Human Movement Science 45 (2016) 119–129
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Various shrug exercises can change scapular kinematics and scapular rotator muscle activities in subjects with scapular downward rotation syndrome Ji-Hyun Lee a, Heon-Seock Cynn a,⇑, Woo-Jeong Choi a, Hyo-Jung Jeong a, Tae-Lim Yoon b a b
Applied Kinesiology and Ergonomic Technology Laboratory, Department of Physical Therapy, The Graduate School, Yonsei University, South Korea Department of Physical Therapy, College of Health Science, Cheongju University, South Korea
a r t i c l e
i n f o
Article history: Received 26 February 2015 Revised 20 November 2015 Accepted 20 November 2015
Keywords: Frontal plane Spinal stabilization Upper trapezius
a b s t r a c t Scapular dyskinesis, characterized by scapular downward rotation syndrome (SDRS) affects scapula-humeral rhythm and results in shoulder dysfunction. Previous study has led to the recommendation of standard shrug exercise to contend with SDRS and strengthen the upper trapezius (UT) muscle. However, few researchers have examined which shrug exercise is most effective. The aim of this research was to compare scapular kinematic changes and scapular rotator muscles activity across three different shrug exercises in SDRS. The amounts of scapular downward rotation were measured by a caliper and the scapular upward rotation angle was measured using two digital inclinometers. Surface electromyography was used to measure EMG amplitude from the UT, lower trapezius (LT), serratus anterior (SA), and levator scapula (LS). Seventeen subjects with SDRS were recruited for this study. The subjects performed three shrug exercises with 30° shoulder abduction (preferred shrug, frontal shrug, and stabilization shrug). The stabilization shrug showed a significantly greater scapular upward rotation angle compared with the preferred shrug (P = 0.004) and frontal shrug (P = 0.006). The UT activity was significantly greater in the frontal shrug than in the preferred shrug (P = 0.002). The UT/LS muscle activity ratio was also significantly greater in the frontal shrug than in the preferred shrug (P = 0.004). The stabilization shrug should be preferred to enhance the upward rotation angle. In addition, the frontal shrug can be used as an effective method to increase UT activity and to decrease LS activity in SDRS. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction The upper trapezius (UT) muscle is the key contributor to normal scapular motion and control and it activates the scapular upward rotators in normal shoulder functions (Moraes, Faria, & Teixeira-Salmela, 2008; Wadsworth & Bullock-Saxton, 1997). Muscle imbalance between the elongated UT muscle and the shortened levator scapulae (LS) may cause scapular dyskinesis, characterized by a drooping scapula and increased downward rotation. This type of dyskinesis is defined as
⇑ Corresponding author at: Department of Physical Therapy, The Graduate School, Yonsei University, 1 Yonseidae-gil, Wonju, Kangwon-do 220-710, South Korea. E-mail address: [email protected] (H.-S. Cynn). http://dx.doi.org/10.1016/j.humov.2015.11.016 0167-9457/Ó 2015 Elsevier B.V. All rights reserved.
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scapular downward rotation syndrome (SDRS) (Sahrmann, 2002). Increased UT muscle length in SDRS does not allow for the transfer of weight from an upper extremity load to the sternoclavicular joint (Johnson, Bogduk, Nowitzke, & House, 1994). Furthermore, increased stiffness of the LS muscle may contribute to an increased compressive load and shear force on the cervical spine during active neck movement (Szeto, Straker, & Raine, 2002). Previous investigations show that LS can be activated simultaneously with UT during shoulder rehabilitation exercises, such as rowing, shoulder abduction, and shrug exercises (Moseley, Jobe, Pink, Perry, & Tibone, 1992). Thus, the UT muscle is the focus of therapeutic exercise protocols for rehabilitation from SDRS. Traditionally, the standard shrug exercise has been prescribed in shoulder rehabilitation programs to strengthen the UT muscle (Burkhead & Rockwood, 1992; Hintermeister, Lange, Schultheis, Bey, & Hawkins, 1998). However, previous studies reported that the shrug, with a 30° shoulder abduction exercise (i.e., completing the exercise with 30° of shoulder abduction, rather than with the arm by the side), generates greater activity in the UT muscle, as well as the serratus anterior (SA) and lower trapezius (LT), than the standard shrug (Pizzari, Wickham, Balster, Ganderton, & Watson, 2014; Watson, Balster, Finch, & Dalziel, 2005; Watson, Pizzari, & Balster, 2010). For this reason, the shrug with a 