The Effect of Traditional Bridging or Suspension-Exercise Bridging on Lateral Abdominal Thickness in Individuals With Low Back Pain

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1 Journal of Sport Rehabilitation, 2012, 21, Human Kinetics, Inc. The Effect of Traditional Bridging or Suspension-Exercise Bridging on Lateral Abdominal Thickness in Individuals With Low Back Pain Rebecca J. Guthrie, Terry L. Grindstaff, Theodore Croy, Christopher D. Ingersoll, and Susan A. Saliba Context: Individuals with low back pain (LBP) are thought to benefit from interventions that improve motor control of the lumbopelvic region. It is unknown if therapeutic exercise can acutely facilitate activation of lateral abdominal musculature. Objective: To investigate the ability of 2 types of bridging-exercise progressions to facilitate lateral abdominal muscles during an abdominal drawing-in maneuver (ADIM) in individuals with LBP. Design: Randomized control trial. Setting: University research laboratory. Participants: 51 adults (mean ± SD age 23.1 ± 6.0 y, height ± 10.5 cm, mass 74.7 ± 14.5 kg, and 64.7% female) with LBP. All participants met 3 of 4 criteria for stabilization-classification LBP or at least 6 best-fit criteria for stabilization classification. Interventions: Participants were randomly assigned to either traditional-bridge progression or suspension-exercise-bridge progression, each with 4 levels of progressive difficulty. They performed 5 repetitions at each level and were progressed based on specific criteria. Main Outcome Measures: Muscle thickness of the external oblique (EO), internal oblique (IO), and transversus abdominis (TrA) was measured during an ADIM using ultrasound imaging preintervention and postintervention. A contraction ratio (contracted thickness:resting thickness) of the EO, IO, and TrA was used to quantify changes in muscle thickness. Results: There was not a significant increase in EO (F 1,47 = 0.44, P =.51) or IO (F 1,47 =.30, P =.59) contraction ratios after the exercise progression. There was a significant (F 1,47 = 4.05, P =.05) group-bytime interaction wherein the traditional-bridge progression (pre = 1.55 ± 0.22; post = 1.65 ± 0.21) resulted in greater (P =.03) TrA contraction ratio after exercise than the suspension-exercise-bridge progression (pre = 1.61 ± 0.31; post = 1.58 ± 0.28). Conclusion: A single exercise progression did not acutely improve muscle thickness of the EO and IO. The magnitude of change in TrA muscle thickness after the traditional-bridging progression was less than the minimal detectable change, thus not clinically significant. Keywords: lumbar stabilization, sonography, therapeutic exercise Low back pain (LBP) is thought to affect 70% to 80% of the population, and 60% to 90% of these individuals have recurrent symptoms. 1 5 Although LBP is a common health problem, the underlying pathology and optimal intervention strategy remain unclear. 6 Since LBP is not a homogeneous entity, individuals with similar symptoms and clinical signs may be more likely to benefit from an exercise program that enhances neuromuscular control of the spine. 7 Individuals with chronic LBP have been shown to benefit from rehabilitation programs that incorporate Guthrie is with the Orthopaedics Dept, Emory Sports Medicine Center, Atlanta, GA. Grindstaff is with the Dept of Physical Therapy, Creighton University, Omaha, NE. Croy is with the Doctoral Program in Physical Therapy, U.S. Army Baylor University, Ft Sam Houston, TX. Ingersoll is with the Office of the Dean, Central Michigan University, Mt Pleasant, MI. Saliba is with the Dept of Human Services, University of Virginia, Charlottesville, VA. therapeutic exercises to enhance neuromuscular control of the spine Neuromuscular control of the lumbar spine is achieved though coordinated activity of a number of muscles surrounding the spine. 12 Individuals with LBP have been shown to demonstrate deficiencies in transversus abdominis (TrA) activation A fundamental exercise in a motor-control exercise program for LBP is the abdominal drawing-in maneuver (ADIM). 16,17 This exercise is used to preferentially activate the TrA while maintaining relaxation of the more superficial musculature (rectus abdominis, external oblique). 17 Individuals with LBP have been shown to demonstrate difficulty in increasing the thickness of the TrA, measured with ultrasound imaging, during an ADIM. 15,18 Once subjects are able to successfully perform this exercise they are advanced to exercises that are dynamic and require coordinated activation of all spine-stabilizing musculature. 11,17,19 These types of exercises have been shown to reduce symptoms associated with LBP and improve function. 11,

