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ASSESSMENT OF THE VALIDITY AND

RELIABILITY OF THE SIDE BRIDGING TEST

Laurence Sinatti

01510368

Maud Van de Casteele

01503162

Louise Van de Walle

01505889

Promotor: Dr. Cedric De Blaiser

Copromotor: Prof. Roel De Ridder

A dissertation submitted to Ghent University in partial fulfilment of the requirements for the degree of Master

of Science in Rehabilitation Sciences and Physiotherapy

(Rehabilitation Sciences and Physiotherapy with Musculoskeletal Afflictions)

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ASSESSMENT OF THE VALIDITY AND

RELIABILITY OF THE SIDE BRIDGING TEST

Laurence Sinatti

01510368

Maud Van de Casteele

01503162

Louise Van de Walle

01505889

Promotor: Dr. Cedric De Blaiser

Copromotor: Prof. Roel De Ridder

A dissertation submitted to Ghent University in partial fulfilment of the requirements for the degree of Master

of Science in Rehabilitation Sciences and Physiotherapy

(Rehabilitation Sciences and Physiotherapy with Musculoskeletal Afflictions)

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EXPRESSION OF GRATITUDE

The master's thesis was a project that was completed over a period of two years. This study is the gathering of hard work during the second master year in Rehabilitation Sciences and Physiotherapy. None of this would have been possible without the help of a number of people and therefore we would like to thank them.

First of all, we would like to thank prof. De Ridder for guiding us through the first year of our thesis. Thank you for your support, patience, advice and the time you always made for us. We would also like to thank Dr. De Blaiser. He was prepared to guide us through the second year of this thesis. A sincere thank you for everything you have done for us! Thank you for the enthusiasm you expressed concerning this subject, for answering our numerous questions and emails and for your time to attend our testing moments.

Secondly, this thesis would not have been possible without the participation of the individuals being tested in this study. Thank you for your precious time and energy. We would also like to thank the University of Ghent for the provision of their facilities and equipment, to make all these measurements and their processing possible.

Finally, we would like to thank our family and friends. Our friends for answering our questions, for the motivating words and the necessary distraction and relaxation. Our family for giving us the opportunity to study, for their unconditional support, for listening to our complaints and for allowing us to grow into who we are today.

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TABLE OF CONTENTS

List of figures and tables

...

List of abbreviations

...

Literature study

... 8

1

Abstract

... 8

1.1

Nederlands ... 8

1.2

English ... 9

2

Introduction

... 10

3

Method

... 12

3.1

Design ... 12

3.2

Participants ... 12

3.3

Protocol ... 12

3.4

Equipment and data analysis ... 14

3.5

Statistical analysis ... 15

3.5.1

Validity ... 15

3.5.2

Reliability ... 16

4

Results

... 17

4.1

Study characteristics: population ... 17

4.2

Validity ... 17

4.2.1

Differences in NMFslope values between oblique abdominal and lumbar muscles on the supporting side versus non-supporting side (Hypothesis one) ... 17

4.2.2

Differences in NMFslope values between oblique abdominal and lumbar muscles versus stabilizing shoulder muscles (Hypothesis two) ... 18

4.2.3

Pearson correlations and Multiple Stepwise Regression (Hypothesis three) ... 19

4.3

Reliability ... 19

5

Discussion

... 20

5.1.1

Comparison NMFslope values oblique abdominal and lumbar muscles (Hypothesis one) ... 20

5.1.2

Comparison NMFslope values oblique abdominal/lumbar muscles and stabilizing shoulder muscles (Hypothesis two) ... 21

5.1.3

Pearson correlation and Multiple Stepwise Regression (Hypothese three) ... 21

5.1.4

Conclusion ... 22

6

References

... 25

Abstract in lekentaal

...

Etics Committee

...

Appendix

...

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LIST OF FIGURES AND TABLES

Figure 1: EMG electrode placement

Figure 2: Starting position of the side bridging test

Figure 3: NMFslope values (%/s) of the oblique abdominal and back muscles on the supporting and non-supporting side

obtained during the side bridging test

Figure 4: NMFslope values (%/s) of the oblique abdominal, back and stabilizing shoulder muscles on the supporting and

non-supporting side obtained during the side bridging test

Table 1: Anthropometric characteristics men Table 2: Anthropometric characteristics women

Table 3: Pearson correlation coefficients (r) between NMFslope values and endurance time

Table 4: Intratester and intertester reliability values of the side bridging test

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LIST OF ABBREVIATIONS

EMG electromyography

ICC intraclass correlation coefficient LT Lower Trapezius

MeF median frequency MF Multifidus

NMFslope normalized amplitude and fatigue slope

OE External Oblique OI Internal Oblique PD Posterior Deltoid QL Quadratus Lumborum RA Rectus Abdominus sEMG surface electromyography SEM standard error of measurement ULT Iliocostalis Lumborum Pars Thoracis UT Upper Trapezius

XXc contralateral side XXs supporting side

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8

LITERATURE STUDY

1

ABSTRACT

1.1

Nederlands

Achtergrond: Rompstabiliteit wordt in de huidige maatschappij beschouwd als een belangrijk onderdeel van zowel trainings- als preventieprogramma's. Om dit aspect op een snelle, veilige en goedkope wijze te kunnen onderzoeken, kan een veldtest gebruikt worden, zoals de side bridging test. Deze test wordt in de praktijk gebruikt om de maximale uithoudingscapaciteit van de rompspieren te beoordelen. Echter werd de validiteit van deze test nog niet vastgelegd en werd er slechts in vijf studies een goede betrouwbaarheid gerapporteerd.