30° shoulder abduction exercise has been recommended for increasing the activation of the scapular upward rotators. Although the shrug with 30° abduction was proven to be superior to the traditional shrug, other shrug exercises could potentially produce even greater benefits for training the UT and upward rotators. Previous investigators reported that UT muscle activity was higher during arm abduction in the frontal plane (abduction plane) compared to the sagittal (flexion) plane (Antony & Keir, 2010). This is likely due to the relatively increased constant moment arm in the frontal plane compared with the flexion plane (Hughes & An, 1996). In addition, Hellwig and Perrin (1991) demonstrated that abduction within the frontal plane requires significant scapular upward rotation. Previous studies also revealed that the trapezius increases its activity as the plane of humeral elevation moves from the sagittal plane to the frontal plane (Bagg & Forrest, 1986; Inman, Saunders, & Abbott, 1944). These findings indicate that the exercise plane, where the shrug exercise is performed, is another critical factor that should be taken into consideration because the UT muscle may activate differently depending on the exercise plane during the shrug exercise. Moreover, previous researchers have focused on altered craniocervical and thoracic postures during scapular movement because spinal alignment is thought to influence scapular position and movement (Cole et al., 2013; Kibler, 1991). Others have reported that scapular upward rotation significantly decreased in flexed-head positions (Ludewig & Cook, 1996) and decreased with greater thoracic kyphosis (Finley & Lee, 2003; Kebaetse, 1999). Cervical stability may require synergistic muscle actions for active shoulder functions (Behrsin & Maguire, 1986). The LS can be particularly activated as a cervical stabilizer (extensor), rather than a scapular elevator, during arm movements because it directly connects the cervical spine to the scapula (Kendall, McCreary, & Provance, 2007). However, no study has determined the influence of the exercise plane and craniocervicothoracic stability during shrugs with 30° shoulder abduction exercises on scapular kinematic changes and scapular rotator muscle activity, even though arm elevation on various planes, and craniocervical and thoracic stabilization have been shown to affect scapular kinematics and scapular rotator muscle activity during shrugs with 30° shoulder abduction exercise. The purpose of this research was to compare the amount of scapular downward rotation (SDR), scapular upward rotation angle, UT, LT, SA, and LS electromyography (EMG) activity, and the EMG activity ratio of UT/LS during three different shrug exercises in SDRS, including the preferred shrug, frontal shrug, and stabilization shrug. The hypothesis is that the amounts of SDR, scapular upward rotation angle, UT, LT, SA, LS EMG activity, and the EMG activity of the UT/LS ratio would differ during three different shrug exercises in SDRS. The results of current study could provide valuable evidence to shoulder rehabilitation strategies for individuals with SDRS. 2. Methods 2.1. Subjects G-power software was used for the power analyses. The necessary sample size of seven subjects was calculated using data obtained from a pilot study of 10 subjects to achieve a power of 0.80 and an effect size of 0.59 (calculated from the partial g2 of 0.26 from the pilot study), with an a level of 0.05. Finally, 17 subjects with SDRS participated including 10 participants of the pilot study (age = 19.82 ± 1.60 years, height = 167.77 ± 6.92 cm, weight = 56.93 ± 6.38 kg, BMI = 20.19 ± 1.52, and amount of SDR before the exercises = 1.05 ± 0.55 cm). The SDR determination was confirmed using a caliper. This SDR measurement was modified using Kibler’s method to measure scapular alignment (Kibler, 1991; Watson et al., 2010). Both sides were measured to determine the amount of SDR in the subject. Next, the side that had greater scapular downward rotation was used in the data collection process (one subject had SDR in the right side and 16 Subjects had SDR in the left side). The exclusion criteria were a history or clinical exam revealing pain or dysfunction that substantially limited shoulder motion or resulted in gross instability of the shoulder during daily activities, signs and symptoms of cervical pain, adhesive capsulitis, thoracic outlet syndrome, or a current complaint of numbness or tingling in the upper extremity, forward head posture, and scoliosis. Prior to participation, the subjects provided written informed consent. The investigation was approved by Yonsei University Wonju Institutional Review Board.
