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Changes in Transversus Abdominis Thickness With Use of the Abdominal Drawing-In Maneuver During a Functional Task
Original Research
Changes in Transversus Abdominis Thickness With Use of the Abdominal Drawing-In Maneuver During a Functional Task Michael K. McGalliard, ScD, Gregory S. Dedrick, ScD, Jean Michel Brismée, ScD, Chad E. Cook, PhD, Gail G. Apte, ScD, Phillip S. Sizer Jr., PhD Objective: The purpose of this study was to examine an individual’s ability to produce an increase in transversus abdominis (TrA) thickness during the performance of a functional task with the use of the abdominal drawing-in maneuver (ADIM). Design: Within-subject repeated measures analysis of variance was used to examine the effects of the ADIM and a loaded forward-reaching activity on the dependent variable of TrA thickness. Setting: Laboratory Participants: Convenience sample of 8 women and 8 men, asymptomatic, with a mean age of 27.6 ⫾ 7.1 years. Interventions: Ultrasound imaging measurements were recorded during 4 conditions: (1) while the patient was standing without the ADIM; (2) while the patient was standing with the ADIM; (3) during a loaded forward-reaching activity without the ADIM; and (4) during a loaded forward-reaching activity with the ADIM. Main Outcome Measures: Thickness of the TrA muscle. Results: The measurement obtained by an investigator blinded to the condition revealed statistically significant differences in the thickness of the TrA between all uncontracted conditions as compared with all contracted conditions. No statistically significant difference in the thickness of the TrA in the contracted states during quiet standing versus loaded forward reach was observed. Conclusion: Subjects in this study demonstrated the ability to voluntarily activate the TrA during upright static and functional tasks. Additionally, the TrA thickness may change in a direction-specific manner. These findings support a protective role of the ADIM during functional activity and may add information to ways for promoting low back pain prevention. Future studies should include the effectiveness in the use of ADIM during functional tasks for the prevention of low back pain. PM R 2010;2:187-194
INTRODUCTION With the ever-increasing societal and personal impact of low back pain, investigators have turned their focus to evaluating the different factors associated with the onset, perpetuation, and treatment of lumbar spine conditions. The high risk and financial costs accompanying medical and surgical management has led many to focus attention on nonsurgical approaches for the prevention of low back injury and pain. Attention has turned to the role of spinal stability in prevention and management of low back injury and pain [1-3]. The contributions of both the locomotor strength and motor control systems have been investigated in the quest for greater understanding of spinal stabilization. As such, controversy exists in the literature regarding the relative importance of each of these components for spinal stability [4]. The function of the transversus abdominis (TrA) in the stabilization of the spine is of particular interest because, in comparison with the other abdominal muscles, the TrA is unique in its capacity to promote segmental spinal stability [5,6] and possibly support the spine in a protective fashion [3,7,8]. Many investigations regarding the TrA have focused on PM&R 1934-1482/10/$36.00 Printed in U.S.A.
M.K.M. Physical Therapy Program, Department of Rehabilitation Sciences, School of Allied Health Sciences, Texas Tech University Health Science Center, 1400 Coulter Street, Amarillo, TX 79106. Address correspondence to M.K.M.; e-mail: [email protected] Disclosure: nothing to disclose G.S.D. Clinical Musculoskeletal Research Laboratory, Department of Rehabilitation Sciences, School of Allied Health Sciences, Texas Tech University Health Science Center, Amarillo, TX Disclosure: nothing to disclose J.M.B. Clinical Musculoskeletal Research Laboratory, Department of Rehabilitation Sciences, School of Allied Health Sciences, Texas Tech University Health Science Center, Amarillo, TX Disclosure: nothing to disclose C.E.C. The Center for Excellence in Surgical Outcomes, Duke University Medical Center, Durham, NC Disclosure: nothing to disclose G.G.A. Clinical Musculoskeletal Research Laboratory, Department of Rehabilitation Sciences, School of Allied Health Sciences, Texas Tech University Health Science Center, Amarillo, TX Disclosure: nothing to disclose P.S.S. Clinical Musculoskeletal Research Laboratory, Department of Rehabilitation Sciences, School of Allied Health Sciences, Texas Tech University Health Science Center, Amarillo, TX Disclosure: nothing to disclose Submitted for publication November 4, 2009; accepted January 26, 2010.
© 2010 by the American Academy of Physical Medicine and Rehabilitation Vol. 2, 187-194, March 2010 DOI: 10.1016/j.pmrj.2010.01.015
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motor control and the sequencing of muscle contractions during the stabilization of the spine [3,6,9-11]. Several trunk muscles function selectively in an anticipatory manner in response to selected postural perturbations. In this respect TrA is unique in its function during routine trunk control activity [6,10-12]. In subjects without low back pain, the TrA has been shown to function as an anticipatory/feedforward trunk stabilizer regardless of the direction of the perturbation [3,8,10,12]. Furthermore, the TrA is consistently the first muscle to contract in an anticipatory/feedforward manner when compared with all other abdominal muscles. Thus, the TrA is thought to serve as the primary, protective preparatory stabilizer of the lumbosacral spinal region. Previous investigators [7,11,12] have used needle electromyography to measure the activation of the TrA. However, this technique is invasive and uncomfortable for the patient. In response, investigators have established the validity of ultrasound (US) imaging as an instrument to detect muscle contractility by demonstrating that changes in the thickness of the TrA correlate to changes in muscle activity, especially with low-level isometric muscle contractions [13-15]. Various clinical strategies for protective “spinal stabilization” have been suggested. Despite the variation in the strategic approaches, most strategies incorporate some form of voluntary preparatory abdominal activation before, as well as during, the performance of a functional task. Some investigators [16,17] have favored studying the effects of such strategies that target the TrA and the multifidus muscles. The basis for this type of strategic lumbar stabilization is the preferential activation of the TrA via the voluntary drawing-in of the lower abdominal wall, or the abdominal drawing-in maneuver (ADIM) [18-22]. Although, the “ideal” ADIM involves attaining an isolated or preferential contraction of the TrA [19,21,22], controversy exists in the literature regarding the ability of an individual to produce an “ideal” ADIM [18,22,23]. This controversy represents a gap in available knowledge and brings into question the proposed mechanism underlying functional trunk stabilization. Although some investigators [24] have demonstrated that an upright functional position may induce changes in TrA activation, the ability to produce TrA activation with the ADIM during a functional task in the upright position has not been directly demonstrated. Therefore, the purpose of this study was to examine an individual’s ability to produce TrA activation during the performance of a functional task with use of the ADIM.
