Abdominal muscle activation: An EMG study of the Sahrmann five-level core stability test

Background: Sahrmann five-level core stability test protocol has been used to evaluate the ability of the core muscles to stabilize the spine. However, validation studies on the Sahrmann protocol are limited. Objective: The purpose of this study was to compare the different levels of Sahrmann five-level core stability (levels 1–5) on the muscle activity of rectus abdominis (RA), external oblique (EO), and transverse abdominis/internal oblique (TrA/IO). Methods: Twenty-two asymptomatic male participants aged 21.36±1.59 years were recruited. Participants were instructed to perform maximum voluntary contraction (MVC) and five levels of Sahrmann five-level core stability test guided with a pressure biofeedback unit (PBU). The surface electromyography (EMG) data of each muscle during five levels of Sahrmann five-level core stability test were normalized as a percentage of MVC. Results: Results showed significant differences in the normalized EMGs of RA [χ2(4) = 64.80, p<0.001], EO [χ2(4) = 58.11, p<0.001], and TrA/IO [χ2(4) = 56.00, p<0.001] between the five levels of Sahrmann five-level core stability test. Post-hoc analysis revealed Sahrmann levels 5 and 3 have significantly higher abdominal EMG signals than levels 4, 2, and 1 (p<0.001). Conclusion: In conclusion, the Sahrmann five-level core stability test differs according to the level of Sahrmann tests. Significantly higher abdominal muscle activities were observed during levels 3 and 5. Therefore, the classification exchange in levels 3 and 4 of the Sahrmann five-level core stability test should be reconsidered in the future.


Introduction
Core stability is de¯ned as the \ability of the lumbopelvic-hip complex to return to equilibrium following a perturbation without buckling of the vertebral column". 1 There are 29 muscles attached to the lumbopelvic-hip complex. 2 The muscles which were frequently mentioned in previous researches are the multi¯dus, which stabilizes the vertebral joints on each segmental level, and the transverse abdominis (TrA), which stabilizes the spine through the increment of intra-abdominal pressure. 3 Other super¯cial trunk muscles that contribute to spinal stability include abdominal muscles [rectus abdominis (RA), abdominal internal oblique (IO), and abdominal external oblique (EO)] and paraspinal muscles (erector spinae and quadratus lumborum). According to Barr et al.,3 these super¯cial muscles are activated to provide additional stability during direction and load-spe-ci¯c activity to prevent unwanted spinal displacement. It is highlighted that the entire core muscles have to be co-activated from all angles and directions to enable all layers of core muscles, deep and super¯cial to be physically bound together enhancing spinal stability and sti®ness to a higher degree. 4 Lower extremity movement protocol with pressure biofeedback transducer has been widely used by researchers to evaluate core stability performance. [5][6][7] One of the commonly used lower extremity movement protocols to measure core stability is the Sahrmann¯ve-level core stability test protocol. 7 Sahrmann 8 initially suggested a lower abdominal muscle progression grading which consists of nine di®erent lower extremity movement protocol maneuvers (nine-level ordinal scale, scored on a scale of 0.10-5.00). However, previous researchers have utilized a modi¯ed version which only adapted¯ve maneuvers from the original Sahrmann protocol. [9][10][11][12] The Sahrmann¯ve-level core stability test consists of¯ve di®erent leg lowering maneuvers (levels 1-5) that progressively increase in di±culty. 7 Details of the Sahrmann¯velevel core stability test was explained in Sec. 2.
Aggarwal et al. 9 have examined the relationship of commonly used core stability tests; Sorensen test, prone plank test, side plank test, abdominal fatigue test, and Sahrmann¯ve-level core stability test. Aggarwal et al. 9 discovered that the Sahrmann's test performance was only signi¯cantly correlated with the performance of prone plank test (Rho ¼ 0.408; p ¼ 0:009). It was highlighted that the signi¯cant correlation of Sahrmann¯ve-level core stability test with the prone plank test indicates that both tests speci¯cally evaluated the core stability performance in the sagittal plane. 9 Although the Sahrmann protocol may indirectly evaluate the ability of the core muscles to stabilize the spine, studies on validation of the Sahrmann protocol are limited. 7 Therefore, there is a need to validate the Sahrmann¯ve-level core stability test by comparing the muscle recruitment pattern amongst the di®erent levels/maneuvers of Sahr-mann¯ve-level core stability test particularly the maximum voluntary contraction percentage (MVC %) of RA, EO, and TrA/IO.

