Approximately 30% of individuals in the United States

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1 [ research report ] DUSTIN R. GROOMS, MEd, ATC, CSCS 1 TERRY L. GRINDSTAFF, PT, PhD, ATC, SCS, CSCS 2 THEODORE CROY, PT, PhD 3 JOSEPH M. HART, PhD, ATC 4 SUSAN A. SALIBA, PT, PhD, ATC, FNATA 4 Journal of Orthopaedic & Sports Physical Therapy Clinimetric Analysis of Pressure Biofeedback and Transversus Abdominis Function in Individuals With Stabilization Classification Low Back Pain 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 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, ; unable to maintain pressure, ) 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 (NCT ). J Orthop Sports Phys Ther 2013;43(3): Epub 16 November doi: /jospt TTKEY WORDS: lumbar stabilization, sonography, trunk control is thought to selectively activate the transversus abdominis (TrA) muscle 19,34,49,55,66-68 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 spine 23,35,42,51,52,57 via improved motor control 38,40-42 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 1 School of Health and Rehabilitation Sciences, The Ohio State University, Columbus, OH. 2 Department of Physical Therapy, Creighton University, Omaha, NE. 3 US Army Medical Specialist Corps; US Army-Baylor University Doctoral Program in Physical Therapy, Fort Sam Houston, TX. 4 Kinesiology 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 Grooms.69@osu.edu t Copyright 2013 Journal of Orthopaedic & Sports Physical Therapy 184 march 2013 volume 43 number 3 journal of orthopaedic & sports physical therapy

2 Journal of Orthopaedic & Sports Physical Therapy 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 al 39 found that only below 20% of the maximal voluntary contraction, the relationship between muscle thicknesses and EMG amplitude was linear. However, McMeeken et al 50 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 al 39 and McMeeken et al 50 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 palpation 11,55 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 reliability 11 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 This 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 journal of orthopaedic & sports physical therapy volume 43 number 3 march

3 [ research report ] Journal of Orthopaedic & Sports Physical Therapy TABLE 1 Able to Maintain Pressure (n = 38) 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 Group Demographics Unable to Maintain Pressure (n = 11) Total (n = 49) P Value Age, y* Height, cm* Mass, kg* Body mass index, kg/m 2 * Pain scale* Modified Oswestry* FABQ* Symptom duration, mo* 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 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. 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 [ICC 3,3 ]>0.98; 95% confidence interval [CI]: 0.90, 0.99) and excellent interrater reliability (ICC 2,3 >0.99; 95% CI: 0.96, 0.99). These reliability estimates have been previously reported 28,59 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 186 march 2013 volume 43 number 3 journal of orthopaedic & sports physical therapy

4 Journal of Orthopaedic & Sports Physical Therapy 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 al 37 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 extremities 37 ). 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 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 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, Yes Yes No Included Included 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. 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 journal of orthopaedic & sports physical therapy volume 43 number 3 march

5 [ research report ] Unable to maintain pressure Able to maintain pressure Pressure, mmhg 45 ADIM Rest ADIM 40 Journal of Orthopaedic & Sports Physical Therapy 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. 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 ,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). Clinically 55,61 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 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 al 1 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 al 20 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. Time, s 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. 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 ,67 Our sample size of 49 powered the ability to detect a correlation between pressure and TrA activation as low as 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 188 march 2013 volume 43 number 3 journal of orthopaedic & sports physical therapy

6 Journal of Orthopaedic & Sports Physical Therapy TABLE 2 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 There were no significant differences (P>.05) between any of the group demographics (TABLE 1). When Lateral Abdominal Muscle Thickness* Able to Maintain Pressure (n = 38) Unable to Maintain Pressure (n = 11) EO relaxed, cm EO contracted, cm IO relaxed, cm IO contracted, cm TrA relaxed, cm TrA contracted, cm TrA contraction ratio Abbreviations: EO, external oblique; IO, internal oblique; TrA, transversus abdominis. *Values are mean SD. Pressure Biofeedback, mmhg Able to maintain pressure Unable to maintain pressure True positive (n = 5) False positive (n = 6) False negative (n = 18) 1.0 True negative (n = 20) Low TrA activation TrA Activation Ratio FIGURE 5. Scatter plot of transversus abdominis activation and pressure. High TrA activation 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, mmhg; unable to maintain pressure, 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 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 The 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 al 39 and McMeeken et al 50 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 al 39 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 al 50 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 journal of orthopaedic & sports physical therapy volume 43 number 3 march

7 [ research report ] Journal of Orthopaedic & Sports Physical Therapy 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 activation 44 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 unit to detect high versus low TrA activation. 66 Performing assessments that are more challenging, such as the abdominal muscle test 26 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 ,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 and activation levels, and the majority of the literature reports levels above ,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 literature 12 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 190 march 2013 volume 43 number 3 journal of orthopaedic & sports physical therapy

8 Journal of Orthopaedic & Sports Physical Therapy 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 assessment 37 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 TrA 2,31 and not an objective cutoff, is a limitation for both screening and providing feedback on performance. CONCLUSION Our 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 KEY POINTS FINDINGS: The pressure biofeedback unit has poor diagnostic accuracy to detect TrA activation during an ADIM. 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