch Phys Med Rehabil. 2005;86:1753-1762. http://dx.doi.org/10.1016/j.apmr.2005.03.033

33. Hides J, Stanton W, Freke M, Wilson S, McMahon S, Richardson C. MRI study of the size, sym-metry and function of the trunk muscles among elite cricketers with and without low back pain. Br J Sports Med. 2008;42:809-813. http://dx.doi.org/10.1136/bjsm.2007.044024

34. Hides J, Wilson S, Stanton W, et al. An MRI investigation into the function of the trans-versus abdominis muscle during “drawing-in” of the abdominal wall. Spine (Phila Pa 1976). 2006;31:E175-E178. http://dx.doi.org/10.1097/01.brs.0000202740.86338.df

35. Hides JA, Jull GA, Richardson CA. Long-term effects of specific stabilizing exercises for first-episode low back pain. Spine (Phila Pa 1976). 2001;26:E243-E248.

36. Hides JA, Miokovic T, Belavy DL, Stanton WR, Richardson CA. Ultrasound imaging assessment of abdominal muscle function during drawing-in of the abdominal wall: an intrarater reliability study. J Orthop Sports Phys Ther. 2007;37:480-486. http://dx.doi.org/10.2519/jospt.2007.2416

37. Hodges P, Richardson C, Jull G. Evaluation of the relationship between laboratory and clini-cal tests of transversus abdominis function. Physiother Res Int. 1996;1:30-40. http://dx.doi.org/10.1002/pri.45

38. Hodges PW. Is there a role for transversus abdominis in lumbo-pelvic stability? Man Ther. 1999;4:74-86. http://dx.doi.org/10.1054/math.1999.0169

39. Hodges PW, Pengel LH, Herbert RD, Gandevia SC. Measurement of muscle contraction with ul-trasound imaging. Muscle Nerve. 2003;27:682-692. http://dx.doi.org/10.1002/mus.10375

40. Hodges PW, Richardson CA. Delayed postural contraction of transversus abdominis in low back pain associated with movement of the lower limb. J Spinal Disord. 1998;11:46-56.

41. Hodges PW, Richardson CA. Feedforward contraction of transversus abdominis is not in-fluenced by the direction of arm movement. Exp Brain Res. 1997;114:362-370.

42. Hodges PW, Richardson CA. Inefficient muscular stabilization of the lumbar spine associated with low back pain. A motor control evaluation of transversus abdominis. Spine (Phila Pa 1976). 1996;21:2640-2650.

43. Kavcic N, Grenier S, McGill SM. Determining the stabilizing role of individual torso muscles during rehabilitation exercises. Spine (Phila Pa 1976). 2004;29:1254-1265.

44. Kiesel KB, Underwood FB, Mattacola CG, Nitz AJ, Malone TR. A comparison of select trunk muscle thickness change between subjects with low back pain classified in the treatment-based

classification system and asymptomatic con-trols. J Orthop Sports Phys Ther. 2007;37:596-607. http://dx.doi.org/10.2519/jospt.2007.2574

45. Kisner C, Colby LA. Therapeutic Exercise: Foun-dations and Techniques. 5th ed. Philadelphia, PA: F.A. Davis; 2007.

46. Koh TJ, Grabiner MD. Evaluation of methods to minimize cross talk in surface electromyogra-phy. J Biomech. 1993;26 suppl 1:151-157.

47. Koppenhaver SL, Hebert JJ, Parent EC, Fritz JM. Rehabilitative ultrasound imaging is a valid measure of trunk muscle size and activation during most isometric sub-maximal contrac-tions: a systematic review. Aust J Physiother. 2009;55:153-169.

48. Manchikanti L, Singh V, Datta S, Cohen SP, Hirsch JA. Comprehensive review of epidemiol-ogy, scope, and impact of spinal pain. Pain Physician. 2009;12:E35-E70.

49. Mannion AF, Pulkovski N, Gubler D, et al. Muscle thickness changes during abdominal hollowing: an assessment of between-day measurement error in controls and patients with chronic low back pain. Eur Spine J. 2008;17:494-501. http://dx.doi.org/10.1007/s00586-008-0589-x

50. McMeeken JM, Beith ID, Newham DJ, Mil-ligan P, Critchley DJ. The relationship between EMG and change in thickness of transversus abdominis. Clin Biomech (Bristol, Avon). 2004;19:337-342. http://dx.doi.org/10.1016/j.clinbiomech.2004.01.007

51. O’Sullivan PB, Phyty GD, Twomey LT, Allison GT. Evaluation of specific stabilizing exercise in the treatment of chronic low back pain with radiologic diagnosis of spondylolysis or spondylolisthesis. Spine (Phila Pa 1976). 1997;22:2959-2967.

52. O’Sullivan PB, Twomey L, Allison GT. Altered abdominal muscle recruitment in patients with chronic back pain following a specific exer-cise intervention. J Orthop Sports Phys Ther. 1998;27:114-124.