2 152 Guthrie et al The use of suspension training, in addition to a multimodal physical therapy intervention program, has been shown to be effective in reducing pain, improving functional status, and improving health-related quality of life for up to 2 years in postpartum women with pelvic pain. 22,23 Suspension-training devices incorporate labile surfaces to increase exercise difficulty and provide a greater challenge to the patient. Exercises performed on labile surfaces, compared with stable surfaces, result in increased muscle-contraction speed and muscle-activity levels A bridging exercise utilizing the suspensionexercise device has been shown to result in greater TrA muscle-thickness changes during exercise than a traditional bridging exercise. 27 It is currently unknown if the facilitation of the TrA during a bridging exercise performed on a suspension-training device carries over to other tasks such as an ADIM after exercise intervention. Therefore, the purpose of this study was to compare changes in lateral abdominal-muscle thickness before and after a single intervention consisting of either a traditional-bridging progression or a suspension-exercisebridging progression. We hypothesized that after subjects performed the suspension-exercise-bridge progression there would be an acute increase in their TrA muscle thickness while performing an ADIM, and these changes would not be present after performing the traditionalbridge progression. Participants Methods Fifty-one adults (18 men, 33 women) with a current episode of LBP participated in this study (Table 1 and Figure 1). Participants were recruited from the university community and from an athletic therapy clinic. Inclusion criteria were based on physical examination and history findings consistent with the stabilization classification, which is a component of a treatment-based classification system for individuals with LBP (Figure 2). 7 Since the clinical predictor rule used to identify individuals with LBP who would be likely to benefit from stabilization exercises has not been validated, other evidence-based clinical examination (best-fit) criteria 28,29 were used to broaden the selection of individuals who may benefit from stabilization exercise. Eligible participants met at least 3 of 4 stabilization-classification criteria that included age less than 40, straight-leg raise greater than 91, positive prone instability test, and instability catch or aberrant movements in flexion/extension motions. 7 If participants did not meet at least 3 of these 4 criteria, they were further evaluated to see if they met at least 6 of the following best-fit criteria: spring-test hypermobility, increased episode frequency, more than 3 previous episodes, positive posterior pelvic pain-provocation test, difficulty performing active straight-leg raise, modified Trendelenburg, or pain with palpation of the long dorsal sacroiliac ligament or pubic symphysis. Exclusion criteria were neurological symptoms distal to the hip, spinal surgery, pregnancy, or known cancer or tumor. The study was approved by the institutional review board of the University of Virginia, and written informed consent was obtained from all participants before testing. Procedures After we obtained informed consent all participants completed a health-history form, modified Oswestry Low Back Pain Disability Questionnaire, and Fear-Avoidance Beliefs questionnaire and underwent a standardized physical examination to determine their eligibility for the study. Next, they underwent a brief training session and were instructed on how to perform an ADIM. Participants were verbally instructed to gently pull their navel to the spine at the end of a normal exhalation and to hold this contraction for 10 seconds while continuing normal respiration. 17 During the training session, the researcher monitored participant progress using ultrasound imaging but did not allow the participant to visualize the ultrasound screen. Practice trials were performed until the researcher determined the participant could perform 3 isolated TrA contractions (thickening and lateral movement of the muscle during ultrasound imaging) without an appreciable increase in external oblique (EO) and internal oblique (IO) muscle thickness. 30 Table 1 Participant Demographics, Mean ± SD Total Traditional bridge Suspension-exercise bridge Age (y) 23.1 ± ± ± 5.1 Height (cm) ± ± ± 10.2 Mass (kg) 74.7 ± ± ± 15.8 Body-mass index (kg/m 2 ) 24.6 ± ± ± 3.2 Oswestry score (%) 25.8 ± ± ± 9.4 Fear-Avoidance Beliefs questionnaire score (physical activity) 14.9 ± ± ± 5.5 Fear-Avoidance Beliefs questionnaire score (work) 5.5 ± ± ± 7.6 Duration of symptoms (mo) 39.0 ± ± ± 19.2

