The Effect of Distractive Function on Volitional Preemptive Abdominal Contraction During a Loaded Forward Reach in Normal Subjects

PM R XXX (2016) 1-9

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Original Research

The Effect of Distractive Function on Volitional Preemptive Abdominal Contraction During a Loaded Forward Reach in Normal Subjects Marwan A. Kublawi, PT, ScD, FAAOMPT, Troy L. Hooper, PT, ATC, LAT, Vittal R. Nagar, MD, Mark P. Wilhelm, PT, DPT, Kevin L. Browne, PT, ScD, ´e, PT, ScD, FAAOMPT, Phillip S. Sizer, PT, PhD, FAAOMPT Jean-Michel Brisme

Abstract Background: Volitional preemptive abdominal contraction (VPAC) is used to protect the spine and prevent injury. No published studies to data have examined the effect of distraction on VPAC use during function. Objective: To examine the effect of an auditory distraction (“Stroop task”) on healthy subjects’ ability to sustain VPAC by use of the abdominal drawing-in maneuver during loaded forward reach. Design: Within-subjects, repeated-measure cohort design. Setting: Clinical laboratory setting. Participants: Convenience sample of 42 healthy individuals (ages 20-57 years). Methods: Transversus abdominis (TrA) thickness was measured with M-mode ultrasound imaging. Each subject performed Stroop versus no Stroop during 4 conditions: (1) without VPAC, quiet standing; (2) with VPAC, quiet standing; (3) without VPAC, forward reach; and (4) with VPAC, forward reach. An investigator blinded to the conditions measured the first 10 subjects to establish intratester reliability of probe/transducer placement and TrA measurement. Data Reduction: TrA thickness (mm) change represented VPAC performance. A single investigator measured onscreen TrA thickness twice at each second from second-6 through -10 on a recorded ultrasound imaging sequence. Results: A 2 (Stroop)
4 (Activity) repeated-measures analysis of variance found no significant Stroop
Activity interaction [F(3, 93) ¼ 0.345, P ¼ .793] and no main effect for Stroop [F (1,31) ¼ 1.324, P ¼ .259] but found a significant main effect for activity [F (3,93) ¼ 17.729, P < .001]. Tukey post-hoc pairwise comparisons demonstrated significant differences between VPAC versus no-VPAC conditions, except between quiet standing/yes-VPAC and loaded forward reach/no-VPAC conditions (P ¼ .051). The interclass correlation coefficient (3,2) for probe/transducer placement reliability was 0.87, 0.91, 0.92, and 0.93 for conditions 1-4, respectively. The interclass correlation coefficient (3,2) for TrA measurement reliability was 0.96, 0.99, 0.99, and 0.99 for conditions 1-4, respectively. Conclusion: A distracting executive function (Stroop task) did not produce a significant negative impact on normal individuals’ ability to sustain a VPAC during quiet standing or loaded forward reach activities.

Introduction In the United States, low back pain (LBP) is the second-leading cause for visits to the physician, the third most common cause for surgical procedures, and the fifth most common cause for hospitalization [1]. Approximately 85% of this population will experience at least 1 episode of LBP during their lifetime [1]. Up to 44% will experience another episode of LBP within

1 year, whereas 80% will experience a second episode within 10 years [1]. Clinicians use muscle-activation strategies that aim to protect the spine for LBP treatment and prevention. Muscle activation can either automatically or volitionally assist muscle cocontraction that promotes lumbar stiffness and control during functional tasks [2-4]. Volitional preemptive abdominal contraction (VPAC) can be used to support spine control and stability. A

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The Effect of Distractive Function on VPAC

VPAC strategy can activate the transversus abdominis (TrA) in individuals with and without LBP [5-7]. Furthermore, LBP sufferers can engage their TrA in various functional positions [6]. Individuals without LBP can produce a VPAC during functional positions such as standing [8-10] and loaded forward reach [11]. Similar responses have been recorded in individuals with a history of LBP but no present symptoms [6]. The TrA and internal obliques promote lumbopelvic stability by enhancing intra-abdominal pressure [12-14]. In addition, studies have shown that VPAC strategies suppress spinal perturbations and movements during lower limb activities [15-17]. These responses elucidate the capacity of a VPAC to stabilize the trunk during functional movements. It is not known, however, how cognitive distraction affects an individual’s ability to use such a protective strategy. Previous investigators found that a distracting executive function (EF, or “Stroop” task) decreases performance during functional activities (such as posture and gait) in healthy subjects [18-20]. Two studies reported that EF alters postural control in patients with LBP [21,22]. These reports merit further examination of the effect of a distracting EF on individuals’ trunk control during a functional task. At this time, no studies have examined how distracting EF influences one’s ability to sustain VPAC during a dynamic postural control activity. We hypothesized that incorporating a distracting EF would decrease a subject’s ability to sustain a VPAC during quiet standing and a loaded functional reach activity. The effect of using a distracting EF on VPAC during a loaded forward reach could represent using the strategy during real-life activities in which spine protection and injury prevention are paramount.