METHODS Design A within-subject repeated-measures design was used to examine the effects of the ADIM and a loaded forward-reaching activity on the dependent variable of TrA thickness.
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Subjects This study was approved by the Texas Tech University Health Sciences Center institutional review board. By the use of a sample of convenience, 17 subjects (8 men and 9 women) with no history of low back pain were recruited for this study. All subjects provided informed consent for their participation. The subjects were recruited from 2 sources: (1) students, faculty, and staff from the local university health science center; and (2) acquaintances of the investigators from the general public. One female subject was excluded from the study because multiple sclerosis was subsequently discovered. This condition has the potential to affect trunk control [25-27]. The remaining 16 subjects ranged in age from 21 to 49 years (mean ⫾ SD, 27.6 ⫾ 7.1 years). The subjects’ height ranged from 157.5 to 193 cm (174.8 ⫾ 9.4 cm), weight ranged from 56.8 to 93.1 kg (74.1 ⫾ 11 kg), and body mass index (BMI) ranged from 20.2 to 29 kg.m2 (24.1 ⫾ 2.5 kg.m2). The subjects were eligible for inclusion in the study if they were between 18 and 65 years of age. Subjects were excluded if they reported any of the following: (1) current spinal pain; (2) diagnosed and presently active abdominal, respiratory, or gastrointestinal condition; (3) pregnancy; (4) significant spinal deformity or condition; (5) known neurological or joint disease affecting the trunk; or (6) current urinary tract infection. Additionally, individuals with a BMI ⬎ 30 were excluded from study participation. While piloting the procedure for this experiment, the investigators found that an obese individual (BMI ⫽ 31) presented with a very thin image of the TrA, and the hyperechoic fascial planes of all of the muscles of the lateral abdominal wall were very unclear, thus causing the US images to be of poor quality and subsequently difficult to interpret. This informal finding agrees with previous reports [28] that BMI is inversely related to the ratio of the TrA thickness to the total thickness of all of the lateral abdominal muscles in both the relaxed and contracted states. Investigators have suggested that future investigations should consider controlling for BMI in US imaging studies of the lateral abdominal muscles [28].
Instrumentation and Materials US was used in this study to measure the thickness of the TrA over time. This imaging modality has been demonstrated to be a reliable [9,15,19,29,30], valid [15,31], and noninvasive tool to study the activity of the lateral abdominal muscles. The US images of the muscles of the lateral abdominal wall were obtained with the use of a Mylab 25 (Biosound Esaote, 8000 Castleway drive, Indianapolis, IN) scanner with a 5-MHz curvilinear transducer (CA621). A water-based hypoallergenic transduction gel was used as a coupling agent between the skin of the patient and the transducer.
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Functional Performance Task A validated functional task related to trunk use and dysfunction was needed for this study. Previous authors [32,33] have established the utility of the “loaded forward-reach task” as a task that is germane to spine use and dysfunction. Thus, the loaded forward-reach task was used as the preferred functional task because it represents a behavior fundamental to daily activity that can be compromised in individuals with low back pain [32].
Training Procedures After recruitment, each subject gave written informed consent and completed a demographics and medical history questionnaire. Subjects then viewed a procedural video that included education regarding the purpose of the study, experimental procedures, and TrA contraction through the ADIM. After watching the instructional video, each subject underwent further individualized ADIM training during 2 positions: (1) the 4-point kneeling position; and (2) the standing position. To control for the effects of respiration, each subject was trained to perform the ADIM while exhaling in the standing position [19,34]. A physical therapist with extensive experience in teaching and evaluating the ADIM assessed the subject’s ability to achieve a proper ADIM in each position according to previously established clinical criteria [18]. The same physical therapist assessed all subjects in the study. Each subject performed a total of 9 contraction attempts each with a 10-second hold (3 contractions in 4-point kneeling, 3 contractions in upright standing, and 3 contractions in upright standing with exhalation) during the training session. When the subject could consistently perform a proper ADIM in each of these conditions, the subject could then continue with functional training. Functional training of each subject incorporated the loaded forward-reach task. After this training, the subject practiced the loaded forward-reach task for 3 repetitions in 2 different conditions: (1) without the ADIM; and (2) with the ADIM. Proper understanding and performance was evaluated by the same investigator as used in the first training component. After assessment of proper performance of the loaded forward reach with ADIM, the subject was allowed to rest for 5 minutes after the completion of training to reduce the influence of fatigue.
Experimental Procedures The US mode was set to simultaneous B/M mode. The display was set to depict a 60-mm depth of view and the M-mode chart set to record 20 seconds of continuous data. Hypoallergenic water-based transducer gel was applied to the transducer. Next, the transducer was positioned along the right lateral abdominal wall at the anterior axillary line midway between the costal margin and iliac crest [5,9,19]. The trans-
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ducer was manipulated to find the site, allowing the best visualization of the fascial planes of the lateral abdominal muscles. This site was then marked with a marker. The power and gain were then adjusted to provide the optimum clarity of the fascial planes between the abdominal layers. To control for respiration, the subject was asked to exhale and hold the exhalation during each measurement condition [34]. A 15-second US image was obtained at the end of exhalation (as identified by visual inspection of the abdominal wall). The total time recorded for each image was 15 seconds; however, the subject only held the exhalation for 10 seconds within the 15-second time frame. Data were collected in this manner in each of the following conditions: Condition 1. In Condition 1, the subject stood quiet and upright while not performing the ADIM (Figure 1A). He or she stood with feet positioned shoulder width apart and was instructed to “Take in a deep breath, then exhale and hold the exhalation.” Condition 2. In Condition 2, the subject stood quiet and upright while performing ADIM (Figure 1B). He or she stood with feet positioned shoulder width apart and was instructed to “Take in a deep breath, draw your belly button up and in towards your spine as you exhale; hold this condition.” Condition 3. In Condition 3, the subject performed loaded forward-reach activity while not performing the ADIM (Figure 2). He or she was instructed to “Take in a deep breath and exhale; hold the exhalation as you hold the stick and reach forward as far as possible without lifting your heels off of the floor.” Condition 4. In Condition 4, the subject performed loaded forward-reach activity while performing the ADIM. He or she was instructed to “Take in a deep breath and exhale; draw your belly button up and in towards your spine as you exhale; hold this condition as you hold the stick and reach forward as far as possible without lifting your heels off of the floor.” Each US image was frozen on the screen of the US machine immediately after the 20-second image was taken for each condition (Figure 3). With the subject present in the room, 5 measurements were taken at 1-second intervals at 5 different points in time. The 5 measurements were standardized by taking the measurements from second 6 to second 10. To control for order effect, the conditions being tested were randomized with the use of a randomization table generated by sampling without replacement. To further protect against measurement bias, the data were collected by a team of 3 investigators. One investigator positioned the transducer and obtained the image. A second investigator, who was blinded to condition and the actual measurement values, measured the thickness of the TrA on the screen. A third investigator read the measurement from the US monitor
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Figure 2. Description of the loaded forward reach activity. The subject holds a 4.5-kg weighted stick using both hands at shoulder height. Without moving the hips backwards or bending the knees, the subject reaches forward as far as possible with elbows straight without lifting the heels off the floor.