3 The Effect of Traditional Bridging 153 Figure 1 CONSORT (Consolidated Standards of Reporting Trials) flowchart. Figure 2 Inclusion criteria. Abbreviations: SLR, straight-leg raise; ROM, range of motion; ASLR, active straight-leg raise; SI, sacroiliac. Ultrasound imaging was used to quantify muscle thickness of the EO, IO, and TrA during an ADIM before and after exercise intervention. Two images were obtained during each ADIM, 1 during rest and 1 during ADIM contraction. All images were obtained after exhalation, and the sequence was repeated 3 times for 6 images (3 resting, 3 contracted). Ultrasound images were obtained using a LOGIQ Book XP (GE Healthcare Products, Milwaukee, WI) with an 8-MHz linear transducer while the participant was in the supine hook-lying position. The transducer was placed on the lateral abdominal wall, in the transverse plane, along the midaxillary line midway between the iliac crest and the inferior angle of the rib cage, corresponding to the side where the participant experienced LBP. If pain was bilateral or central, the transducer was placed on the right lateral abdominal wall. The transducer

4 154 Guthrie et al was oriented transversely perpendicular to the abdominal musculature, with a slight tilt toward the pubis to align with the fibers of the TrA to optimize the image. Transducer position was adjusted so the lateral insertion of the TrA, on the thoracolumbar fascia, was approximately 2 cm from the edge of the image. 30 Images were stored with no indication of group assignment and were measured by a blinded researcher. Before enrolling participants, both researchers underwent a supervised training program for ultrasound imaging by an experienced physical therapist. The training program included transducer placement, image acquisition, and image-measurement techniques. Measures of within-session intratester reliability, standard error of measure (SEM), and the minimal detectable change using a 95% confidence interval for resting and contracted muscle-thickness values were obtained on 10 separate participants. After training, both researchers demonstrated excellent intratester reliability for all muscle-thickness measures (at rest and contracted; Table 2). Reliability was consistent with previous reports of this measurement technique The preintervention and postintervention images were measured after data collection by a researcher who was blinded to the exercise condition. Thicknesses of the EO, IO, and TrA were measured using Image J software (version 1.41o, Wayne Rasband, National Institutes of Health, USA). EO, IO, and TrA thicknesses were measured using the distance between the superior edge of the deep hyperechoic fascial line to the inferior edge of the superior fascial line. 32 The thickness ratio of each muscle (EO, IO, and TrA) was calculated by dividing the contracted thickness during an ADIM by the resting thickness (contracted thickness/resting thickness). 33 Intervention After baseline measurement of lateral abdominal-muscle thickness, participants were randomly assigned to either the traditional-bridging group or the suspension-exercisebridging group using a computer-generated randomization schedule and preprinted cards enclosed in sealed envelopes. Four sets of 5 repetitions were performed, and each exercise progression consisted of 4 levels of difficulty. Each repetition was held for 5 seconds, and a 1-minute rest was allowed between sets. All participants started at the first level. Both progressions were nearly identical in terms of limb position, movement, and hold time, but the suspension-exercise-bridge group had their legs suspended in every level. Criteria for exercise progression required the exercise to be performed in a pain-free manner with correct technique. If the participant could not perform the exercise correctly or pain resulted, assistance was provided via elastic cords or the progression level of the exercise was decreased. Immediately after intervention, images were obtained during 3 ADIMs using the same methods previously described. Traditional Bridge. The traditional bridge exercise consisted of 4 possible levels (Figure 3). The researcher monitored success at each level before advancing the participant to the next stage of the exercise. The researcher observed the alignment and/or any compensatory movements and asked participants about whether the activity increased their pain. Each exercise began with the participant in the supine hook-lying position on the treatment table with the knees bent to 90, feet flat on the table, and arms crossed over the chest. Participants were then instructed to push through their heels to lift their hips into the air while maintaining straight alignment of the knees, hips, and shoulders. No specific instructions were given regarding activation of abdominal musculature (ie, performing the bridging exercise in conjunction with an ADIM). In the first level, the participants held this position for 5 seconds and then were instructed to lower back to the starting position. The second level consisted of the