Doelstelling: Het doel van deze studie was te bepalen of de side bridging test tot uitputting een valide veldtest is en te bekijken of de huidige testmethode van de side bridging test betrouwbaar is.

Onderzoeksdesign: Enerzijds werd op basis van cross-sectioneel onderzoek de validiteit geverifieerd, anderzijds werd de betrouwbaarheid onderzocht door middel van een test-hertest design.

Methode: Dertig gezonde en sportieve participanten (15 mannen en 15 vrouwen) werden geïncludeerd in dit onderzoek. Om de validiteit van de side bridging test te bepalen, werd oppervlakte elektromyografie uitgevoerd op 11 verschillende buik-, rug-, en schouderspieren. De genormaliseerde mediane frequentie van het elektromyografische power spectra - een maatstaf voor de spiervermoeidheid - werd gebruikt in de statistische analyse. De vermoeidheid tussen de spieren werd onderling vergeleken en de correlatie tussen de vermoeidheid en de uithoudingstijd werd onderzocht. Deze analyse onderzocht de verschillen tussen de spieren onderling en hun correlatie met de uithoudingstijd.

Om de intertester betrouwbaarheid te onderzoeken, werden de participanten op twee afzonderlijke dagen door twee verschillende onderzoekers geëvalueerd. De intratester betrouwbaarheid werd onderzocht door één onderzoeker die deze test twee keer afnam op dezelfde dag.

Resultaten: Na statistisch onderzoek werd gezien dat er significante verschillen zijn tussen de genormaliseerde mediane frequentie (NMFslope) van de onderzochte buik- en rugspieren van zowel de steun- als niet steunzijde, als tussen

de NMFslope van deze spieren met de schoudermusculatuur. Er werd geen correlatie gevonden tussen de NMFslope van de

onderzochte spieren en de uithoudingstijd. Geen enkele NMFslope kon deze tijd voorspellen. De enige voorspellende factor

van de uithoudingstijd was het geslacht.

Qua betrouwbaarheid, toonde de statistische analyse aan dat er een excellente intratester (ICC = 0,89) en een excellente intertester (ICC = 0,88) betrouwbaarheid was.

Conclusie: De isometrische side bridging test tot uitputtig is geen valide test om de maximale uithoudingscapaciteit van de onderzochte spieren te beoordelen. Dit onderzoek rapporteert wel een excellente betrouwbaarheid.

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1.2

English

Background: In today's society, core stability is considered an important component in athletic training and injury prevention. A field test is a fast, safe and inexpensive way to measure this aspect, such as the side bridging test, used to assess the maximum endurance capacity of the core muscles. The validity of this test has not yet been established, however a good reliability was reported in five studies.

Objectives: The aim of this study was to establish whether the side bridging test to failure is a valid field test and to investigate if the current method of performing the side bridging test is reliable.

Study design: The validity was examined based on a cross-sectional design and the reliability was investigated using a test-retest design.

Methods: Thirty healthy and athletic participants (15 men and 15 women) were included in this study. To determine the validity of the side bridging test, surface electromyography (sEMG) was performed on 11 different oblique abdominal, back, and stabilizing shoulder muscles. The normalised median frequency slope (NMFslope) of the electromyography

(EMG) power spectra - an indicator of muscle fatigue - was used in statistical analysis. The muscle fatigue was compared and the correlation between fatigue and endurance time was investigated. This analysis examined the differences between the muscles interplay and their correlation with endurance time.

To investigate the intertester reliability, the participants were evaluated on two separate days by two different researchers. The same tester examined the test two times on the same day, to investigate the intratester reliability. Results: After statistical analysis, significant differences were observed between the NMFslope of the abdominal and

dorsal muscles, both on the supporting and non-supporting sides, and between the NMFslope of these muscles and the

stabilizing shoulder musculature. No correlation was found between the NMFslope of the investigated muscles and the

endurance time and no NMFslope is able to predict the endurance time until failure. The only predictor of the time was

gender.

The statistical analysis concerning the reliability, showed an excellent intratester (ICC = 0.89) and an excellent intertester (ICC = 0.88) reliability.

Conclusion: The isometric side bridging test to failure is not a valid test to assess the maximum endurance capacity of the investigated muscles. However, there is excellent reliability to report.

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2 INTRODUCTION

Core stability has become a popular concept in the sport environment (Norris, 1993; Butcher et al., 2007). This may be defined as a complex anatomical unit that combines core muscle strength, endurance, proprioception and neuromuscular control. Required to maintain the position and motion of the trunk throughout the kinetic chain (Bergmark, 1989; Richardson & Jull, 1995; Hodges & Richardson, 1996; Panjabi, 2003, Ireland et al., 2003; Kibler et al., 2006; Hammill et al., 2008).

An optimal functioning core system ensures proper force distribution and maximum force generation with minimal compressive, translational or shearing forces in the joints of the kinetic chain during functional movement (Fredericson, 2005). Furthermore, considerable benefits have been touted, from improving athletic performance and preventing injuries, to relieving low back pain (Akuthota et al., 2008).