and recorded the data in a spreadsheet for analysis at a later date. The investigators used SPSS statistical software version 15.0 (SPSS, Chicago, IL) to analyze the data. Intrarater reliability was assessed for condition 1 and 3 by use of the first 11 subjects enrolled in the study. The subjects were measured on 2 separate occasions 4 days apart with the use of the reliability principles described by Rankin and Stokes [35]. At the first measurement session, the site of the transducer placement was marked with a marker, and the subjects were assessed under all 4 conditions according to the protocol of the main study. After the collection of data on the first day this site was covered with a bandage, and the subject was
Figure 1. (A). Condition 1: Without the ADIM during quiet upright standing. The subject stood with feet positioned shoulder width apart and was instructed to “Take in a deep breath, then exhale and hold the exhalation.” (B) Condition 2: With the ADIM during quiet upright standing. The subject stood with feet positioned shoulder width apart and was instructed to “Take in a deep breath; draw your belly button up and in towards your spine as you exhale; hold this condition.”
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Table 2. ANOVA summary with Greenhouse-Geisser correction
Source
df
F
Sig.
Within-subject 1.899 13.606 P ⬍ .001 effects
Partial Eta Observed Squared Power 0.476
0.995
ANOVA ⫽ analysis of variance.
Figure 3. The read B/M-mode US image is at the top of this composite image. It gives a reference point for the M-mode image at the bottom of the composite image. The line coursing through the middle of the B-mode image corresponds to the scaled line at the right side of the M-mode image.
asked to avoid washing the site of the marking before returning for remeasurement. The subjects returned 4 days later and were reassessed for Conditions 1 (quiet standing without the ADIM) and 3 (loaded forward reach without the ADIM) by the use of the same procedure and after undergoing the same preliminary training. The examiners used the previous skin markings on subject returning 4 days later for repeated measures. Testing was performed at the same time of day for each of the testing sessions. The means for the 5 measurements were calculated and analyzed with analysis of variance (ANOVA). The values from the ANOVA were then used to perform an intraclass correlation coefficient (ICC 3,1). To test for differences between contractile states and differences between conditions, the means of the 4 conditions were analyzed by the use of a 2 ⫻ 2 repeated-measures ANOVA. Paired t-tests with a Bonferroni correction (␣FW ⫽ 0.0083) was used post-hoc to locate significant differences.
RESULTS Intratester reliability was calculated for the baseline uncontracted conditions only. Precedence for this has been demonstrated in the current literature [36]. The ICCs for CondiTable 1. Muscle thickness for each of the 4 conditions (n ⫽ 16)
Condition 1: Standing without ADIM Condition 2: Standing with ADIM Condition 3: Loaded forward reach without ADIM Condition 4: Loaded forward reach with ADIM ADIM ⫽ abdominal drawing-in maneuver.
Mean, mm
SD, mm
6.10 8.22 6.79
1.37 2.71 1.84
8.67
2.78
tions 1 and 3 were 0.82 and 0.95, respectively (n ⫽ 11; CI ⫽ 95%). Table 1 shows the means, SD, and ranges of muscle thickness for each of the 4 conditions. A Mauchly test of Sphericity revealed a lack of sphericity (P ⫽ .002). Consequently, a Greenhouse-Geisser correction was used to control for potential type 1 error. The ANOVA revealed significant differences (F(1.899,28.844) ⫽ 13.06, P ⫽ .0001; Table 2) Additionally, a large observed power (0.995) and a large effect size (f ⫽ 0.54) were found. Significant differences in TrA thickness found were between all contracted and uncontracted states. No significant difference in TrA thickness was found as the result of functional activity. The paired t tests with a Bonferroni correction used to analyze group differences revealed statistically significant difference between: Conditions 1 and 2 (P ⫽ .001), Conditions 1 and 4 (P ⫽ .001), Conditions 2 and 3 (P ⫽ .005), and Conditions 3 and 4 (P ⬍ .001). No significant differences were found between Conditions 1 and 3, and Conditions 2 and 4 (Table 3).
DISCUSSION The study authors elected to use a previously validated measure of functional performance to examine how subjects were able to produce a TrA contraction during a functional task. Measures of functional performance for individuals with low back pain have been studied by other researchers [32,33]. These measures typically are applied as a clinical battery of tests that include tasks that are fundamental to day-to-day activity and are compromised by low back pain. Simmonds et al [32] studied the correlation between several commonly used physical performance measures and self report of disability and external measures of pain. These authors found that, of the physical performance measure studied, a patient’s Table 3. Comparison of between-group differences: paired t-test summary Pair Comparison Pair Pair Pair Pair Pair Pair
1: 2: 3: 4: 5: 6:
Cond Cond Cond Cond Cond Cond
1-Cond 1-Cond 1-Cond 2-Cond 2-Cond 3-Cond
2 3 4 3 4 4
*Denotes significant difference.