53. Peschers UM, Gingelmaier A, Jundt K, Leib B, Dimpfl T. Evaluation of pelvic floor muscle strength using four different techniques. Int Uro-gynecol J Pelvic Floor Dysfunct. 2001;12:27-30. http://dx.doi.org/10.1007/s001920170090

54. Pulkovski N, Mannion AF, Caporaso F, et al. Ul-trasound assessment of transversus abdominis muscle contraction ratio during abdominal hollowing: a useful tool to distinguish between patients with chronic low back pain and healthy controls? Eur Spine J. 2012;21:S750-S759. http://dx.doi.org/10.1007/s00586-011-1707-8

55. Richardson C, Hodges P, Hides J. Therapeutic Exercise for Lumbopelvic Stabilization: A Motor Control Approach for the Treatment and Preven-tion of Low Back Pain. 2nd ed. Edinburgh, UK: Churchill Livingstone; 2004.

56. Richardson CA, Hides JA, Wilson S, Stanton W, Snijders CJ. Lumbo-pelvic joint protec-tion against antigravity forces: motor control and segmental stiffness assessed with mag-netic resonance imaging. J Gravit Physiol.

43-03 Grooms.indd 192 2/20/2013 3:21:28 PM

journal of orthopaedic & sports physical therapy | volume 43 | number 3 | march 2013 | 193

MORE INFORMATIONWWW.JOSPT.ORG@

2004;11:P119-P122. 57. Richardson CA, Snijders CJ, Hides JA, Damen

L, Pas MS, Storm J. The relation between the transversus abdominis muscles, sacroiliac joint mechanics, and low back pain. Spine (Phila Pa 1976). 2002;27:399-405.

58. Saal JA. Dynamic muscular stabilization in the nonoperative treatment of lumbar pain syn-dromes. Orthop Rev. 1990;19:691-700.

59. Saliba SA, Croy T, Guthrie R, Grooms D, Welt-man A, Grindstaff TL. Differences in transverse abdominis activation with stable and unstable bridging exercises in individuals with low back pain. N Am J Sports Phys Ther. 2010;5:63-73.

60. Souza GM, Baker LL, Powers CM. Electromyo-graphic activity of selected trunk muscles dur-ing dynamic spine stabilization exercises. Arch Phys Med Rehabil. 2001;82:1551-1557. http://dx.doi.org/10.1053/apmr.2001.26082

61. Stabilizer Pressure Biofeedback Unit Instruction Booklet. Hixson, TN: Chattanooga Group; 1995.

62. Storheim K, Bø K, Pederstad O, Jahnsen R. Intra-tester reproducibility of pressure biofeed-

back in measurement of transversus abdominis function. Physiother Res Int. 2002;7:239-249.

63. Strohl KP, Mead J, Banzett RB, Loring SH, Kosch PC. Regional differences in abdominal muscle activity during various maneuvers in humans. J Appl Physiol. 1981;51:1471-1476.

64. Teyhen DS, George SZ, Dugan JL, Williamson J, Neilson BD, Childs JD. Inter-rater reliability of ultrasound imaging of the trunk muscula-ture among novice raters. J Ultrasound Med. 2011;30:347-356.

65. Teyhen DS, Gill NW, Whittaker JL, Henry SM, Hides JA, Hodges P. Rehabilitative ultrasound imaging of the abdominal muscles. J Orthop Sports Phys Ther. 2007;37:450-466. http://dx.doi.org/10.2519/jospt.2007.2558

66. Teyhen DS, Miltenberger CE, Deiters HM, et al. The use of ultrasound imaging of the abdominal drawing-in maneuver in subjects with low back pain. J Orthop Sports Phys Ther. 2005;35:346-355. http://dx.doi.org/10.2519/jospt.2005.1780

67. Teyhen DS, Rieger JL, Westrick RB, Miller AC, Molloy JM, Childs JD. Changes in deep

abdominal muscle thickness during common trunk-strengthening exercises using ultra-sound imaging. J Orthop Sports Phys Ther. 2008;38:596-605. http://dx.doi.org/10.2519/jospt.2008.2897

68. Urquhart DM, Hodges PW, Allen TJ, Story IH. Ab-dominal muscle recruitment during a range of voluntary exercises. Man Ther. 2005;10:144-153. http://dx.doi.org/10.1016/j.math.2004.08.011

69. von Garnier K, Koveker K, Rackwitz B, et al. Reliability of a test measuring transversus abdominis muscle recruitment with a pressure biofeedback unit. Physiotherapy. 2009;95:8-14. http://dx.doi.org/10.1016/j.physio.2008.10.003

70. Wohlfahrt D, Jull GA, Richardson CA. The rela-tionship between the dynamic and static func-tion of abdominal muscles. Aust J Physiother. 1993;39:9-12.

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