participants extending their right knee for 3 seconds followed by their left knee for 3 seconds and then lowering back to the starting position. In the third level, participants had an air-filled balance disc placed between their scapulae to perform the exercise on an unstable surface, and in the fourth level they were instructed to extend their knees, Table 2 Intratester Reliability (ICC 3,3 ) and 95% Confidence Intervals (95% CI), Standard Error of Measurement (SEM), and Minimal Detectable Change (MDC) Using Estimates for Ultrasound- Imaging Measurements of Lateral Abdominal Muscles (cm) at Rest and During an Abdominal Drawing-In Maneuver Examiner 1 Examiner 2 ICC (95% CI) SEM MDC ICC SEM MDC External oblique resting.95 (.79.99) (.71.98) External oblique contracted.93 (.70.98) (.52.97) Internal oblique resting.98 ( ) (.75.99) Internal oblique contracted.94 (.77.99) (.88.99) Transversus abdominis resting.99 ( ) (.90.99) Transversus abdominis contracted.99 ( ) ( ).02.05

5 The Effect of Traditional Bridging 155 (A) (B) (C) (D) Figure 3 Traditional-bridge-exercise progression. (A) Two-leg. (B) Single-leg. (C) Two-leg with unstable surface under back. (D) Single-leg with contralateral hip abduction. as well as abduct the hip 45. Participants performed 5 repetitions on each level of the exercise. The participants were progressed to each level based on the clinician s subjective clinical opinion. If participants could perform the exercise correctly, with no pain, and without difficulty, they were progressed to the next level. If they could not perform the exercise correctly, it was painful, or it was difficult, the level of exercise was decreased or modified to enable 4 sets of 5 repetitions. Suspension-Exercise Bridge. he suspension-exercise device used in this study was a Redcord Workstation Professional (Redcord AS, Staubo, Norway). This device consists of a rope, sling, and pulleys that are used to suspend the legs during the bridging exercise. There were 4 possible levels of the suspension-bridging exercise (Figure 4). The participants in the suspensionexercise-bridge group began by laying supine on the treatment table with their hips and knees bent to 90. The participants knees were placed in a sling that was suspended from the ceiling. During the exercise, no specific instructions were given regarding activation of abdominal musculature (ie, performing the bridging exercise in conjunction with an ADIM). The first level consisted of participants lifting their hips while maintaining straight alignment of the knees, hips, and shoulders. The participants held this position for 5 seconds and then were instructed to lower back to the starting position. In the second level, each participant s left knee remained in the sling and the right knee was not in the sling. The participant was instructed to hold the right leg at the same level as the left and to lift the hips into the air while maintaining straight alignment of the knees, hips, and shoulders. Participants held this position for 5 seconds and then lowered back to the starting position. In the third level, they had both knees placed in the sling and a disc was placed between their scapulae to provide an unstable surface on which to perform the bridge. In the fourth level, the participants ankles were placed in 2 separate slings and they were instructed to perform the bridge and then abduct their legs 1 at a time before lowering back to the starting position. The participants performed 5 repetitions

6 156 Guthrie et al (A) (B) (C) (D) Figure 4 Suspension-exercise-bridge progression. Sling was placed on the posterior aspect of the knee. (A) Two-leg. (B) Singleleg. (C) Two-leg with unstable surface under back. (D) Single-leg with contralateral hip abduction. on each level of the exercise. The same subjective criteria were used to progress them through each level of exercise in the suspension-exercise-bridging intervention as was used in the traditional-bridge intervention. Data Analysis A randomized control trial with 1 between factor, exercise group (traditional, suspension), and 1 within factor, time (preintervention and postintervention) was used to examine the effects of exercise on the contraction ratios (muscle-contracted thickness/muscle-relaxed thickness) of the EO, IO, and TrA. Participant demographics between groups were compared using independent t tests. Separate repeatedmeasures ANOVAs were performed to compare contraction ratios of the EO, IO, and TrA before and after exercise (traditional, suspension). The level of statistical significance was set a priori at P <.05. Statistical analyses were performed with SPSS Version 16.0 (SPSS Inc, Chicago, IL). Sample-size calculations indicated that the number of participants required to detect a 0.3 difference in TrA contraction ratio between groups was 24 