Df
Sig. (2-tailed)
15 15 15 15 15 15
.001* .082 .001* .005* .263 ⬍.001*
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performance in the loaded reach task was the most strongly correlative with the patient’s self report of disability and external measures of pain (r ⫽ ⫺0.57 and r ⫽ ⫺0.42, respectively). Spinal instability has been implicated in perpetuating low back pain [37,38]. Furthermore, the ADIM has been demonstrated to increase both sacroiliac [39,40] and lumbar stiffness [40]. An individual’s ability to activate a protective stabilization strategy for the lumbar spine during a functional task has yet to be examined. Engaging in a successful spinal stabilization strategy for the lumbar spine often begins with ADIM performance in the 4-point kneeling or supine position. This approach is typically progressed from low loads with minimal body weight to more functional body positions and gradual increases in external loads [17,18]. Several studies [19,20,22,31,39] have demonstrated an individual’s ability to achieve changes in the thickness of the TrA with the ADIM in the prone, supine hook-lying, and 4-point kneeling positions. However, these are not positions common to daily functional tasks. It may be assumed that a voluntary preparatory abdominal muscle contraction can trigger the desired, protective TrA activation. However, an individual’s ability to produce TrA contraction through the ADIM during a functional task has not been previously examined. In response, the authors’ findings show that a healthy individual can produce changes in TrA muscle thickness through the use of the ADIM while performing a functional task. The study authors observed a statistically significant difference in the thickness of the TrA in all uncontracted conditions as compared with all contracted conditions. This demonstrates that subjects have the ability to voluntarily create changes in the thickness of the TrA during upright static standing and during a functional task, thus engaging in a potentially protective contractile strategy and promoting trunk stability during the performance of functional tasks that are fundamental to day-to-day activity [32]. Furthermore, the findings may suggest that when individuals are trained to use a protective ADIM during specific functional tasks, one can be fairly confident that the protective contractile behavior is actually occurring. As such, one could surmise that a preparatory ADIM could contribute to stabilizing the spine in a protective fashion. The results of earlier investigations have demonstrated the TrA to be active in healthy individuals in a nondirectionspecific, anticipatory fashion [12,40]. These studies have largely used needle electromyography to quantify the activation of the deep abdominal muscle [7,11,12]. The invasiveness and cost of this method has prompted investigators to use US as an alternative method of measuring muscle activation. Evidence exists demonstrating a correlation between the electrical activation of the TrA to changes in its thickness [13-15]. This study did not investigate electrical muscle activity or feedforward (automatic) behavior and sequencing of the TrA. Instead, US was used to indirectly measure
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abdominal muscle recruitment via the quantification of changes in the muscle’s thickness. The findings here indicate that there is no statistically significant difference in the thickness of the TrA in the contracted states during quiet standing versus loaded forward reach. These results suggest that the loaded forward-reach activity did not attenuate a change in TrA contraction in context with the more complex motor behavior. Therefore, although previous findings illustrate a feedforward activation of TrA in nonfunctional tasks, failure to do so significantly during a functional task may imply need for considering volitional TrA activation during functional task as a learned skill. Similarly, Ainscough-Potts et al [24] used US to measure the thickness of the TrA in response to changes in posture. Those authors found the TrA to have greater automatic activation (muscle thickness) as the result of decreased postural stability. In contrast, no statistically significant difference in TrA thickness was found as the postural stability of the current study subjects decreased during Condition 1 (without the ADIM during quiet upright standing) versus Condition 3 (without the ADIM during performance of the loaded forward-reach activity). This finding indicates that the activation of the TrA, as measured by muscle thickness changes, does not necessarily change in response to changes in functional activity. This agrees with the findings of other recent studies [41,42]. Furthermore, one may gather from the findings of this study that to achieve a significant contraction of the TrA in a loaded forward-reaching task, one needs to volitionally contract the TrA in a protective preparatory manner.
LIMITATIONS This study had several limitations. First, according to Simmonds et al [32], the loaded forward reach was the physical performance measure most strongly correlated to both self report of disability and pain intensity. Despite this fact, the correlations were not strong: ⫺0.57 and ⫺0.42, respectively. Second, whereas the subjects in the current study had no difficulty with the loaded forward reach, the 4.5-kg weight used in this task may be problematic for some people, especially those with low back pain. Third, to control for the effects of respiration, the subject in the current study were asked to exhale completely and hold the exhalation. Although this is a necessary procedure to accurately assess the thickness of the abdominal muscles with diagnostic US [5,24,34], it does create problems for applications to function as most individuals do not hold an expired state during functional tasks. Fourth, this study may not be applicable to patients with low back pain or previous injury because these individuals have demonstrated an impaired function of the TrA. Future research is needed to investigate thickness changes in the TrA in response to the ADIM in the context of a functional task in both symptomatic and asymptomatic
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subjects [43,44]. Furthermore, the utility of ultrasonography for the measurement of TrA thickness with strong muscle contractions may be limited. Earlier investigations have established the validity of measuring muscle thickness changes with US [13,31]. Hodges et al [13], however, found that although large changes in muscle thickness of the TrA and the internal oblique occur with changes in muscle electrical activity, these changes tend to plateau around 20% of a maximal voluntary effort for the TrA and internal oblique muscles. This curvilinear relationship during an isometric activation may limit the effectiveness of US as an indication of the strength of muscle contraction when stronger contractions are engaged.
CONCLUSION Asymptomatic individuals can produce changes in TrA thickness through the use of the ADIM while performing a functional task. To obtain significant changes in TrA muscle thickness in a forward reaching task, an individual needs to volitionally contract the TrA with a protective preparatory abdominal contraction. In addition, these findings suggest that the thickness of the TrA activation does not necessarily occur automatically during changes in postural stability associated with a functional task. This finding may have implications for the protection of the low back during functional tasks, thus serving as a foundation for future studies evaluating the role of this strategy in injury prevention and correction.
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32. Simmonds MJ, Olson SL, Jones S, et al. Psychometric characteristics and clinical usefulness of physical performance tests in patients with low back pain. Spine 1998;23:2412-2421. 33. Smeets RJ, Hijdra HJ, Kester AD, Hitters MW, Knottnerus JA. The usability of six physical performance tasks in a rehabilitation population with chronic low back pain. Clin Rehabil 2006;20:989-997. 34. 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. 35. Rankin G, Stokes M. Reliability of assessment tools in rehabilitation: An illustration of appropriate statistical analyses. Clin Rehabil 1998;12:187-199. 36. Endleman I, Critchley DJ. Transversus abdominis and obliquus internus activity during pilates exercises: Measurement with ultrasound scanning. Arch Phys Med Rehabil 2008;89:2205-2212. 37. Panjabi MM. The stabilizing system of the spine. Part I. Function, dysfunction, adaptation, and enhancement. J Spinal Disord 1992;5: 383-389; discussion 397. 38. Panjabi MM. The stabilizing system of the spine. Part II. Neutral zone and instability hypothesis. J Spinal Disord 1992;5:390-396; discussion 397. 39. 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 2002;27:399-405. 40. Stanton T, Kawchuk G. The effect of abdominal stabilization contractions on posteroanterior spinal stiffness. Spine 2008;33:694-701.