faculty, and staff from the local university; and (2) the investigators’ acquaintances. Inclusion and exclusion criteria can be found in Table 1. Subjects with a body mass index (BMI) greater than or equal to 30 were excluded because of the challenges in USI data collection as per previous investigators [6,11,23]. Instrumentation and Materials A LOGIQ P5 ultrasound system with a 4C-RS 2.0-5.5 MHz curvilinear probe (GE Healthcare, Milwaukee, WI) was incorporated. A water-based hypoallergenic gel was used for USI coupling. Ultrasound is a reliable, valid, and noninvasive tool for studying abdominal muscle performance [24,25] during loaded functional activities [11,20,26]. The M-mode USI provides instantaneous, visualized feedback and optimizes TrA measurement across time, which may best represent a contractile response during function [24,27,28]. The M-mode USI performs these measures reliably when subjects are positioned in supine lying, standing, and walking [8,29-33]. Preparatory Procedures After subjects signed an approved informed consent at Texas Tech University Health Sciences Center, they completed a demographics and medical history questionnaire. Then, subjects watched an instructional video regarding experimental procedures [34,35]. The subjects stood upright with feet positioned shoulder width apart. An investigator (P.S.) who is a physical therapist with extensive experience in VPAC implementation assessed the subject’s ability to achieve a proper ADIM according to previous investigators

Methods Research Design and Variables This study incorporated a 2 (Stroop)
4 (Activity) within-subjects design with repeated measures for all factors. The Stroop variable included 2 levels: yes-Stroop versus no-Stroop. The Activity variable included 4 levels: (1) Quiet standing, no-VPAC; (2) quiet standing, yes-VPAC using the abdominal drawing in maneuver (ADIM); (3) loaded forward reach, no-VPAC; and (4) loaded forward reach, yes-VPAC using the ADIM. The dependent variable was change in TrA muscle thickness (representing a contractile response) as measured by ultrasound imaging (USI) motion mode (M-mode) during each condition over second-6 through second-10 of each 10-second trial. Sampling and Subjects Forty-two subjects (both male and female) between ages 20 and 57 years were recruited from: (1) students,

Table 1 Subject inclusion and exclusion criteria Inclusion criteria Age 18-65 years Ability to activate TrA during a loaded forward reach activity Ability to stand at least 60 minutes independently Ability to follow instructions. Exclusion criteria Existing active spinal pain History of diagnosed LBP that required professional health care management Diagnosed and presently active abdominal, respiratory, or gastrointestinal condition Pregnancy, based on subject report Diagnosed spinal conditions to include: scoliosis, spina bifida, spinal pathologies, tumors, present fractures, and/or rheumatologic disorders Known neurologic or joint disease affecting the trunk Current urinary tract infection Hearing loss Cognitive disorders that hinder understanding simple directions Body mass index greater than or equal to 30. TrA ¼ transversus abdominis; LBP ¼ low back pain.

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[11,34,35]. The subject practiced this for 3 repetitions with a 10-second hold. The subject was then trained in the loaded (4.5 kg) forward reach activity while looking straight forward as per previous investigators [11,31]. The subject practiced the loaded forward reach activity for 5 repetitions, where he or she attained the forward reach limit in 5 seconds and remained in that position for 5 seconds. The subject was then asked to practice the activity for 10 additional repetitions while applying the ADIM. The investigator provided further demonstration and tactile cuing during all training when needed. The subject rested for 5 minutes after training completion. For data collection, baseline M-mode USI measurements were taken during the same subject position as described previously both with and without ADIM. The USI display was adjusted to a 60-mm view depth and the M-mode chart set to 15 seconds of continuous data. The USI probe/transducer with gel was positioned along the right lateral abdominal wall at the anterior axillary line, midway between the costal margin and iliac crest (Figure 1) [25,35,36]. The abdominal muscles were visualized and identified with the subject in quiet standing. The optimal transducer placement site was marked with a nontoxic marker. Each trial image was saved for later measurement.