The link between core stability and injuries has been established several times. Multiple researchers have observed low back pain as a recurring phenomenon when a deficiency, in the functioning of the core muscles, can be found (Wilson et al., 2005). De Blaiser et al. (2018C) and Hamill et al. (2008) proved the preliminary connection between impaired aspects of core stability (core strength, core endurance, core proprioception and neuromuscular control) and the development of lower extremity injuries in healthy athletes. Therefore, the stability of the core muscles should be considered when screening athletes (De Blaiser et al., 2018A, 2018C, 2019; Rivera, 2016).

Core stability can be screened in several manners going from laboratory settings using expensive materials that require a certain experience, to field tests. Field tests are defined as on the spot assessments of a physical characteristic (Vaara et al., 2012). They are commonly used thanks to their many advantages such as time efficiency, low cost, the possibility to screen several participants simultaneously and the absence of the need for specific equipment (Evans et al., 2007). Field tests can be used for many purposes such as epidemiological studies, fitness assessments, in school and military settings and injury risk assessment (Vaara et al., 2012). Despite intensive research, only four tests measuring core stability were found reliable and valid: the prone bridging test to measure abdominal muscle endurance capacity (De Blaiser et al., 2018A), the hand held dynamometry to measure isometric trunk flexion and extension strength (De Blaiser et al., 2018B), the Biering-Sørensen to measure trunk extension muscle endurance capacity (Casto et al., 2018) and the flexion-rotation trunk test to measure trunk flexion muscle endurance capacity (Casto et al., 2018).

Furthermore research on validity and reliability of the widely used side bridging test has not yet been established. The side bridging test is an isometric holding test in side position and is often used to measure the endurance capacity of abdominal and back muscles (Escamilla et al., 2014; Ekstrom et al., 2007). Despite the fact that there is little evidence concerning the side bridging test, it is often used as an exercise in rehabilitation and as a testing method (Friedrich et al., 2017; Sandrey & Mitzel, 2013).

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11 A high reliable test provides comparable results under consistent conditions, meaning that the same results are obtained when the testing procedure is performed at a different moment by the same or different tester. In five different studies good (Friedrich et al., 2017) to excellent (Chan et al., 2005; Evans et al., 2007; Palmer & Uhl, 2011; McGill et al., 1999) reliability was found following the interpretation of intraclass correlation coefficient (ICC) values based on Cicchetti and Sparrow (1981). It is also key to know whether this test gives an accurate representation of the core stability of the subject. At present, there is no evidence of the validity concerning the side bridging test.

The aim of this study is to investigate whether the current method of performing the side bridging test is valid to evaluate the maximal endurance capacity of the oblique abdominal and thoracolumbar muscles. To provide an answer to this aim, three distinct hypotheses are defined. First, it is hypothesized that the side bridging test elicits significant fatigue of the oblique abdominal and lumbar muscles on both sides. This in contrast with the belief that only the muscles on the unilateral testing side will be activated (Kavcic et al., 2004). Second, it is hypothesized that the stabilizing shoulder muscles on the supporting side show a similar magnitude of fatigue as the core muscle corset during the test. Third, it is hypothesized that the endurance time of the side bridging test to failure is not a good indicator of the maximal endurance capacity of the abdominal and thoracolumbar muscles. As such we hypothesize that the side bridging test is not a valid test to evaluate maximal endurance capacity of the abdominal and thoracolumbar muscles given the complexity of the exercise and the involvement of other muscle groups that might be a limiting factor to the test performance.

In addition, it was hypothesized that the standardized protocol of the side bridging test, used in this study, by means of visual inspection, tactile feedback and assessment criteria, is reliable. This hypothesis will be investigated based on the interpretation of the reliability analyses.

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3 METHOD

3.1

Design

The validity of the side bridging test was investigated by a cross-sectional design. During the first testing moment the sEMG activity of the involved muscles was measured under supervision of examiner one. A test-retest design was used

to investigate reliability. These measurements were performed in two different phases: phase one was performed at the

first test moment, where the side bridging test was performed without sEMG under the supervision of examiner one. One week later, in phase two, the same protocol was repeated by examiner two.

3.2

Participants

Thirty healthy participants (15 men and 15 women) were selected to participate in this study. The participants were recruited voluntarily through social media and through advertising of the researchers in their social circles. All participants were at least 18 years old and participated in competitive sports activities requiring regular training sessions. The following exclusion criteria were used: history of low back pain requiring a consultation of the general practitioner and/or physiotherapist, injury of the upper and/or lower limb (less than three months ago) or known systemic pathology present, physical discomfort at the time of testing (anywhere on the body) or inability to assume the side bridging test position correctly.

3.3

Protocol

The study was approved by the ethics committee of Ghent University. All subjects signed an informed consent. To determine the validity of the side bridging test, bilateral sEMG was performed on the External Obliquus (EO), Internal Obliquus (IO), Iliocostalis Lumborum Pars Thoracis (ULT) and Multifidus (MF) muscles and unilaterally on the dominant side of the Posterior Deltoid (PD), Lower Trapezius (LT) and Upper Trapezius (UT) muscles. In addition, the intratester and intertester reliability of this test was examined by organizing two different test moments separated by one week. All participants were asked not to perform any sport activities - if possible - for a period of 24 hours before testing. Each participant was evaluated under the same conditions in a research laboratory of Ghent University Hospital (UZ Gent, Ghent, Belgium).