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41. Allison GT, Morris SL, Lay B. Feedforward responses of transversus abdominis are directionally specific and act asymmetrically: Implications for core stability theories. J Orthop Sports Phys Ther 2008;38: 228-237. 42. Carpenter MG, Tokuno CD, Thorstensson A, Cresswell AG. Differential control of abdominal muscles during multi-directional supportsurface translations in man. Exp Brain Res 2008;188:445-455. 43. Hides JA, Boughen CL, Stanton WR, Strudwick MW, Wilson SJ. A magnetic resonance imaging investigation of the transversus abdominis muscle during drawing-in of the abdominal wall in elite Australian football league players with and without low back pain. J Orthop Sports Phys Ther 2010;40:4-10. 44. Teyhen DS, Bluemle LN, Dolbeer JA, et al. Changes in lateral abdominal muscle thickness during the abdominal drawing-in maneuver in those with lumbopelvic pain. J Orthop Sports Phys Ther 2009;39:791-798.
This CME activity is designated for 1.0 AMA PRA Category 1 Credit™ and can be completed online at me.aapmr.org. Log on to www.me.aapmr.org, go to Lifelong Learning (CME) and select Journal-based CME from the drop down menu. This activity is FREE to AAPM&R members and $25 for non-members.
Changes in Transversus Abdominis Thickness With Use of the Abdominal Drawing-In Maneuver During a Functional Task Michael K. McGalliard, ScD, Gregory S. Dedrick, ScD, Jean Michel Brismée, ScD, Chad E. Cook, PhD, Gail G. Apte, ScD, Phillip S. Sizer Jr., PhD Objective: The purpose of this study was to examine an individual’s ability to produce an increase in transversus abdominis (TrA) thickness during the performance of a functional task with the use of the abdominal drawing-in maneuver (ADIM). Design: Within-subject repeated measures analysis of variance was used to examine the effects of the ADIM and a loaded forward-reaching activity on the dependent variable of TrA thickness. Setting: Laboratory Participants: Convenience sample of 8 women and 8 men, asymptomatic, with a mean age of 27.6 ⫾ 7.1 years. Interventions: Ultrasound imaging measurements were recorded during 4 conditions: (1) while the patient was standing without the ADIM; (2) while the patient was standing with the ADIM; (3) during a loaded forward-reaching activity without the ADIM; and (4) during a loaded forward-reaching activity with the ADIM. Main Outcome Measures: Thickness of the TrA muscle. Results: The measurement obtained by an investigator blinded to the condition revealed statistically significant differences in the thickness of the TrA between all uncontracted conditions as compared with all contracted conditions. No statistically significant difference in the thickness of the TrA in the contracted states during quiet standing versus loaded forward reach was observed. Conclusion: Subjects in this study demonstrated the ability to voluntarily activate the TrA during upright static and functional tasks. Additionally, the TrA thickness may change in a direction-specific manner. These findings support a protective role of the ADIM during functional activity and may add information to ways for promoting low back pain prevention. Future studies should include the effectiveness in the use of ADIM during functional tasks for the prevention of low back pain. PM R 2010;2:187-194
INTRODUCTION With the ever-increasing societal and personal impact of low back pain, investigators have turned their focus to evaluating the different factors associated with the onset, perpetuation, and treatment of lumbar spine conditions. The high risk and financial costs accompanying medical and surgical management has led many to focus attention on nonsurgical approaches for the prevention of low back injury and pain. Attention has turned to the role of spinal stability in prevention and management of low back injury and pain [1-3]. The contributions of both the locomotor strength and motor control systems have been investigated in the quest for greater understanding of spinal stabilization. As such, controversy exists in the literature regarding the relative importance of each of these components for spinal stability [4]. The function of the transversus abdominis (TrA) in the stabilization of the spine is of particular interest because, in comparison with the other abdominal muscles, the TrA is unique in its capacity to promote segmental spinal stability [5,6] and possibly support the spine in a protective fashion [3,7,8]. Many investigations regarding the TrA have focused on PM&R 1934-1482/10/$36.00 Printed in U.S.A.
M.K.M. Physical Therapy Program, Department of Rehabilitation Sciences, School of Allied Health Sciences, Texas Tech University Health Science Center, 1400 Coulter Street, Amarillo, TX 79106. Address correspondence to M.K.M.; e-mail: [email protected] Disclosure: nothing to disclose G.S.D. Clinical Musculoskeletal Research Laboratory, Department of Rehabilitation Sciences, School of Allied Health Sciences, Texas Tech University Health Science Center, Amarillo, TX Disclosure: nothing to disclose J.M.B. Clinical Musculoskeletal Research Laboratory, Department of Rehabilitation Sciences, School of Allied Health Sciences, Texas Tech University Health Science Center, Amarillo, TX Disclosure: nothing to disclose C.E.C. The Center for Excellence in Surgical Outcomes, Duke University Medical Center, Durham, NC Disclosure: nothing to disclose G.G.A. Clinical Musculoskeletal Research Laboratory, Department of Rehabilitation Sciences, School of Allied Health Sciences, Texas Tech University Health Science Center, Amarillo, TX Disclosure: nothing to disclose P.S.S. Clinical Musculoskeletal Research Laboratory, Department of Rehabilitation Sciences, School of Allied Health Sciences, Texas Tech University Health Science Center, Amarillo, TX Disclosure: nothing to disclose Submitted for publication November 4, 2009; accepted January 26, 2010.