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Data Collection For each trial, a 10-second M-mode USI image of the abdominal wall was obtained at the end of exhalation during all 8 conditions (Table 2). Specific instructions were issued regarding the no-VPAC and yes-VPAC responses, as well as the quiet standing versus loaded forward reach responses during the respective conditions in concordance with previous studies [6,11]. A previously investigated auditory Stroop program [37] was used in the present study to produce a distractive function during specific conditions (5-8). Auditory Stroop effects have been validated previously, where subjects were asked to judge the gender of an individual’s voice as the male or female voice stated words such as “man” or “girl.” When the gender of the voice was incongruent with the corresponding descriptor, subjects required longer times to classify the voice’s gender [37-39]. The present Stroop application carried the distraction and subsequent choice one step further by requiring the subjects to not only cognitively attend to auditory stimuli and first judge the gender of the individual’s voice (cognitive distraction), but then subsequently choose (cognitive distraction) the correct finger that corresponded to the voice and raise it (secondary motor task). This combination was aimed at increasing the level of distraction during the primary motor tasks of the experiment. Each 5-word Stroop combination in the present study came from lists of male or female names, as well as stereotypical masculine, feminine, or gender-neutral words (Table 3). Male and female voices stated the words and names into separate digital files. Congruent and incongruent word-voice pairs were delivered to the subject through headphones that were positioned over the subject’s ears. Congruent word-voice pairs were those in which a female voice stated a stereotypically feminine word or name and a male voice stated a stereotypically masculine word or name (such as “Amy” or “George,” respectively). Incongruent word voice pairs were those in which a female voice stated a stereotypically masculine word or name and a male voice stated a stereotypically feminine word or name (such as “Football” or “Lipstick,” respectively). Subjects were instructed to classify the gender of each voice as quickly as possible by lifting their left index finger for a male voice and right index finger for a female voice. Each Table 2 Testing conditions for all phases No-Stroop

Figure 1. Test position for data collection, showing an investigator with the ultrasound probe placed over the right TrA while an individual performs a loaded forward reach with VPAC. The image shows the subject raising the right index finger, which represents the subject hearing a female voice during the trial segment. The subject was not able to view the on-screen data, because of the angle of the screen.

Response

No-VPAC

Yes-Stroop Yes-VPAC

No-VPAC

Yes-VPAC

Quiet standing Condition 1 Condition 2 Condition 5 Condition 6 Loaded forward Condition 3 Condition 4 Condition 7 Condition 8 reach VPAC ¼ volitional preemptive abdominal contraction.

The Effect of Distractive Function on VPAC

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Table 3 Words and names that were used in the auditory Stroop task Female Stereotyped Words

Male Stereotyped Words

Gender-Neutral Words

Female Names

Male Names

Bracelet Cheerleader Doll Lipstick Lovely Makeup Nurse Perfume Pink Pretty

Baseball Captain Football Gun Pirate Punch Rough Soldier Tackle Tough

Apple Door Draw Paper Pencil Spoon Table Taste Walk Window

Amy Cindy Jenny Jill Julie Laurie Nancy Rachel Sarah Susan

Brian David George Henry Jason John Matthew Michael Peter Robert

word onset was separated by 2 seconds. The word trails continued beyond the physical completion of each trial. A 10-second M-mode USI image was obtained during each of the 8 conditions (Table 2) by use of the previously described data collection strategy. During conditions with a distractive EF (yes-Stroop task; Conditions 5 through 8), the subjects were encouraged to achieve the greatest accuracy possible in identifying the voice’s gender. Conditions 1 through 8 were randomized to control for practice effect and fatigue. A 10-second M-mode USI image was obtained during each of the 8 conditions at the end of exhalation, when the abdominal wall ceased to move inward, by use of the previously described data collection sequence. Each M-mode USI image was recorded and saved for later analysis. A single investigator blinded to measurement outcomes measured each image on the screen, where each measurement represented a 1-second interval at 5 different points in time (seconds 6 through 10) on the M-mode USI recording. A second investigator observed the posted measurement value and logged the data for subsequent analysis. Intratester reliability was established for both probe/ transducer placement and TrA measurement for a single investigator early in the study by exclusively using the first 10 enrolled subjects [26]. To establish reliability for probe/transducer placement, we analyzed the investigator’s consistency in placing the USI probe/transducer across different test days. Sufficient rest time (2 days) was placed between testing sessions to blind the rater and eliminate learning effects. Subjects returning 2 days later were reassessed for conditions 1 through 4 via the same procedure, after undergoing a brief training review. The examiners used the skin markings that were made 2 days previously. Testing was performed at the same time of day for each testing session. Data Reduction and Statistical Analyses For each trial, mean thickness was calculated from 5 measurements that were captured at 1-second intervals during second-6 through second-10 of the