The first test moment was used to assess the validity and intertester reliability. The subject was measured and weighed and then prepared for application of the EMG electrodes. This process included shaving, scrubbing and cleaning with alcohol to lower the impedance between skin and the electrodes. The placement of the electrodes is shown in Figure 1.

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Figure 1: EMG electrode placement

Before each testing moment a standardized cardiovascular warm-up was performed. Between the warm-up and the first endurance test, the participants were informed on the correct position of the side bridging test. This position can be observed in Figure 2. It was indicated to hold this position as long as possible till fatigue or pain forced the participant to stop the test, meaning the test is performed until failure. The subjects were allowed a familiarization attempt. During the performance of the side bridging test, the examiner always gavethe same verbal instructions and standardized encouragement for every subject.

Figure 2: Starting position of the side bridging test

The side bridging test was performed on the left and right side (McGill et al., 1999), starting with the dominant side of the participant. The initial position was executed as follows: the feet were lying on top of each other, the legs were straightened, the forearm was perpendicular to the mat, resulting in a 90° angle between the upper arm and the mat and a closed fist was incorporated. From this position the participant was asked to push him/herself up so that only the supporting arm (elbow-forearm-fist) and the feet made contact with the ground. Special attention was given to not touching the ground by the calves and a straight axis made by the position of the head, torso, legs and feet. The hand of the non-supporting side was placed on the opposite shoulder (the arm crossed the chest). As soon as this position was reached, the time started. When one of the following termination criteria was fulfilled, the chronometer was stopped. The first reason was that the person could no longer hold the position correctly, secondly the test subject ended the test

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14 him/herself and finally the subject was no longer able to hold the correct position for two seconds after giving adjustments. These corrections, tactile and/or verbal, were standardized and permitted. Tactile feedback to correct the position was given at the pelvic crest, when the pelvis was lowered to the ground or raised. During this test, EMG measurements were performed. There were five minutes rest between testing the left and right side and 20 minutes between the first and second test series. In the second test series the same protocol was repeated without EMG measurements.

The second test moment was used to assess the intratester reliability and was conducted one week after the first testing moment. After a standardized cardiovascular warm-up, the first test series on the left and right side were performed with five minutes rest intersession. After a 20-minute rest period, the second test series was executed.

3.4

Equipment and data analysis

During this study, EMG signals were recorded with a 18-channel surface EMG system (Ultium EMG; Noraxon USA Inc., Scottsdale, Arizona, USA). The processing of the data has been performed analogue to the processing of data in the article of the prone bridging test (De Blaiser et al., 2018a). The raw EMG signals were analogue bandpass-filtered between 10 and 500 Hz, amplified (common mode rejection ratio > 100 dB, overall gain 1000, noise <1 μv RMS) and converted analogue to digital (12-bit) at 1000 Hz sampling rate.For these measurements 11 Blue sensor electrodes were used. The registration and processing were carried out in Noraxon’s Myoresearch v3.14 (Noraxon USA Inc.) and Matlab R2020a (MathWorks USA Inc., Natick, Massachusetts, USA).

The raw data of the EMG signals were ECG reducted, full-wave rectified and smoothed using a root mean square with a moving average window of 100 ms. Using Noraxon’s Myoresearch and Matlab as mentioned earlier, each recorded EMG signal during the side bridging test was divided into intervals of one second. Each one second interval of the median frequency (MeF) of the EMG power spectrum was calculated using the Fast Fourier transform (FFT) algorithm. The NMFslope of the EMG signal of each muscle during the side bridging test was calculated. The MeF slope was used to

represent muscle fatigue. Median frequency can be defined as the frequency dividing the spectrum into two equal areas. The fatigue in the muscles causes a decrease of the frequency content of the EMG signal, often described as a decline of the parameter MeF (Coorevits et al., 2008; Manninon et al., 1998) On the calculated MeF of the EMG signal of each side bridging test, linear regression analyses were performed in function of time. In addition, the intercept of the regression line was determined as the initial MeF and the slope of the regression line as the MeF slope. The MFslope was automatically normalized related to the interception of the regression with the formula (MFslope/MFinit)x100 (Coorevits et

al., 2008), because EMG parameters can be influenced by differences in subcutaneous tissue layers (between muscle locations of the same subject). This is further referred as the NMFslope.

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3.5

Statistical analysis

3.5.1 Validity

The following statistical analyses were performed using the software program SPSS v26.0 (SPSS Inc., Chicago, Illinois, USA).

Two approaches were used to define the normality of the variables. The first approach was the Shapiro-Wilk test. The normality was established on the basis of the p-value exceeding 0,05. Secondly, the distribution was visually assessed through Q-Q plots.Since all the variables used in this study had an approximately normal distribution, parametric tests were used for statistical analyses.

In order to verify the hypotheses, the following statistical analyses were performed.

For hypothesis one, the One-Way Repeated Measure ANOVA was used, after all the assumptions of the test were required, to compare the NMFslopes of the abdominal/thoracolumbar muscles on the supporting side with the values on

the contralateral side. If the result was significant, a Post-Hoc Pairwise Comparison with Bonferroni correction was executed to identify significant differences. A One Sample T-test was performed to see if these NMFslopes deviated

significantly from the horizontal. The steeper the decline - deviation from zero - the more the muscle is fatigued.