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motor control and the sequencing of muscle contractions during the stabilization of the spine [3,6,9-11]. Several trunk muscles function selectively in an anticipatory manner in response to selected postural perturbations. In this respect TrA is unique in its function during routine trunk control activity [6,10-12]. In subjects without low back pain, the TrA has been shown to function as an anticipatory/feedforward trunk stabilizer regardless of the direction of the perturbation [3,8,10,12]. Furthermore, the TrA is consistently the first muscle to contract in an anticipatory/feedforward manner when compared with all other abdominal muscles. Thus, the TrA is thought to serve as the primary, protective preparatory stabilizer of the lumbosacral spinal region. Previous investigators [7,11,12] have used needle electromyography to measure the activation of the TrA. However, this technique is invasive and uncomfortable for the patient. In response, investigators have established the validity of ultrasound (US) imaging as an instrument to detect muscle contractility by demonstrating that changes in the thickness of the TrA correlate to changes in muscle activity, especially with low-level isometric muscle contractions [13-15]. Various clinical strategies for protective “spinal stabilization” have been suggested. Despite the variation in the strategic approaches, most strategies incorporate some form of voluntary preparatory abdominal activation before, as well as during, the performance of a functional task. Some investigators [16,17] have favored studying the effects of such strategies that target the TrA and the multifidus muscles. The basis for this type of strategic lumbar stabilization is the preferential activation of the TrA via the voluntary drawing-in of the lower abdominal wall, or the abdominal drawing-in maneuver (ADIM) [18-22]. Although, the “ideal” ADIM involves attaining an isolated or preferential contraction of the TrA [19,21,22], controversy exists in the literature regarding the ability of an individual to produce an “ideal” ADIM [18,22,23]. This controversy represents a gap in available knowledge and brings into question the proposed mechanism underlying functional trunk stabilization. Although some investigators [24] have demonstrated that an upright functional position may induce changes in TrA activation, the ability to produce TrA activation with the ADIM during a functional task in the upright position has not been directly demonstrated. Therefore, the purpose of this study was to examine an individual’s ability to produce TrA activation during the performance of a functional task with use of the ADIM.
METHODS Design A within-subject repeated-measures design was used to examine the effects of the ADIM and a loaded forward-reaching activity on the dependent variable of TrA thickness.
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Subjects This study was approved by the Texas Tech University Health Sciences Center institutional review board. By the use of a sample of convenience, 17 subjects (8 men and 9 women) with no history of low back pain were recruited for this study. All subjects provided informed consent for their participation. The subjects were recruited from 2 sources: (1) students, faculty, and staff from the local university health science center; and (2) acquaintances of the investigators from the general public. One female subject was excluded from the study because multiple sclerosis was subsequently discovered. This condition has the potential to affect trunk control [25-27]. The remaining 16 subjects ranged in age from 21 to 49 years (mean ⫾ SD, 27.6 ⫾ 7.1 years). The subjects’ height ranged from 157.5 to 193 cm (174.8 ⫾ 9.4 cm), weight ranged from 56.8 to 93.1 kg (74.1 ⫾ 11 kg), and body mass index (BMI) ranged from 20.2 to 29 kg.m2 (24.1 ⫾ 2.5 kg.m2). The subjects were eligible for inclusion in the study if they were between 18 and 65 years of age. Subjects were excluded if they reported any of the following: (1) current spinal pain; (2) diagnosed and presently active abdominal, respiratory, or gastrointestinal condition; (3) pregnancy; (4) significant spinal deformity or condition; (5) known neurological or joint disease affecting the trunk; or (6) current urinary tract infection. Additionally, individuals with a BMI ⬎ 30 were excluded from study participation. While piloting the procedure for this experiment, the investigators found that an obese individual (BMI ⫽ 31) presented with a very thin image of the TrA, and the hyperechoic fascial planes of all of the muscles of the lateral abdominal wall were very unclear, thus causing the US images to be of poor quality and subsequently difficult to interpret. This informal finding agrees with previous reports [28] that BMI is inversely related to the ratio of the TrA thickness to the total thickness of all of the lateral abdominal muscles in both the relaxed and contracted states. Investigators have suggested that future investigations should consider controlling for BMI in US imaging studies of the lateral abdominal muscles [28].
Instrumentation and Materials US was used in this study to measure the thickness of the TrA over time. This imaging modality has been demonstrated to be a reliable [9,15,19,29,30], valid [15,31], and noninvasive tool to study the activity of the lateral abdominal muscles. The US images of the muscles of the lateral abdominal wall were obtained with the use of a Mylab 25 (Biosound Esaote, 8000 Castleway drive, Indianapolis, IN) scanner with a 5-MHz curvilinear transducer (CA621). A water-based hypoallergenic transduction gel was used as a coupling agent between the skin of the patient and the transducer.
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Functional Performance Task A validated functional task related to trunk use and dysfunction was needed for this study. Previous authors [32,33] have established the utility of the “loaded forward-reach task” as a task that is germane to spine use and dysfunction. Thus, the loaded forward-reach task was used as the preferred functional task because it represents a behavior fundamental to daily activity that can be compromised in individuals with low back pain [32].
Training Procedures After recruitment, each subject gave written informed consent and completed a demographics and medical history questionnaire. Subjects then viewed a procedural video that included education regarding the purpose of the study, experimental procedures, and TrA contraction through the ADIM. After watching the instructional video, each subject underwent further individualized ADIM training during 2 positions: (1) the 4-point kneeling position; and (2) the standing position. To control for the effects of respiration, each subject was trained to perform the ADIM while exhaling in the standing position [19,34]. A physical therapist with extensive experience in teaching and evaluating the ADIM assessed the subject’s ability to achieve a proper ADIM in each position according to previously established clinical criteria [18]. The same physical therapist assessed all subjects in the study. Each subject performed a total of 9 contraction attempts each with a 10-second hold (3 contractions in 4-point kneeling, 3 contractions in upright standing, and 3 contractions in upright standing with exhalation) during the training session. When the subject could consistently perform a proper ADIM in each of these conditions, the subject could then continue with functional training. Functional training of each subject incorporated the loaded forward-reach task. After this training, the subject practiced the loaded forward-reach task for 3 repetitions in 2 different conditions: (1) without the ADIM; and (2) with the ADIM. Proper understanding and performance was evaluated by the same investigator as used in the first training component. After assessment of proper performance of the loaded forward reach with ADIM, the subject was allowed to rest for 5 minutes after the completion of training to reduce the influence of fatigue.