Materials and Methods
Participants Twenty-two asymptomatic male students from the Sports Centre took part in this study. The mean AE standard deviation of the age was 21.36 AE 1.59 years, of their weight was 65.83 AE 8.37 kg, of their height was 1.71 AE 0.06 cm, of their waist circumference was 74.39 AE 5.23 cm, of their hip circumference was 92.80 AE 4.59 cm, of their body mass index was 22.59 AE 2.2 kg/m 2 , and of their waisthip ratio was 0.80 AE 0.03. The participants were screened using the self-reported Nordic musculoskeletal questionnaire (NMQ). Individuals were excluded if they are having acute low back pain (LBP) (<7 days of LBP) or chronic low back pain (>12 months of LBP). Individuals with a waist circumference greater than 94 cm were also excluded to reduce surface electromyography (EMG) artifact due to adipose tissue. 13 The University Malaya Research Ethics Committee (UMREC) granted ethical approval for this study (Reference No. UM. TNC2/UMREC-338) and all participants provided written informed consent before their participation.

Data acquisition
The wireless Trigno TM system (Trigno, Delsys Inc., USA) was used to record and process the EMG signals of RA, EO, and TrA/IO. Surface electrodes were attached to the muscle¯bers on the righthand sides of the body based on the guidelines from the previous studies [14][15][16][17][18]  The raw EMG signals from the three muscle sites were ampli¯ed (common-mode rejection ratio (CMMR) of 130 dB, input impedance of 20,000 kÞ using a di®erential ampli¯er (Trigno Wireless System, Delsys Inc., Boston, USA) and¯ltered using a Butterworth band-pass¯lter of bandwidth between 20 Hz and 450 Hz. The signals were converted from analog to digital at a sampling rate of 2,000 Hz for data processing and analysis (EMGworks 3.0, Delsys Analysis Software, Boston, USA). In order to minimize speci¯c interfering frequency (i.e., electrical and radio frequencies), the EMG data acquisition was conducted in a controlled and contained laboratory.

Normalization exercises
To standardize the action potential of each of the three abdominal muscles, all participants performed three MVCs against manual resistance as suggested by Vera-Garcia et al. 19 The following are the MVC maneuvers for respective abdominal muscles: (1) Rectus abdominis: Participants were in a sit-up posture positioned on a bench with the knees bent. They then attempted to°ex the upper trunk in the sagittal plane while their thorax was manually braced by the experimenter.
(2) External oblique: Participants attempted to side bend the upper trunk in the frontal plane while they were in a side-lying position, with the knees bent, and thorax and arms were manually braced by the experimenter. (3) Transverse abdominis/internal oblique: Participants maintained a right-side bridge position while a maximally resisted downward pressure on the pelvis was applied by the experimenter.
During the MVC testing, the participants were instructed to avoid any jerky contractions to decrease the chance of injury. To prevent muscle fatigue while the MVC was measured, each maneuver was performed in a random order, with each being held for 5 s and repeated twice with a 2-min rest between trials to prevent muscle fatigue. 13 Sahrmann¯ve-level core stability test All participants were trained to perform the¯ve levels of the standard protocol of the Sahrmann ve-level core stability test as suggested by Aggarwal et al. 9 correctly before the measurement as described below. Each participant completed three trials at each of the¯ve levels, with at least a 1-min rest between trials.
The in°atable pad of a stabilizer pressure biofeedback unit (PBU) (Chattanooga Group, Inc., Hixson, TN) was placed under the lumbar spine at approximately L4-L5. While the participant was lying in a crook-supine position, the PBU was in°ated to 40 mmHg. Participants were told to perform the Sahrmann¯ve-level test while drawing-in their abdomen to avoid pressure deviation of more than 10 mmHg. A deviation of pressure more than 10 mmHg indicates that the stabilization action of stabilizer muscle has been lost. 9,10 After the familiarization phase, the participants performed random orders of Sahrmann ve-level core stability test with abdominal muscle EMG testing and utilizing the PBU as their biofeedback. Participants were given a 2-min rest in between two test levels to reduce muscle post-activation potential carryover e®ects. Participants performed three trials for each level with at 1-min rest between trials. The average percentages of MVC derived from the three trials of each level were used in data analysis.
The¯ve levels of Sahrmann test are described in Table 1 and illustrated in Fig. 2.