subjects per group. This was based on standard deviations (0.36) from a previous study, 33 an a priori significance level of P =.05, and a power of.80. Results The traditional-bridge-exercise group and the suspensionexercise-bridge group did not differ in terms of demographic stabilization-classification criteria or preintervention TrA thickness ratios (Table 1). It was not possible to obtain clear images for 2 participants; therefore, data for those participants were not included in the analysis. All participants were able to progress through the first 2 levels of their respective exercise progressions. One individual was not able to progress to Level 3 when performing the traditional bridge, but all were able to complete Level 3 for the suspension-exercise-bridge progression. Seven individuals were not able to complete the traditional-

7 The Effect of Traditional Bridging 157 bridging exercise, while 4 individuals were not able to complete the suspension-exercise-bridge progression. No individual in either group reported pain during the exercise intervention at any level, and all individuals who progressed to subsequent levels performed each exercise with correct technique. Descriptive data regarding resting and contracted muscle thickness, as well as muscle-contraction ratios, are presented in Table 3. There was not a significant groupby-time interaction for the EO (F 1,47 = 0.44, P =.51) or IO (F 1,47 = 0.30, P =.59) muscle-thickness contraction ratios. When groups were pooled there were no significant differences between preintervention and postintervention contraction ratios for the EO (F 1,47 = 0.41, P =.53) or IO (F 1,47 = 0.40, P =.53). There was a significant difference (F 1,47 = 4.05, P =.05) in TrA thickness ratios by group. The traditionalbridge progression (pre = 1.55 ± 0.22, post = 1.65 ± 0.21) resulted in greater TrA activation (P =.03) after exercise than the suspension-exercise-bridge progression (P =.51; pre = 1.61 ± 0.31, post = 1.58 ± 0.28; Table 3). Discussion This study demonstrates that a single bridging-exercise intervention, traditional or suspension, does not have an immediate effect on EO or IO muscle-thickness ratios during an ADIM. The traditional-bridge-exercise progression did significantly improve the ability to perform an ADIM in individuals with LBP who met stabilizationclassification criteria, but findings should be interpreted with caution. Although the traditional-bridge-exercise progression resulted in a 6% increase in the TrA contraction ratio, the changes in muscle thickness were less than the minimal detectable change (>.05 cm) 34 and thus not thought to be clinically significant. Although a single intervention would not be expected to alter muscle morphology, it is plausible to acutely facilitate muscle function after a single intervention. A lumbopelvic-joint manipulation has been shown to acutely increase muscle thickness of the TrA in individuals with LBP. 34,35 The acute increase in muscle function is thought to be due to increased neural excitability resulting in muscle facilitation. Although those studies examined the effects of spinal manipulation, the results demonstrate that facilitation of the TrA is possible after a single intervention. Neither bridging progression had an immediate clinical effect on EO, IO, or TrA musclethickness ratios during an ADIM. These results do not support the hypothesis of the study, which proposed that the suspension-exercise-bridge progression would result in greater TrA activation than the traditional-bridge progression. Facilitation of TrA activation may be beneficial for individuals with LBP since they have been shown to demonstrate deficiencies in TrA activation measured using ultrasound imaging. 14,15 The exercise progressions used in this study were nearly identical in terms of limb positions and movements, with the exception of the legs being suspended in Table 3 Lateral Abdominal-Muscle Thickness, Mean ± SD Group Preintervention Postintervention External oblique relaxed (cm) Traditional 0.62 ± ± 0.25 Suspension 0.73 ± ± 0.30 External oblique contracted (cm) Traditional 0.66 ± ± 0.24 Suspension 0.76 ± ± 0.31 External oblique contraction ratio Traditional 1.05 ± ± 0.22 Suspension 1.08 ± ± 0.19 Internal oblique relaxed (cm) Traditional 1.01 ± ± 0.26 Suspension 1.08 ± ± 0.33 Internal oblique contracted (cm) Traditional 1.14 ± ± 0.28 Suspension 1.22 ± ± 0.41 Internal oblique contraction ratio Traditional 1.14 ± ± 0.12 Suspension 1.13 ± ± 0.14 Transversus abdominis relaxed (cm) Traditional 0.40 ± ± 0.07 Suspension 0.40 ± ± 0.10 Transversus abdominis contracted (cm) Traditional 0.61 ± ± 0.12 Suspension 0.63 ± ± 0.17 Transversus abdominis contraction ratio Traditional* 1.55 ± ± 0.21 Suspension 1.61 ± ± 0.28 *Significant difference (P =.03) preintervention to postintervention.