Experimental Procedures The US mode was set to simultaneous B/M mode. The display was set to depict a 60-mm depth of view and the M-mode chart set to record 20 seconds of continuous data. Hypoallergenic water-based transducer gel was applied to the transducer. Next, the transducer was positioned along the right lateral abdominal wall at the anterior axillary line midway between the costal margin and iliac crest [5,9,19]. The trans-
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ducer was manipulated to find the site, allowing the best visualization of the fascial planes of the lateral abdominal muscles. This site was then marked with a marker. The power and gain were then adjusted to provide the optimum clarity of the fascial planes between the abdominal layers. To control for respiration, the subject was asked to exhale and hold the exhalation during each measurement condition [34]. A 15-second US image was obtained at the end of exhalation (as identified by visual inspection of the abdominal wall). The total time recorded for each image was 15 seconds; however, the subject only held the exhalation for 10 seconds within the 15-second time frame. Data were collected in this manner in each of the following conditions: Condition 1. In Condition 1, the subject stood quiet and upright while not performing the ADIM (Figure 1A). He or she stood with feet positioned shoulder width apart and was instructed to “Take in a deep breath, then exhale and hold the exhalation.” Condition 2. In Condition 2, the subject stood quiet and upright while performing ADIM (Figure 1B). He or she stood with feet positioned shoulder width apart and was instructed to “Take in a deep breath, draw your belly button up and in towards your spine as you exhale; hold this condition.” Condition 3. In Condition 3, the subject performed loaded forward-reach activity while not performing the ADIM (Figure 2). He or she was instructed to “Take in a deep breath and exhale; hold the exhalation as you hold the stick and reach forward as far as possible without lifting your heels off of the floor.” Condition 4. In Condition 4, the subject performed loaded forward-reach activity while performing the ADIM. He or she was instructed to “Take in a deep breath and exhale; draw your belly button up and in towards your spine as you exhale; hold this condition as you hold the stick and reach forward as far as possible without lifting your heels off of the floor.” Each US image was frozen on the screen of the US machine immediately after the 20-second image was taken for each condition (Figure 3). With the subject present in the room, 5 measurements were taken at 1-second intervals at 5 different points in time. The 5 measurements were standardized by taking the measurements from second 6 to second 10. To control for order effect, the conditions being tested were randomized with the use of a randomization table generated by sampling without replacement. To further protect against measurement bias, the data were collected by a team of 3 investigators. One investigator positioned the transducer and obtained the image. A second investigator, who was blinded to condition and the actual measurement values, measured the thickness of the TrA on the screen. A third investigator read the measurement from the US monitor
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Figure 2. Description of the loaded forward reach activity. The subject holds a 4.5-kg weighted stick using both hands at shoulder height. Without moving the hips backwards or bending the knees, the subject reaches forward as far as possible with elbows straight without lifting the heels off the floor.
and recorded the data in a spreadsheet for analysis at a later date. The investigators used SPSS statistical software version 15.0 (SPSS, Chicago, IL) to analyze the data. Intrarater reliability was assessed for condition 1 and 3 by use of the first 11 subjects enrolled in the study. The subjects were measured on 2 separate occasions 4 days apart with the use of the reliability principles described by Rankin and Stokes [35]. At the first measurement session, the site of the transducer placement was marked with a marker, and the subjects were assessed under all 4 conditions according to the protocol of the main study. After the collection of data on the first day this site was covered with a bandage, and the subject was
Figure 1. (A). Condition 1: Without the ADIM during quiet upright standing. The subject stood with feet positioned shoulder width apart and was instructed to “Take in a deep breath, then exhale and hold the exhalation.” (B) Condition 2: With the ADIM during quiet upright standing. The subject stood with feet positioned shoulder width apart and was instructed to “Take in a deep breath; draw your belly button up and in towards your spine as you exhale; hold this condition.”
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Table 2. ANOVA summary with Greenhouse-Geisser correction
Source
df
F
Sig.
Within-subject 1.899 13.606 P ⬍ .001 effects
Partial Eta Observed Squared Power 0.476
0.995
ANOVA ⫽ analysis of variance.
Figure 3. The read B/M-mode US image is at the top of this composite image. It gives a reference point for the M-mode image at the bottom of the composite image. The line coursing through the middle of the B-mode image corresponds to the scaled line at the right side of the M-mode image.
asked to avoid washing the site of the marking before returning for remeasurement. The subjects returned 4 days later and were reassessed for Conditions 1 (quiet standing without the ADIM) and 3 (loaded forward reach without the ADIM) by the use of the same procedure and after undergoing the same preliminary training. The examiners used the previous skin markings on subject returning 4 days later for repeated measures. Testing was performed at the same time of day for each of the testing sessions. The means for the 5 measurements were calculated and analyzed with analysis of variance (ANOVA). The values from the ANOVA were then used to perform an intraclass correlation coefficient (ICC 3,1). To test for differences between contractile states and differences between conditions, the means of the 4 conditions were analyzed by the use of a 2 ⫻ 2 repeated-measures ANOVA. Paired t-tests with a Bonferroni correction (␣FW ⫽ 0.0083) was used post-hoc to locate significant differences.
RESULTS Intratester reliability was calculated for the baseline uncontracted conditions only. Precedence for this has been demonstrated in the current literature [36]. The ICCs for CondiTable 1. Muscle thickness for each of the 4 conditions (n ⫽ 16)
Condition 1: Standing without ADIM Condition 2: Standing with ADIM Condition 3: Loaded forward reach without ADIM Condition 4: Loaded forward reach with ADIM ADIM ⫽ abdominal drawing-in maneuver.
Mean, mm
SD, mm
6.10 8.22 6.79
1.37 2.71 1.84
8.67
2.78
tions 1 and 3 were 0.82 and 0.95, respectively (n ⫽ 11; CI ⫽ 95%). Table 1 shows the means, SD, and ranges of muscle thickness for each of the 4 conditions. A Mauchly test of Sphericity revealed a lack of sphericity (P ⫽ .002). Consequently, a Greenhouse-Geisser correction was used to control for potential type 1 error. The ANOVA revealed significant differences (F(1.899,28.844) ⫽ 13.06, P ⫽ .0001; Table 2) Additionally, a large observed power (0.995) and a large effect size (f ⫽ 0.54) were found. Significant differences in TrA thickness found were between all contracted and uncontracted states. No significant difference in TrA thickness was found as the result of functional activity. The paired t tests with a Bonferroni correction used to analyze group differences revealed statistically significant difference between: Conditions 1 and 2 (P ⫽ .001), Conditions 1 and 4 (P ⫽ .001), Conditions 2 and 3 (P ⫽ .005), and Conditions 3 and 4 (P ⬍ .001). No significant differences were found between Conditions 1 and 3, and Conditions 2 and 4 (Table 3).