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2.2. Instrumentation Surface EMG data were collected using a Noraxon TeleMyo-DTS (Noraxon, Inc., Scottsdale, AZ, USA) and analyzed using the Noraxon MyoResearch 1.06 XP software. The EMG signals were amplified, band pass-filtered (10 and 450 Hz), and notchfiltered (60 Hz, 120 Hz) before being recorded digitally at 1000 Hz and processed into root-mean-square data. Input impedance was 1 megaOhm. The common mode rejection rate ratio was 92 dB at 60 Hz. The data were collected from the UT, LT, SA, and LS on the tested side. After shaving and rubbing the skin with alcohol, disposable Ag/AgCl surface electrodes were placed on each muscle at standardized sites (Criswell, 2010). The UT electrodes were placed at one-half the distance between the cervical spine at C7 and the acromion (Hermens, Freriks, Disselhorst-Klug, & Rau, 2000). The LT electrodes were placed at an oblique vertical angle with one superior and one inferior electrode at a point 5 cm inferomedial from the root of the spine of the scapula. The SA (lower part) electrodes were attached just below the axillary area at the level of the inferior tip of the scapula and medial to the latissimus dorsi. The LS electrodes were placed between the posterior margin of the sternocleidomastoid and the anterior margin of the UT (Ludewig & Cook, 1996). Two electrodes were placed approximately 20 mm apart in the direction of the muscle fibers. The electrode pairs were placed parallel to the target muscle fibers. Inter-electrode distance was 10 mm (Holtermann & Roeleveld, 2006). Correct electrode placement was confirmed by visual inspection of the EMG signals on a computer screen during specific muscle testing. EMG data were collected for 5 s during the isometric phase and calculated using the middle 3 s of each exercise to avoid any connecting element of the skin electrode, as well as possible starting or ending effects (Ayotte, Stetts, Keenan, & Greenway, 2007). 2.3. Testing procedure First, the SDR determination was confirmed using a caliper. The subjects were standing with their arms relaxed by their sides. With a pen, the examiner marked three anatomical landmarks (Lewis, Green, Reichard, & Wright, 2002), including (1) the root of the scapular spine, (2) inferior angle of the scapula, and (3) the second and seventh thoracic spinous processes. The amounts of SDR were calculated using the following equation: distance between the second thoracic spinous process and spine of the scapula minus the distance between the seventh thoracic spinous process and inferior angle of the scapula. Positive values indicate downward rotation (Fig. 1). This study measured inter- and intra-rater reliabilities. Inter- and intrarater reliabilities of SDR determination using a caliper was examined by two raters who were experienced physical therapists with 1 and 2 years of clinical experience. Before data collection, the raters underwent a 2-h training session to ensure standardization of the caliper and accurate location of reference points by the principal investigator (JHL), who had 10 years of clinical experience. To examine intra-rater reliability, rater 1 measured two sessions of SDR determination. To avoid measurement recall, 1 h was allowed between sessions. The interclass correlation coefficient (ICC) of the inter-rater reliability was 0.85 (95% confidence interval [CI]: 0.63–0.94, standard error of measurement [SEM]: 0.04 cm). The ICCs of the intrarater reliabilities were 0.88–0.96 (95% CI: 0.70–0.98, SEM: 0.02–0.05 cm).
Fig. 1. Scapular downward rotation measurement. Scapular downward rotation index was calculated using the equation: distance between spinous process and spine of the scapula – distance between spinous process and inferior angle of scapula. Positive values mean downward rotation and negative values refer to upward rotation.