One-Way ANOVA was re-used for hypothesis two to compare NMFslopes of the abdominal/thoracolumbar muscles with

NMFslopes of the stabilizing shoulder muscles. To identify the differences, a Post-Hoc Pairwise Comparison was used in

case of a significant result of the One-Way ANOVA. Analogue to hypothesis one, the One Sample T-Test was used to determine the fatigue.

Hypothesis three was verified based on two calculations. First, the Pearson correlation coefficient was determined between the NMFslopes of the different muscles and the endurance time. This test was conducted to examine their

interconnection. The interpretation of the correlation coefficients (r) has been established in accordance with Cohen (1998): low = 0.10-0.30, moderate = 0.30-0.50 and high >0.50. The same approach was applied to the negative correlation coefficients (Cohen J. 1998). Secondly, a multiple backward linear regression analysis was performed to determine if a certain NMFslope is able to predict the endurance time.

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3.5.2 Reliability

The reliability was examined based on the endurance times performed during the side bridging test. In order to quantify the reliability, an ICC with a 95% confidence interval was used to illustrate the degree of agreement between the values. A two-way random effects model with single measure reliability (ICC [2,1]) was performed. To assess these ICC values the interpretation according to Cicchetti and Sparrow (1981) was used with the following values of reliability: low value <0.40, moderate value 0.40-0.59, good value 0.60-0.75 and excellent value if >0.75. Finally, the standard error of measurement (SEM) was calculated. This is a way to measure how the test scores are spread around a "true" value or how the scores vary when repeating the measurements (SEM = SD 1 − 𝐼𝐶𝐶).

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4 RESULTS

4.1

Study characteristics: population

During the side bridging study 30 participants (mean age 22,3 ± 1,8 years old; mean height 1,75 ± 0,112 m; mean weight 69,5 ± 9,5 kg and mean BMI 22,6 ± 1,8 kg/m²), including 15 men and 15 women, were involved. The anthropometric characteristics are given by gender in Tables 1 and 2.

Table 1: Anthropometric characteristics men

N 15

Mean age (years) 23,33 ± 1,80

Mean height (m) 1,83 ± 0,076

Mean weight (kg) 75,72 ± 6,23

Mean BMI (kg/m²) 22,56 ± 1,85

Table 2: Anthropometric characteristics women

N 15

Mean age (years) 22,60 ± 1,85

Mean height (m) 1,67 ± 0,077

Mean weight (kg) 63,27 ± 8,04

Mean BMI (kg/m²) 22,60 ± 1,79

4.2

Validity

4.2.1 Differences in NMFslope values between oblique abdominal and lumbar muscles on the

supporting side versus non-supporting side (Hypothesis one)

First, when all assumptions were fulfilled, the One-Way Repeated Measures ANOVA was conducted on the different NMFslopes of the lumbar and oblique abdominal muscles of both the supporting and non-supporting side. An overall

significant difference was found (p < 0,05). To further investigate this, a Post-Hoc Pairwise Comparison with Bonferroni correction was performed (with Bonferroni p-value < 0,003). A significant difference in NMFslopes was found between the

External Oblique on the supporting side (OEs) and External Oblique on the contralateral side (OEc), the OEs and ULTs, OEc and MFc and lastly the OIc and MFc. Subsequently, in the results of the One Sample T-test, the OEs, OIs, ULTs, ULTc, MFs and MFc, differed significantly from zero and the OEc and OIc showed no significant difference. In other words the most exhausted muscles were the OEs, OIs, MFs and MFc. The muscles that showed the least fatigue were the OEc and OIc.

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Figure 3: NMFslope values (%/s) of the oblique abdominal and back muscles on the supporting and non-supporting

side obtained during the side bridging test

(OEs: External Oblique supporting side, OEc: External Oblique contralateral side, OIs: Internal Oblique supporting side, OIc: Internal Oblique contralateral side, ULTs: Iliocostalis Lumborum Pars Thoracis supporting side, ULTc: Iliocostalis Lumborum Pars Thoracis contralateral side, MFs: Multifidus supporting side, MFc: Multifidus contralateral side. The error bars indicate the standard deviation of 95%). *Significant differences after Post-Hoc Pairwise Comparison with Bonferroni correction (p < 0,003).

4.2.2 Differences in NMFslope values between oblique abdominal and lumbar muscles versus

stabilizing shoulder muscles (Hypothesis two)

Parallel to supporting versus non-supporting side, the One-Way Repeated Measures ANOVA was performed on the NMFslope of the oblique abdominal and lumbar muscles versus the stabilizing shoulder muscles, where a significant

difference was found (p < 0,05). The Post-Hoc Pairwise Comparison with Bonferroni correction (with Bonferroni p-value < 0,003) showed a significant difference in the NMFslope of the OEs and LT, MFs and LT as well as the UT and LT. The

NMFslope of UT and PD differs significantly from zero. This in contrast to the LT NMFslope. The most exhausted muscles were

UT and PD and the LT, was least exhausted.