Data processing
The EMG signals generated during both MVCs testing and Sahrmann¯ve-level core stability testing were analyzed and¯ltered using root-meansquare (RMS) technique (EMGworks 3.0, Delsys Analysis Software, Boston, USA). The MVC for each muscle was determined by calculating the peak EMG signal throughout the 5-s period of MVC. The starting and end of each EMG signal of the Sahrmann test were manually determined by assessing the muscle activities at baseline (rest periods), during the test, and at the end when the muscle activity returned to baseline. 20 Lastly, the EMG activity during Sahrmann¯ve-level core stability test was normalized by the MVCs for each muscle.

Statistical analysis
Statistical analysis was carried out using IBM SPSS Statistics for Windows, version 25.0 (IBM, Armonk, NY). The normality of data was tested using the Shapiro-Wilk test. Categorical and continuous data were then analyzed using the most appropriate statistical tests based on data distribution. The Friedman two-way analysis of variance (ANOVA) by ranks was performed to test for the di®erences in EMG activities. Where a signi¯cant di®erence emerged, a multiple comparison procedure with the Wilcoxon signed-ranked test was used to test which pairwise di®erences were signi¯cant. A Bonferroni-adjusted alpha level was used to safeguard for the Type-1 error to be accepted as signi¯cant. The signi¯cance value for all was set at p < 0.05.

Sample size calculation
An a priori analysis was conducted for sample size calculation of repeated-measures ANOVA (within factors) using GÃPower for Windows, version 3.1.7. 21 The e®ect size from past-related used was from Ref. 22. The power analysis showed that F -test with an e®ect size of 0.328, alpha and statistical power equal to 0.05 and 0.95, respectively, with one group and¯ve repeated measures yielded a sample size of 19 participants with an actual power of 0.96. A 15% sample size calculation adjustment for non-parametric Friedman test was added for the¯nal sample size of this study to become 22 participants. 23 Table 1. Five progressive levels of Sahrmann¯ve-level core stability test. 9 Level Description Level 1 The participant slowly raised one leg to a position of approximately 90 hip°exion with 90 knee°exion. The participant then attempted to bring the opposite leg also to the same position in the same manner. Level 2 From the¯nal position of the previous level, the participant slowly lowered one leg such that the heel contacted the ground/plinth. Then the leg slid out to fully extend the knee. Level 3 From the end position of level 1, the participant slowly lowered one leg such that the heel reached approximate 12 cm above the ground. Then the leg slid out to fully extend the knee. Level 4 From the¯nal position of level 1, the participant slowly lowered both legs together such that the heels contacted the plinth. Then the legs slid out to fully extend the knees. Level 5 From the¯nal position of level 1, the participant slowly lowered both legs simultaneously such that the heels reached 12 cm above the ground. The legs then slid out to fully extend the knees.

Results
Comparison of EMG activity within ve levels of Sahrmann¯ve-level core stability test  Abdominal muscle activation: EMG study of Sahrmann¯ve-level core stability test 93

Comparison of EMG activity within muscles
This study also compared EMG activities within abdominal muscles (RA, EO, and TrA/IO) during each level of Sahrmann¯ve-level core stability tests. No signi¯cant di®erence was found between RA, EO, and TrA/IO EMG activities during each level of Sahrmann¯ve-level core stability test (p > 0.05).