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Second, verification of EMG signal quality was completed for each muscle by having the subject performs maximal voluntary isometric contractions (MVICs) recommended by previous studies (Ekstrom, Soderberg, & Donatelli, 2005; Zanca, Oliveira, Ansanello, Barros, & Mattiello, 2014). MVICs were collected to normalize the EMG data from the UT, LT, SA, and LS. To determine the MVIC value for UT, each subject was tested in the sitting position with no back support. The subject elevated shoulder abduction at 90° with simultaneous resistance to the head after the neck was first side-bent to the same side, rotated to the opposite side, and then extended (Ekstrom et al., 2005; Zanca et al., 2014). To determine the MVIC value for LT, each subject was tested in the prone position. The subject’s arm was placed diagonally overhead, in line with the lower fibers of the trapezius muscle during external rotation while resistance was applied distal to the elbow (Ekstrom et al., 2005). To obtain the MVIC value for SA, the subject was seated on a treatment table with no back support. The subject’s shoulder was rotated internally and abducted to 125° in the scapular plane, while resistance was applied proximal to the subject’s elbow by the investigator. To obtain the MVIC value for LS, each subject was tested in the prone position. The subject adducted and elevated the scapula with medial rotation of the inferior angle with the elbow flexed. The examiner applied pressure with one hand against the patient’s arm in the direction of the abducting scapula, while rotating the inferior angle laterally and against the patient’s shoulder with the other hand in the direction of the depression (Kendall et al., 2007). Each contraction was held for 5 s, with maximal effort against manual resistance, and a 2 min rest was given between trials to minimize muscle fatigue (Vera-Garcia, Moreside, & McGill, 2010). The mean MVIC value of the two trials was calculated. The first and last seconds of the EMG data from each MVIC trial were discarded, and the remaining 3 s of the data were used. The gathered EMG amplitudes for UT, LT, SA, and LS during the exercises were expressed as percentages of the mean MVIC (% MVIC). Third, the subjects were uniformly instructed by a primary investigator (JHL) on the standardized position of the three exercises and how to perform each of the exercises. The subjects were then allowed to familiarize themselves with the exercises for approximately 10 min until the proper motion and timing were achieved. A 5 min resting period was allowed after the familiarization period and before the data collection began. Fourth, each subject first performed the preferred shrug exercise before completing the other shrug exercises. The frontal shrug and stabilization shrug were performed in randomization using the random number generator in Microsoft Excel (Microsoft Corp., Redmond, WA, USA). Each exercise was maintained for 5 s before slowly returning to the starting position. The subjects performed two trials of each exercise with a 1 min resting period between trials. A 10 min resting period was allowed between the three conditions to avoid fatigue. To ensure that each subject performed the exercises at a standard speed, a metronome was set to one beat per second (Nyland, Kuzemchek, Parks, & Caboru, 2004). For the data collection, the mean value of the two trials for each exercise was used for the data analysis. The amount of SDR and scapular upward rotation angle was measured immediately following the three shrug exercises. 2.3.1. Scapular upward rotation angle measurement after three shrug exercises The subjects were in a standing position, while the scapular upward rotation was measured after three shrug exercises using two inclinometers. One inclinometer was used to measure the shoulder abduction angle, and a second inclinometer was used to measure the upward rotation of the scapula. For measuring 30° shoulder abduction, the first inclinometer was placed on the shaft of the humerus, just above the lateral humeral epicondyle. At 30° shoulder abduction position, the degree of upward rotation of the scapula was measured using a second inclinometer. This was achieved by manually aligning the base of the second inclinometer along the spine of the scapula. The degree of upward rotation of the scapula was confirmed by measuring between the horizontal plane and the spine of the scapula. Previous study reported the intra-rater reliabilities at total shoulder abduction range. The overall ICC was 0.88 (Watson et al., 2005). 2.3.2. Preferred shrug with 30° shoulder abduction (preferred shrug) The subjects stood with their feet positioned shoulder-width apart. Two target bars were positioned at the subjects’ sides bilaterally to control the height of the shrug (subject’s maximal shrug height). The inclinometer was used to maintain the 30° shoulder abduction angle. During the exercise period, the subjects were instructed to move both of their shoulders until their acromion touched the target bars. Although only the SDRS side was being measured, both shoulders performed the shrug exercise simultaneously in order to avoid the tendency to lean to the other side. The subjects were instructed to shrug both of their shoulders in their preferred way with 30° shoulder abduction (Fig. 2). 2.3.3. Shrug with 30° shoulder abduction in the frontal plane (frontal shrug) The subjects stood with their feet positioned shoulder-width apart. The height of target bars was the same as ‘preferred shrug’. They also performed in the same way as in the preferred shrug exercise except the frontal plane was added. Two plastic guides were used to maintain the frontal plane. The subjects were instructed to touch slightly the plastic guides by the radial borders of their wrists to maintain the frontal plane (Fig. 3). 2.3.4. Shrug with 30° shoulder abduction in the frontal plane with craniocervicothoracic stabilization (stabilization shrug) They performed in the same way as in the frontal shrug, excluding the craniocervicothoracic stabilization. The principal investigator (JHL) pushed subject’s chin and most prominent spinous process of thoracic to maintain craniocervicothoracic stabilization, the investigator applied all the craniocervicothoracic stabilization to control the pushing pressure (Fig. 4).