DISCUSSION The study authors elected to use a previously validated measure of functional performance to examine how subjects were able to produce a TrA contraction during a functional task. Measures of functional performance for individuals with low back pain have been studied by other researchers [32,33]. These measures typically are applied as a clinical battery of tests that include tasks that are fundamental to day-to-day activity and are compromised by low back pain. Simmonds et al [32] studied the correlation between several commonly used physical performance measures and self report of disability and external measures of pain. These authors found that, of the physical performance measure studied, a patient’s Table 3. Comparison of between-group differences: paired t-test summary Pair Comparison Pair Pair Pair Pair Pair Pair
1: 2: 3: 4: 5: 6:
Cond Cond Cond Cond Cond Cond
1-Cond 1-Cond 1-Cond 2-Cond 2-Cond 3-Cond
2 3 4 3 4 4
*Denotes significant difference.
Df
Sig. (2-tailed)
15 15 15 15 15 15
.001* .082 .001* .005* .263 ⬍.001*
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performance in the loaded reach task was the most strongly correlative with the patient’s self report of disability and external measures of pain (r ⫽ ⫺0.57 and r ⫽ ⫺0.42, respectively). Spinal instability has been implicated in perpetuating low back pain [37,38]. Furthermore, the ADIM has been demonstrated to increase both sacroiliac [39,40] and lumbar stiffness [40]. An individual’s ability to activate a protective stabilization strategy for the lumbar spine during a functional task has yet to be examined. Engaging in a successful spinal stabilization strategy for the lumbar spine often begins with ADIM performance in the 4-point kneeling or supine position. This approach is typically progressed from low loads with minimal body weight to more functional body positions and gradual increases in external loads [17,18]. Several studies [19,20,22,31,39] have demonstrated an individual’s ability to achieve changes in the thickness of the TrA with the ADIM in the prone, supine hook-lying, and 4-point kneeling positions. However, these are not positions common to daily functional tasks. It may be assumed that a voluntary preparatory abdominal muscle contraction can trigger the desired, protective TrA activation. However, an individual’s ability to produce TrA contraction through the ADIM during a functional task has not been previously examined. In response, the authors’ findings show that a healthy individual can produce changes in TrA muscle thickness through the use of the ADIM while performing a functional task. The study authors observed a statistically significant difference in the thickness of the TrA in all uncontracted conditions as compared with all contracted conditions. This demonstrates that subjects have the ability to voluntarily create changes in the thickness of the TrA during upright static standing and during a functional task, thus engaging in a potentially protective contractile strategy and promoting trunk stability during the performance of functional tasks that are fundamental to day-to-day activity [32]. Furthermore, the findings may suggest that when individuals are trained to use a protective ADIM during specific functional tasks, one can be fairly confident that the protective contractile behavior is actually occurring. As such, one could surmise that a preparatory ADIM could contribute to stabilizing the spine in a protective fashion. The results of earlier investigations have demonstrated the TrA to be active in healthy individuals in a nondirectionspecific, anticipatory fashion [12,40]. These studies have largely used needle electromyography to quantify the activation of the deep abdominal muscle [7,11,12]. The invasiveness and cost of this method has prompted investigators to use US as an alternative method of measuring muscle activation. Evidence exists demonstrating a correlation between the electrical activation of the TrA to changes in its thickness [13-15]. This study did not investigate electrical muscle activity or feedforward (automatic) behavior and sequencing of the TrA. Instead, US was used to indirectly measure
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abdominal muscle recruitment via the quantification of changes in the muscle’s thickness. The findings here indicate that there is no statistically significant difference in the thickness of the TrA in the contracted states during quiet standing versus loaded forward reach. These results suggest that the loaded forward-reach activity did not attenuate a change in TrA contraction in context with the more complex motor behavior. Therefore, although previous findings illustrate a feedforward activation of TrA in nonfunctional tasks, failure to do so significantly during a functional task may imply need for considering volitional TrA activation during functional task as a learned skill. Similarly, Ainscough-Potts et al [24] used US to measure the thickness of the TrA in response to changes in posture. Those authors found the TrA to have greater automatic activation (muscle thickness) as the result of decreased postural stability. In contrast, no statistically significant difference in TrA thickness was found as the postural stability of the current study subjects decreased during Condition 1 (without the ADIM during quiet upright standing) versus Condition 3 (without the ADIM during performance of the loaded forward-reach activity). This finding indicates that the activation of the TrA, as measured by muscle thickness changes, does not necessarily change in response to changes in functional activity. This agrees with the findings of other recent studies [41,42]. Furthermore, one may gather from the findings of this study that to achieve a significant contraction of the TrA in a loaded forward-reaching task, one needs to volitionally contract the TrA in a protective preparatory manner.
LIMITATIONS This study had several limitations. First, according to Simmonds et al [32], the loaded forward reach was the physical performance measure most strongly correlated to both self report of disability and pain intensity. Despite this fact, the correlations were not strong: ⫺0.57 and ⫺0.42, respectively. Second, whereas the subjects in the current study had no difficulty with the loaded forward reach, the 4.5-kg weight used in this task may be problematic for some people, especially those with low back pain. Third, to control for the effects of respiration, the subject in the current study were asked to exhale completely and hold the exhalation. Although this is a necessary procedure to accurately assess the thickness of the abdominal muscles with diagnostic US [5,24,34], it does create problems for applications to function as most individuals do not hold an expired state during functional tasks. Fourth, this study may not be applicable to patients with low back pain or previous injury because these individuals have demonstrated an impaired function of the TrA. Future research is needed to investigate thickness changes in the TrA in response to the ADIM in the context of a functional task in both symptomatic and asymptomatic
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subjects [43,44]. Furthermore, the utility of ultrasonography for the measurement of TrA thickness with strong muscle contractions may be limited. Earlier investigations have established the validity of measuring muscle thickness changes with US [13,31]. Hodges et al [13], however, found that although large changes in muscle thickness of the TrA and the internal oblique occur with changes in muscle electrical activity, these changes tend to plateau around 20% of a maximal voluntary effort for the TrA and internal oblique muscles. This curvilinear relationship during an isometric activation may limit the effectiveness of US as an indication of the strength of muscle contraction when stronger contractions are engaged.
CONCLUSION Asymptomatic individuals can produce changes in TrA thickness through the use of the ADIM while performing a functional task. To obtain significant changes in TrA muscle thickness in a forward reaching task, an individual needs to volitionally contract the TrA with a protective preparatory abdominal contraction. In addition, these findings suggest that the thickness of the TrA activation does not necessarily occur automatically during changes in postural stability associated with a functional task. This finding may have implications for the protection of the low back during functional tasks, thus serving as a foundation for future studies evaluating the role of this strategy in injury prevention and correction.
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