M-mode trace. Measures for central tendency (means) and dispersion (standard deviations, 95% confidence interval) were established for both subject characteristics and USI measurements. An interclass correlation coefficient (3,2) was used to establish intratester reliability of both probe/ transducer placement and TrA measurement for a single investigator. A 2 (Stroop)
4 (Activity) repeatedmeasures analysis of variance was used to establish significant interactions and main effects for TrA thickness across the 8 resulting conditions. Tukey post-hoc tests with a Bonferroni correction were used to locate significant differences. Significance was set at a ¼ .05. Results A convenience sample of 42 healthy individuals, with an age range of 20-57 years, was recruited for the study. Five male and 5 female subjects were enrolled for the reliability portion of the study, whereas 25 female and 8 male subjects were recruited for the experimental phase of the study. All subjects had no previous history of abdominal or lumbosacral spine surgery. One female subject was dismissed from the study for poor TrA image quality, where the USI operating investigator could not successfully read the M-mode USI image during all trials because of the subject’s unique tissue that clouded the image during selected contractile states. This left our sample size to a total of 24 for experimental phase analysis. The mean and standard deviation for age, height, weight, and BMI are found on Table 4. The mean width and standard deviation of the TrA for each of the test conditions were documented in millimeters (Table 5). For intrarater reliability, the measuring investigator measured the saved image 2 times without replacement to randomize the order. Results can be viewed on Table 6. A 2 (Stroop)
4 (Activity) repeated-measures analysis of variance demonstrated no significant interactions for Stroop versus Activity [F(3, 93) ¼ 0.345, P ¼ .793; partial h2 ¼ .045; power ¼ 0.13]. No significant main effect was observed for Stroop [F(1,31) ¼ 1.324, P ¼ .259; partial h2 ¼ .041; power ¼ 0.20]. A significant

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Table 4 Subject descriptive data Age, y

Height, m

BMI, kg/m2

Weight, kg

Gender

N

Mean

SD

Mean

SD

Mean

SD

Mean

SD

P1-Female P1-Male P2-Female P2-Male

4 6 24 8

24.8 26.5 27.4 30.4

11.4 3.9 7.70 14.1

1.79 1.66 1.63 1.82

1.3 0.74 0.09 0.06

60.2 87.8 58.7 87.5

27.2 11.9 5.68 14.9

22.0 25.2 22.0 26.3

9.9 3.2 2.4 3.1

BMI ¼ body mass index; SD ¼ standard deviation; P1 ¼ Phase 1; P2 ¼ Phase 2.

main effect was observed for Activity [F(3,93) ¼ 17.729, P ¼ .001; partial h2 ¼ .62; power ¼ 1.000]. Pairwise comparisons demonstrated significant differences between VPAC and no-VPAC states (Table 7), except between Condition 2 versus Condition 3. Discussion To the best of our knowledge, this is the first study to examine the effect of a distractive cognitive event on a subject’s ability to maintain a VPAC during a functional activity. Previous investigators observed decreased posture and balance control in cognitively distracted subjects [9,20,40,41]; however, contrary to the present hypotheses, this study found that a cognitive distracter does not appear to correspond with a change in one’s ability to volitionally contract the TrA during a functional activity, which has meaningful clinical implications. Considering the high value for prevention in health care, the present study suggests that the use of VPAC can be applied to healthy individuals as a protective measure without great concern for the effects of distraction on the resulting muscle response. As clinicians, we educate individuals in using a VPAC strategy to protect their spine for injury prevention purposes and reduced potential for developing pain. Although the effectiveness of VPAC during a quiet position and various postures has been documented [5,42,43], the evidence supporting one’s ability to use a VPAC during a combination of distraction and functional activity is limited. The present results suggest that a person can incorporate a protective VPAC when performing functional tasks, such as reaching for a weighted object while concurrently attending to a conversation. Even though a person is attending to the distracting event, the distraction does not appear to prevent one from maintaining a VPAC for protecting the spine during function. Different strategic approaches to volitionally activating TrA have been investigated [44-47]. This study chose the ADIM because it can successfully create a deep volitional abdominal muscle contraction [35,48], emphasizing TrA and internal oblique activation [35,47,48]. This muscle activation changes during different postures in subjects without LBP [34,49], increasing when they are positioned upright against