Discussion
The purpose of this study was to evaluate the abdominal muscle activity (RA, EO, and TrA/IO) during di®erent levels/maneuvers of Sahrmann ve-level core stability test. The Sahrmann¯velevel core stability test was assumed to evaluate the abdominal muscle function as they act by isometrically contracting into°exion to maintain lumbar spine°exion by posteriorly tilting the pelvis against an assumed increasing resistance. The abdominal muscle activity levels are supposed to increase from Sahrmann levels 1 to 5.
The results of this study suggest that the abdominal muscle activity varies between the¯ve di®erent maneuvers of Sahrmann core stability test, with the highest abdominal muscle activities recorded during level 5, followed by level 3, level 4, level 2, and level 1. To our best knowledge, this is one of the few studies which explore abdominal muscle activity during non-heel contact and heel contact during leg lowering maneuvers. This study showed that the abdominal muscle activity was higher when heels do not contact the ground as noted in levels 3 and 5. A similar study by Gilleard and Brown 24 showed that thigh unsupported leg lowering yields greater abdominal muscle activity than thigh supported with hands leg lowering. This shows less recruitment of abdominal muscles may be needed during supported lowering (levels 2 and 4) as compared to unsupported lowering (levels 3 and 5) because participants may have utilized the lower limb muscles to support the abdominal muscles to maintain the neutral lumbar curvature.
Our study also reveals similar abdominal muscle synergy pattern in unilateral leg lowering (level 3) and bilateral leg lowering (level 5). Our¯ndings in this study are supported by those of Richardson et al. 25 who also reported no signi¯cant di®erence in abdominal muscle activity between pelvic tilt with unilateral leg lowering and pelvic tilt with bilateral leg lowering. However, this study contradicts with the¯ndings from Ref. 24 which observed lower RA activity but higher IO and EO muscle activities in bilateral leg lowering as compared to unilateral leg lowering. Shields and Heiss 26 proposed two muscle synergy patterns during bilateral leg lowering. The¯rst pattern included high RA, high EO, and low IO muscle activations, while the second pattern includes low RA, high EO, and low IO muscle activations. The Sahrmann level-5 bilateral leg lowering test di®ers from the conventional leg lowering test as the leg lowering maneuvers are started from 90 hip and knee°exion compared to conventional leg lowering test which started in a 90 hip°exion with knees extended, therefore di®erent muscle synergy patterns may occur. During conventional leg lowering, when legs are lowered from a vertical position to a horizontal position, high external torque is created by the mass of the limbs (about 30% of the body weight) resulting in a great challenge to the abdominal muscles. 22,27 This study has several limitations that needed to be addressed. First, the cross-talks between surface electrodes are minimized in this study by using standardized surface EMG electrode position as described by previous researchers. [14][15][16][17][18] McGill et al. 15 proposed the location of surface TrA/IO electrodes which accurately re°ects the muscle activity of deep abdominals. However, the TrA/IO electrodes are still susceptible to cross-talk as they lie underneath super¯cial muscles. 20 Furthermore, the recorded EMG signal from the TrA/IO electrode would come largely from IO rather than TrA because IO is more super¯cial than the TrA. 28  higher RA and EO muscle activities among women compared to their male counterparts. 29 Additionally, higher RA and EO with lower IO muscle activities were reported among patients with low back pain compared to healthy individuals. 30 Further exploration of abdominal muscle activity pattern during the Sahrmann core stability test among women and individuals with musculoskeletal conditions is warranted. Recommendations for future research include: (1) examining the abdominal muscle activity pattern of Sahrmann¯ve-level core stability test in di®erent populations, i.e., females and individuals with musculoskeletal conditions; and (2) investigating the e®ects of adding upper limb movements during performing Sahrmann¯velevel core stability test.

Conclusion
To our best knowledge, this is one of the few studies which investigated the Sahrmann¯ve-level core stability test on abdominal muscle activity.
The results of this study showed that the abdominal muscle responds di®erently in supported (levels 2 and 4) or unsupported (levels 3 and 5) positions. Therefore, consideration should be taken in the future for the classi¯cation exchange in levels 3 and 4 of the Sahrmann¯ve-level core stability test. Correspondingly, classi¯cation of the Sahr-mann¯ve-level core stability test into two smaller subcategories can also be considered, i.e., Poor (levels 1, 2, and 4) and Good (levels 3 and 5). As all the abdominal muscle sites were recruited below 40% of maximal isometric contraction, endurance bene¯t could be obtained with an appropriate number of repetitions. This may indicate that the Sahrmann¯ve-level core stability test can be used as a screening tool and an abdominal endurance training regime.