Figure 4: NMFslope values (%/s) of the oblique abdominal, back and stabilizing shoulder muscles on the

supporting and non-supporting side obtained during the side bridging test

(OEs: External Oblique supporting side, OIs: Internal Oblique supporting side, ULTs: Iliocostalis Lumborum Pars Thoracis supporting side, MFs: Multifidus supporting side, UT: Upper Trapezius, LT: Lower Trapezius, PD: Posterior Deltoid. The error bars indicate the standard deviation of 95%).*Significant differences after Post-Hoc Pairwise Comparison with Bonferroni correction (p < 0,003).

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4.2.3 Pearson correlations and Multiple Stepwise Regression (Hypothesis three)

In order to establish the correlation between the NMFslopes of the muscles and the endurance time, the Pearson

correlation coefficient was calculated. The values can be found in Table 3. The coefficients range between –0,07 and 0,302. Values below 0,30 are considered minimal to non-existent correlations according to Cohen (1988), meaning there is no significant correlation.

Table 3: Pearson correlation coefficients (r) between NMFslope values and endurance time

Muscle OEs OEc OIs OIc ULTs ULTc MFs MFc UT LT PD

Pearson correlation coefficient (r) 0,302 0,016 0,279 0,144 0,227 -0,002 0,120 0,260 0,262 -0,007 0,118 (OEs: External Oblique supporting side, OEc: External Oblique contralateral side, OIs: Internal Oblique supporting side, OIc: Internal Oblique contralateral side, ULTs: Iliocostalis Lumborum Pars Thoracis supporting side, ULTc: Iliocostalis Lumborum Pars Thoracis contralateral side, MFs: Multifidus supporting side, MFc: Multifidus contralateral side, UT: Upper Trapezius, LT: Lower Trapezius, PD: Posterior Deltoid)

To examine whether an independent variable is capable of predicting the endurance time of the test, the Multiple Stepwise Regression was used. The normalized NMFslope was the independent variable and the endurance time was the

dependent variable. None of the NMFslopes could predict the endurance time, since the values were not significant. If

gender was chosen as an independent variable, a significant result was obtained (p < 0,023). The R square value of 0,171 indicates that 17% of the time can be determined by the linear relation with gender.

4.3

Reliability

The overall mean of all endurance times measured during this study was 101,15 ± 36,70 seconds, with a minimum recorded endurance time of 44,32 seconds and a maximum recorded endurance time of 222,40 seconds.

For the side bridging test, the intratester and intertester reliability assessments were conducted with 30 subjects and two assessors. The values shown in Table 4 were used to calculate reliability. The endurance times of the tester one on day one (mean time: 107,6 ± 38,2 seconds) and tester one on day two (mean time: 97,5 ± 37,2 seconds). Subsequently, the endurance time of tester two day two (mean time: 98,4 ± 34,3 seconds), was used. Table 4showed an excellent intratester reliability with ICC = 0,89 with a SEM of 12,5 seconds and an excellent intertester reliability with ICC = 0,88 with a SEM of 12,4 seconds.

Table 4: Intratester and intertester reliabilities of the side bridging test

N (ICC 2,1) 95% CI SEM (s)

Intratester (tester 1) 30 0,89 0,68-0,96 12,5

Intertester (tester 1 and 2) 30 0,88 0,76-0,94 12,4

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5 DISCUSSION

The side bridging test is frequently used as an exercise during training and rehabilitation, but also as a test to assess core stability, in particular core muscle endurance. However, there is only limited evidence regarding the reliability and no scientific evidence concerning validity. The aim of this study was to determine whether the standard protocol for the side bridging test to failure, with tactile feedback, visual inspection and assessment criteria, may be considered as a valid and reliable test.

5.1

Validity

5.1.1

Comparison NMFslope values oblique abdominal and lumbar muscles (Hypothesis one)

To start, it was examined whether the side bridging test equally fatigues the muscles of the supporting side as the muscles on the non-supporting side. For this purpose the NMFslope of the oblique abdominal and lumbar muscles of both

sides were compared. The One Sample T-test showed that the side bridging causes fatigue of all dorsal muscles, i.e. both supporting and non-supporting side. This in contrast to the oblique abdominal muscles where only the supporting side showed a significant difference in NMFslope, causing only fatigue on the supporting side. Thus the oblique abdominal

muscles on the contralateral side, namely the OE and the OI, had no significant difference from zero, meaning that these muscles did not experience noticeable fatigue during this test. The first hypothesis should therefore be rejected. The OE on the supporting side had the greatest decline, therefore indicating that this muscle experienced the most fatigue. Looking at the results of the Post-Hoc Pairwise Comparison, one can observe that the following muscle pairs show a significant difference in persistence, namely the OEs with OEc, the OEs with ULTs, OEc with MFc and similarly the OIc with the MFc. This was confirmed by the above-mentioned result of the One Sample T-test.

The side bridging test can be used as a method of testing, but also as a method of training the endurance capacity of the lumbar musculature and the homolateral oblique abdominal muscles, namely the OE and OI. McGill et al. (1996A) and Evans et al. (2007) already confirmed that the side bridging test is the most effective way to train the abdominal obliques. Another study by Juker et al. (1998) showed that a high activity of Quadratus lumborum (QL) during the side bridging exercise could be measured. The QL was not included in this study as its muscle activity cannot be captured by sEMG electrodes. However, McGill et al. (1996B) already stated that the measured activity of the ULT is strongly correlated to the activity of the QL and therefore the superficial muscles can be used as a representation of the deep musculature. In McGill et al. (1999), a general conclusion was formulated, namely that the isometric bridging exercise challenges the QL and the muscles of the abdominal wall to enhance spine stability.