gravity [8,9,43]. Investigators reported that subjects can engage a VPAC strategy during functional activities [11,50], and most of this study’s VPAC conditions produced a significantly greater TrA contraction versus the no-VPAC conditions. One may question the ceiling effect of the ADIM for increasing deep abdominal responses during the chosen function activity, as the activity itself may maximize the available muscle response not allowing for more changes with a VPAC. This study, however, found that the ADIM increased the TrA activation during the VPAC versus No-VPAC forward reach conditions. This suggests that a VPAC may produce greater muscle contractility versus the functional activity itself, which concurs with previous research [6,11]. Also, because TrA activation did not increase during the no-VPAC forward reach condition, a person may need to use a VPAC in a protective preparatory manner so to gain the benefits from increased abdominal activity. This study showed that it can be done and distraction does not appear to alter that ability. Limitations and Future Research The magnitude of the chosen auditory Stroop may not have been a large enough distraction to sufficiently change muscle contraction. Gregg et al [51] demonstrated that the use of gender-related incongruent words created a gradient in the distracting EF, subsequently enhancing an experimental Stroop effect over the use of congruent words with no gender bias. A similar gradient was used in this study by asking a male voice to speak the female terms such as “lipstick,” versus a female voice speaking a male terms such as “football.” Moreover, the time interval between each word initiation in our auditory Stroop was within the range of other investigators [52], who found that such a Stroop effect increases region-specific brain activity in the anterior cingulate cortex, dorsolateral prefrontal cortex, and presupplementary motor area. These areas are important in error detection [53], response selection [54], and the control of conflict management [55-57], implying the validity for an auditory Stroop using similar time intervals for challenging specific cognitive functions. Future research should examine the effect of changing auditory Stroop gradient on the variables that were measured in the present study.

The Effect of Distractive Function on VPAC

QS ¼ Quiet Standing; NV ¼ No-VPAC; YV ¼ Yes-VPAC, FR ¼ forward reach; VPAC ¼ volitional preemptive abdominal contraction; SD ¼ standard deviation; CI ¼ confidence interval.

6.958 6.653 6.8055 5.432 5.181 5.3065 6.196 5.917 6.056 4.877 4.873 4.875 No-Stroop Yes-Stroop Mean, mm

4.343 4.375 4.359

1.48 1.38 1.43

3.807 3.876 3.8415

5.853 5.741 5.797

1.76 1.86 1.81

5.217 5.071 5.144

6.488 6.411 6.4495

5.08 4.877 4.978

1.76 1.52 1.64

4.446 4.329 4.3875

5.713 5.425 5.569

2.12 2.04 2.08

Upper Lower Mean, mm Upper Lower Mean, mm Upper Lower Upper Mean, mm

SD  mm

Lower

Mean, mm

SD  mm

95% CI, mm QS/YV 95% CI, mm QS/NV, mm

Table 5 Descriptive data for Stroop versus no Stroop and activities during quiet standing and forward reach

FR/NV, mm

SD  mm

95% CI, mm

FR/YV

SD  mm

95% CI, mm

6

Table 6 Intrarater reliability for probe/transducer placement and TrA measurement Intrarater Reliability for Probe/Transducer Placement

Intrarater Reliability for TrA Measurement

Activity/VPAC Condition

ICC (3,1)

Activity/VPAC Condition

ICC (3,1)

QS/NV QS/YV FR/NV FR/YV

0.87 0.91 0.92 0.93

QS/NV QS/YV FR/NV FR/YV

0.96 0.99 0.99 0.99

TrA ¼ Transverse abdominis; VPAC ¼ volitional preemptive abdominal contraction; ICC ¼ intraclass correlation coefficient; QS ¼ quiet standing; NV ¼ No-VPAC; YV ¼ Yes-VPAC; FR ¼ forward reach.