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Fig. 2. Preferred shrug with 30° shoulder abduction (preferred shrug).
2.4. Statistical analysis PASW Statistics 18 software (SPSS, Chicago, IL, USA) was used to perform all of the statistical analyses. A one-way, repeated-measures ANOVA was used to assess the statistical significance of the amounts of SDR, scapular upward rotation angle, UT, LT, SA, LS muscle activity, and the UT/LS muscle activity ratio during the three shrug exercises (preferred shrug vs. frontal shrug vs. stabilization shrug). The level of significance was set at 0.05. A Bonferroni adjustment was performed if a significant difference was found (with a = 0.05/3 = 0.017). 3. Results 3.1. Amounts of SDR and scapular upward rotation angles There were no significant differences in the amounts of SDR during the three shrug exercises (F2, 15 = 2.578, P = .109) (Fig. 5). However, there were significant differences in the scapular upward rotation angles (F2, 15 = 8.677, P = 0.003) among three shrug exercises. The stabilization shrug (3.50 ± 11.23°) showed significantly greater scapular upward rotation angle than the preferred shrug ( 2.04 ± 10.58°, P = 0.004). The stabilization shrug also showed a significantly greater scapular upward rotation angle than the frontal shrug ( 1.82 ± 9.96°, P = 0.006) (Fig. 6). 3.2. UT, LT, SA, and LS EMG activity There was a significant difference in UT muscle activity (F2, 15 = 13.978, P = 0.000) during the three shrug exercises. The frontal shrug (35.46 ± 20.10%MVIC) showed significantly greater UT muscle activity than the preferred shrug (22.89 ± 9.46% MVIC, P = 0.002) (Fig. 7). However, there were no significant differences between the LT (F2, 15 = 2.184, P = 0.081), SA (F2, 15 = 3.350, P = 0.063), and LS muscle activity (F2, 15 = 3.634, P = .052) during the three shrug exercises.
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Fig. 3. Shrug with 30° shoulder abduction in the frontal plane (frontal shrug).
Fig. 4. Shrug with 30° shoulder abduction in the frontal plane with craniocervicothoracic stabilization (stabilization shrug).
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Amounts of Scapular Downward Rotation (cm)
3
2
1
0 Preferred shrug
Frontal shrug
Stabilization shrug
Fig. 5. Scapular downward rotation across three shrug exercises (Mean ± SD). Amounts of scapular downward rotation were calculated using the equation: distance between spinous process and spine of the scapula – distance between spinous process and inferior angle of scapula. None of the depicted differences was significant after Bonferroni adjustment (P < .017).
3.3. UT/LS muscle activity ratio There was a significant difference in the UT/LS muscle activity ratio (F2, 15 = 5.406, P = 0.016) during the three shrug exercises. The frontal shrug (1.80 ± 1.52) showed a significantly greater UT/LS muscle activity ratio, compared to the preferred shrug (1.24 ± 1.15, P = 0.004) (Fig. 8). 4. Discussion The purpose of this study was to compare the amounts of SDR, scapular upward rotation angle, UT, LT, SA, and LS muscle activities, and the muscle activity ratio of UT/LS during three different shrug exercises (preferred shrug vs. frontal shrug vs. stabilization shrug) in SDRS. The findings indicate that the stabilization shrug demonstrated a significantly greater scapular upward rotation angle compared with the preferred and frontal shrugs. The UT muscle activity and the UT/LS muscle activity ratios were significantly greater in the frontal shrug than in the preferred shrug in subjects with SDRS. To the best of our knowledge, this is the first study to compare these three shrugs with 30° shoulder abduction exercises in terms of the amounts of SDR, scapular upward rotation angle, UT, LT, SA, and LS, and UT/LS muscle activity ratios in subjects with SDRS. Although the amount of SDR measured at the resting position decreased by 0.46 cm (39.66%) and 0.31 cm (26.72%) following the frontal and stabilization shrugs, compared with the preferred shrug, the decrement of SDR was not significant. However, the degree of the scapular upward rotation angle was significantly greater (increasing by 5.54°and 5.32°, respectively) in the stabilization shrug compared with the preferred and frontal shrugs. These findings partially support our research hypothesis. The mechanism behind the changes in the scapular upward rotation angle observed in this study is likely the proximal stabilization of the trunk, which occurred during the stabilization shrug. Craniocervicothoracic stabilization used during the stabilization shrug in this study may have activated the deep segmental stabilizing muscle of the craniocervicothoracic spine, which provided a stable base when the UT initiated and maintained the scapular upward rotation. Given that the UT is an axioscapular muscle, the stable spine would enhance scapular movement rather than unwanted spine movement. A previous study stated that distal mobility requires a stable base (proximal stability) for shoulder rehabilitation (Oliver, Sola, Dougherty, & Huddleston, 2013). Cools et al. (2013) demonstrated that it is important to integrate scapular
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Fig. 6. Scapular upward rotation across three shrug exercises (Mean ± SD). Positive values refer to scapular upward rotation and negative values to scapular downward rotation. * indicates a significant difference by Bonferroni adjustment (P < .017).