Ekstrom et al. (2007) describes, besides intense activity of the EO muscle, a high activity of the Gluteus medius muscle. Escamilla et al. (2016) reports the activity of the Rectus Abdominus (RA) muscle. The hip muscles and RA were not included in this review.

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21 In general, when looking at the spinal muscle corset, the MF and ULT muscles of both sides are depleted during the test, as well as the oblique abdominal muscles on the supporting side. These muscles show exhaustion, which is important to take into account when using this exercise in a testing or training context. Other studies have reported activity of the RA, QL and Gluteus Medius muscle. When performing the isometric side bridging test, all these muscles will be trained.

5.1.2 Comparison NMFslope values oblique abdominal/lumbar muscles and stabilizing shoulder

muscles (Hypothesis two)

The NMFslope values of the oblique abdominal and back muscles were compared to those of the stabilizing shoulder

muscles to examine if these were equally exhausted during the test. This hypothesis has been investigated in a similar manner as described in the previous paragraph. The decline of the NMFslope of three stabilizing shoulder muscles, namely

the LT, UT and PD, were assessed with the One Sample T-test. Based on the results, it can be concluded that the PD and UT fatigued while performing the side bridging test, while the LT did not. In the Post-Hoc Pairwise Comparison test a significant difference was seen between the UT and LT, which confirms the above mentioned result of the One Sample T-test. Between the UT and the oblique abdominal muscles of the supporting side, namely the OI and OE, a significant difference could be found, meaning these muscles did not fatigue at the same level.

It was noted that the activity of the stabilizing shoulder musculature was barely reported in other side bridging studies, although it may be suspected that these muscles are an influencing factor on the outcome of the side bridging test.

5.1.3 Pearson correlation and Multiple Stepwise Regression (Hypothese three)

The third hypothesis was based on two approaches. First, the correlation between the NMFslopes and the endurance time

was established. Second, it was examined if the endurance time could be predicted based on a NMFslope.

For the first approach, no correlation could be found in the side bridging test, which indicates that no NMFslope can predict

the endurance time. The comparison can be made with the article of the prone bridging test by De Blaiser et al. (2018A), where a strong correlation was found between the endurance time and the NMFslope of the RA, the OE and OI during the

prone bridging test. Meaning that the endurance time can be predicted by the maximum fatigue of those muscles. In other words, the longer one is able to maintain the prone bridging test, the more tired those muscles will be. This is in contrast to the side bridging test.

Secondly, it was explored if the endurance time can be predicted through a variable, such as the NMFslope. The results

showed no endurance time could be predicted by a NMFslope,meaning that no muscle activated during this test has the

ability to predict the maximum endurance duration. The only predictor of the outcome of the test, namely the duration, was the independent variable gender.

A similar conclusion was found during a subjective questioning. After each endurance test, the participant was asked about the reason for termination, namely pain or exhaustion, with respectively the Visual Analogue Scale (0-10) and

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22 Borg Rating of Perceived Exertion Scale (0-10) used as measurement tool, followed by the most prominent location of pain or exhaustion (scales can be found in the appendix). Of all participants, 86,6% indicated fatigue as the reason for termination, while only 13,3% stated pain. Concerning the fatigued location, 36,67% indicated the deltoid region and 20% the abdominal region. The other participants indicated combinations of different locations. The pain locations were different for each participant. Based on these questions there appeared to be no uniform answer, either for the reason of terminating the test or for the painful/fatigued location.

5.1.4 Conclusion

The general conclusion concerning validity is that the side bridging test elicits fatigue in all lumbar and oblique abdominal muscles, with the exception of the OEc and OIc, as well as in the stabilizing shoulder muscles, except the LT. According to these results, the side bridging test performed to failure is not a good test to judge the maximum endurance capacity of these muscles, since no objective fatigue of these muscles was correlated to the endurance time, nor could it predict the endurance time. Only gender was a good predictor of the outcome of the side bridging test.

5.2

Reliability

The side bridging test according to the current standardized protocol, including visual inspection, tactile feedback and assessment criteria, showed an excellent intratester (ICC = 0,89) and an excellent intertester (ICC = 0,88) reliability. The reliability of the side bridging test was already investigated in the past. The following three studies (Chan et al., 2005; Evans et al., 2007; McGill et al., 1999) all used a protocol similar to the one used in this study.

Chan et al. (2005) compared the ICC values between the side bridging performance on the left (ICC = 0.89) and right (ICC = 0.76) side of a healthy population who all were dominant on the right side. The side of executing the side bridging test had no influence on the endurance time. The ICC values reported by Chan et al. (2005) are similar to those found in this study. McGill et al. (1999) found ICC values ≥ 0,96. However, it should be noted that the population used in this study only consisted of five test subjects. In Evans et al. (2007), the same protocol was used as in McGill et al. (1999). The excellent ICC-values for intratester reliability (ICC = 0,85; left side and ICC = 0,82; right side) and intertester reliability (ICC = 0,82; left side and ICC = 0,91; right side) obtained by subjecting 24 participants to the side bridging test, were lower than those reported in McGill et al. (1999) and more similar to those found in this study.