Second, the chosen loaded functional reach activity may not have sufficiently challenged the subjects’ abilities to experience a change in VPAC. Although studies have demonstrated that this activity is related to both self-disability and pain intensity [58,59], future studies could use more complex movements to increase the challenge and broaden applicability. Because the use of USI for TrA measurement may be limited during more complex functional tasks by difficulty in probe access to the abdominal region, surface electromyography to the internal and external oblique muscles could be used to register dynamic abdominal activity during the task. Third, this study excluded individuals with a BMI greater than 30. Considering that many individuals who develop persistent LBP are challenged by increased body weight [60], this may confront the present results’ generalizability to individuals with a higher BMI. The only comparison that was not significantly different between a yes-VPAC versus no-VPAC condition was between conditions 2 and 3. A retrospective power analysis revealed that testing five more subjects might have produced statistical significance between these conditions. The established effect size and power analysis for activity in this study, per Portney and Watkins [61], demonstrated a high power of 1.000 and high effect size of h2 ¼ .62. Furthermore, the low Table 7 Pairwise comparison for activities conditions 95% Confidence Interval for Difference

Pairwise Comparisons for Activities

Sig

SE

Lower

Upper

QS/NV vs QS/YV QS/NV vs FR/NV QS/NV vs FR/YV QS/YV vs FR/NV QS/YV vs FR/YV FR/NV vs FR/YV

.001* .065 .001* .051 1.00 .013*

.210 .229 .289 .291 .201 .321

2.029 1.265 2.512 0.002 0.827 1.983

0.847 0.025 0.884 1.639 0.307 0.173

SE ¼ standard error; QS ¼ quiet standing; NV ¼ No-VPAC; YV ¼ YesVPAC; VPAC ¼ volitional preemptive abdominal contraction; FR ¼ forward reach. * Denotes significant difference at P < .05.

M.A. Kublawi et al. / PM R XXX (2016) 1-9

effect size and power for Stroop (h2 ¼ .041; power ¼ 0.20) and Stroop versus Activity (h2 ¼ .045; power ¼ 0.13) suggest that more subjects would not have produced significant differences in those relevant comparisons. Moreover, the comparison between conditions 2 and 3 was not related to any of the present research questions. A VPAC often is used as a clinical strategy to help control and decrease LBP. Nagar et al [6] reported that subjects with a LBP history but no current symptoms produced increased TrA activity when using a VPAC strategy during loaded forward reach, again suggesting that VPAC use can potentially protect the spine for similar individuals. The present findings, however, cannot be extrapolated to individuals with current LBP or a LBP history with no current symptoms. Thus, future studies should examine the effects of the same parameters on those groups.

9.

10.

11.

12.

13. 14.

15.

Conclusion This study demonstrates that a distracting EF (Stroop task) does not produce a significant negative impact on a subject’s ability to sustain a VPAC using the ADIM in normal subjects during quiet standing and loaded forward reach activities. Thus, these findings suggest that an individual may be able to activate a VPAC while performing a functional task, such as reaching for an object while distracted. This outcome implies that individuals can use VPAC to protect their spine during a functional activity without influence from daily life distractions.

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18. 19. 20.

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Disclosure M.A.K. Center for Rehabilitation Research, School of Allied Health Sciences, TTUHSC, Lubbock, TX Disclosure: nothing to disclose

V.R.N. Center for Rehabilitation Research, School of Allied Health Sciences, TTUHSC, Lubbock, TX Disclosure: nothing to disclose

T.L.H. Center for Rehabilitation Research, School of Allied Health Sciences, TTUHSC, Lubbock, TX Disclosure: nothing to disclose

M.P.W. Center for Rehabilitation Research, School of Allied Health Sciences, TTUHSC, Lubbock, TX Disclosure: nothing to disclose

M.A. Kublawi et al. / PM R XXX (2016) 1-9 K.L.B. Center for Rehabilitation Research, School of Allied Health Sciences, TTUHSC, Lubbock, TX Disclosure: nothing to disclose J.-M.B. Center for Rehabilitation Research, School of Allied Health Sciences, TTUHSC, Lubbock, TX Disclosure: nothing to disclose

9

P.S.S. Center for Rehabilitation Research, School of Allied Health Sciences, TTUHSC, 3601 4th St., Room 2B 138 - Mail Stop 6226, Lubbock, TX 79430. Address correspondence to: P.S.S.; e-mail: [email protected] Disclosure: nothing to disclose This study protocol was approved by the Institutional review Board at Texas Tech University Health Sciences Center. IRB Number L13-008. Submitted for publication May 12, 2015; accepted March 30, 2016.