8 158 Guthrie et al the suspension-exercise-bridging group. The suspensionexercise progression has been shown to result in higher TrA activation during exercise than the traditional-bridging exercise. 27 Although higher TrA activation occurs during the suspension-exercise bridge, 27 changes in the ability to perform an ADIM after exercise intervention were not immediate in this study. Facilitation of muscle activation before performing therapeutic exercise has been shown to improve clinical outcomes relative to traditional rehabilitation programs. 36 If TrA deficits are due to reflexive muscle inhibition, traditional strengthening interventions may not fully address the neurological dysfunction Although neurological adaptations and motor-pattern efficiency and timing are thought to occur in less than 6 weeks, it is possible that changes in muscle thickness may take longer than 6 weeks with consistent intervention sessions. 21 Rehabilitation programs that incorporate therapeutic exercises to enhance neuromuscular control of the spine are typically performed a number of times per week for multiple weeks (>3). 8 11,21 Future research should examine the cumulative effects of exercises on lateral abdominal-muscle thickness, motorpattern efficiency, timing, and patient-oriented outcomes such as pain relief, function, and quality of life. The participants in this study were recruited from the university community and from an athletic therapy clinic. They responded to a flier that stated eligibility criteria. The flier specifically targeted individuals with LBP 18 to 50 years of age who had increasing symptom frequency, more than 3 previous episodes of LBP, no symptoms distal to the knee, and no history of spine surgery. Most of the participants met the inclusion criteria since the recruitment methods provided prescreening of participants who were likely to already meet at least 3 criteria for a best-fit classification. Although the individuals in this study were currently experiencing an episode of LBP, they were not actively seeking medical care. Individuals who meet stabilization-classification criteria are thought to demonstrate the greatest deficit in the TrA s ability to thicken during an ADIM. 15 The average TrA thickness ratio for study participants before exercise intervention was 1.58, which was consistent with a previous study examining individuals with LBP who met stabilization-classification criteria. 15 Although a deficit in TrA activation may exist, the role of the TrA in neuromuscular control of the lumbar spine has been questioned Spine biomechanical research indicates that no single muscle is solely responsible for neuromuscular control of the lumbar spine, which occurs due to coordinated activity of a number of muscle surrounding the spine. 12 Although previous studies have demonstrated improvements in pain and function, this may have been due to the inability to completely target specifically one of the muscles surrounding the lumbar spine. In addition, the inability to adequately perform an ADIM (static controlled task) may not accurately reflect neuromuscular control of the spine during dynamic tasks. Future research should attempt to better identify individuals with LBP who would benefit from exercises to improve neuromuscular control of the spine. Conclusions Neither traditional- nor suspension-exercise-bridge progression has an immediate clinical effect on EO, IO, or TrA activation immediately after a single exercise intervention. Previous studies have shown bridging exercise utilizing the suspension-exercise device resulting in greater TrA muscle-thickness changes during exercise than a traditional-bridging exercise, indicating a facilitation of muscle activation. 27 The bridging exercise, regardless of surface stability, can still be considered an important component of a rehabilitation program for individuals with LBP, but its mechanism of effectiveness does not appear to be related to immediate changes in TrA activation. It is possible that changes would be evident after a comprehensive and progressive therapeutic exercise intervention program. Acknowledgments 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. References 1. Deyo RA, Mirza SK, Martin BI. 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