The next two studies also investigated the reliability of the side bridging test, however, this was performed with an adapted version of the starting position and/or testing protocol. In both Palmer & Uhl (2011) and Friedrich et al. (2017), adjustments were made to the testing position. In the first-mentioned study, the feet were placed on top of each other and the angle of the supporting arm was reduced to less than 90° (adjusted to 80°- 85°). For the second study mentioned, three different positions were selected from bending the legs to lifting the upper leg. In Palmer & Uhl (2011) excellent intertester (ICC = 0,96; left side and ICC = 0,91; right side) values were reported. Friedrich et al. (2017) reported a correlation coefficient of 0,69 indicating a good reliability. However, this study focused more on the scoring of the side

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23 bridging test, rather than testing the maximum endurance capacity of the core muscles. The results found in previous studies showed that good to excellent reliability could be reported when performing these modified side bridging tests.

Overall, a high intratester and intertester reliability of this test may be reported. The test is a simple static test to perform in which the starting position is determined and the testing ends when the subject is no longer able to maintain the correct position. These limited guidelines help to improve the reliability of this test. Based on the results of both previous studies and this study, it can be concluded that for the side bridging test in its current form, good to excellent reliability can be reported.

5.3

Limitations

Some methodological considerations need to be taken into account when assessing the results of this study. Firstly, it is worthwhile to mention that the participants included in this study were young, all in their 20's, athletic and healthy. It could be interesting to investigate the side bridging test in other populations, such as patients or elderly, as it may yield different results.

Further, choices had to be made regarding which muscles would be submitted to sEMG as a limited number of channels were available. At shoulder level only the muscle activity of the UT, LT and PD were registered. Besides these muscles, other important stabilizing muscles in the shoulder joint can be noted such as the rotator cuff musculature. Of these muscles, the Subscapularis muscle and the Infraspinatus muscle are difficult to reach by means of sEMG due to their deep position. Previously found research indicated that measuring the activity of the following muscles - M. latissimus dorsi, M. pectoralis major and M. triceps - is of little relevance when performing the side bridging test. Parallel to the above-mentioned musculature, the same limitations should be seen regarding the measurements of the RA activity on both sides and the activity of the supporting hip muscles.

During this research the measurements were based on the dominant side of the test subjects. The question is whether a difference can be noticed if one would perform the test on the non-dominant side. Comparison between these results may uncover differences.

Finally, the intertester reliability was based on the measurements of two assessors. Three or more assessors would provide a better representation of the reliability of this test.

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24

5.4

Conclusion

According to this study, including a young and healthy population, it can be concluded that the side bridging test until failure is reliable, but not valid for evaluating maximal endurance capacity. Excellent reliability was reported, both inter- and intratester reliability, implying that when the test is repeated at a different time or assessed by another examiner, comparable results will be obtained. It was concluded that the side bridging test is not a valid test, meaning that the maximum endurance capacity of the examined muscles cannot be determined based on this test. However, it was seen that the oblique abdominal muscles on the supporting side, the back muscles on both sides and the stabilizing shoulder muscles of the supporting side fatigued, while performing the side bridging test. Therefore, these muscles could be trained by using the side bridging test.

Further research regarding the validity of this field test, involving the straight abdominal muscles and the hip muscles, is needed.

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25

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ABSTRACT IN LEKENTAAL

In dit onderzoek werd getracht een antwoord te geven op de vraag of de zijwaartse plank, bekend als de side bridging test, een goede test is om de vermoeidheid van bepaalde spieren te onderzoeken en dus valide is. Daarnaast werd onderzocht of deze test betrouwbaar is om uit te voeren, onafhankelijk van het aantal uitgevoerde pogingen of de evaluator.

Dertig personen namen deel aan onderstaand onderzoek, ieder op twee afzonderlijke testmomenten. Op het eerste testmoment werd via elektroden - rechtstreeks aangebracht op de huid ter hoogte van vooraf bepaalde schuine buik-, rug- en schouderspieren - de elektromyografische activiteit gemeten. Uit deze signalen kon de vermoeidheid van bepaalde spieren worden nagegaan. Daarnaast werd in beide testmomenten telkens 2 linker en 2 rechter zijwaartse planken uitgevoerd en beoordeeld door twee verschillende personen omwille van de betrouwbaarheid.

Algemeen kan besloten worden dat het uitvoeren van de zijwaartse plank tot uitputting, geen valide test is om de maximale uithoudingscapaciteit van de onderzochte spieren te beoordelen. Daarnaast is er wel een excellente betrouwbaarheid aangetoond, wat aangeeft dat onafhankelijk van de tester of het herhalen van de testen, vergelijkbare resultaten zullen worden bekomen.

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APPENDIX

1. Visual Analogue Scale (0-10)

2. Borg Rating of Perceived Exertion Scale (0-10)

0 helemaal niet vermoeid

1 zeer weinig 2 3 4 matig 5 6 7 8 zeer sterk 9 10 maximaal vermoeid

Afbeelding

Figure 2: Starting position of the side bridging test
Table 1: Anthropometric characteristics men
Figure  4:  NMFslope  values  (%/s)  of  the  oblique  abdominal,  back  and  stabilizing  shoulder  muscles  on  the  supporting and non-supporting side obtained during the side bridging test
Table 4: Intratester and intertester reliabilities of the side bridging test

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