orientation with spinal posture correction, especially in patients who have a forward head posture with an increasing thoracic kyphosis and protraction of the shoulder girdle. Kibler (1991) also demonstrated that altered craniocervical and thoracic posture influence scapular position and movement. Additionally, Ludewig and Cook (1996) reported that scapular upward rotation significantly decreased in flexed head positions. Other studies revealed that scapular upward rotation decreased with increased thoracic kyphosis (Finley & Lee, 2003; Kebaetse, 1999). Thus, the stabilization shrug may be preferred to enhance the upward rotation angle. The UT activity was significantly greater during the frontal shrug than in the preferred shrug by 35.45%. However, there were no significant differences in LT, SA, and LS activity among the three shrug exercises, although there were strong trends towards significance with P-values of 0.081, 0.063, and 0.052, respectively. The LT increased by 13.66% and 28.95% in the stabilization shrug compared to the preferred shrug and frontal shrug. The SA increased by 31.02% and 39.70% in the stabilization shrug compared to the preferred shrug and frontal shrug. Especially, the LS decreased by 17.81% and 24.80% in the frontal shrug compared to the preferred shrug and stabilization shrug. These findings partially support our research hypothesis that UT, LT, SA, and LS activity would be different among the three shrug exercises. There are several possible explanations for the increased UT activity in the frontal shrug compared to the preferred shrug and the lack of significant differences in the LT, SA, and LS. First, this may be due to the direction of the UT muscle contraction in the frontal plane. In other words, the line of pull in UT muscle (as a fusiform muscle) coincides with the frontal plane to optimize the activation of the muscle. Other researchers have verified that muscle activity increases when the direction of the muscle contraction is parallel with the muscle fiber orientation (Kang, Jeon, Kwon, Cynn, & Choi, 2013; Smidt & Rogers, 1982; Soderberg, 1983). Second, the shrug exercise and initial scapular upward rotation motion focus on improving UT muscle functioning in order to correct SDR (Watson et al., 2010). Third, performing a shrug with 30° shoulder abduction might have placed the UT at the optimal length and position, rather than the preferred plane. These results are in accordance with previous findings, which reported greater UT activity in the frontal plane. Previous investigators have reported that trapezius activity increases as the plane of humeral elevation moves from the sagittal plane to the frontal plane (Bagg & Forrest, 1986; Inman et al., 1944). Other studies have revealed that UT muscle activity is higher during arm abduction in the frontal plane than in the flexion plane (Antony & Keir, 2010). In addition, Hellwig et al. (1991) demonstrated that abduction within the frontal plane requires significant scapular upward rotation. Previous researchers have explained that the moment arm of UT may be decreased in the flexion plane compared with the frontal plane (Antony & Keir, 2010; Hughes & An, 1996). Pizzari et al. (2014) also demonstrated that UT activity was significantly greater in shrug exercises with 30° shoulder abduction than in standard shrug exercises for subjects with multi-directional instability; however, SA muscle activity was not
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100
% Maximal Voluntary Isometric Contraction
80
*
60
40
20
0 Preferred shrug
Frontal shrug
Stabilization shrug
Fig. 7. Muscle activity in the upper trapezius across three shrug exercises (Mean ± SD normalized electromyographic signal amplitude). * indicates a significant difference by Bonferroni adjustment (P < .017).
statistically different between the two exercises. The current findings indicate that the UT muscles (35.46% MVIC) were more active than the other muscles (LT: 3.42% MVIC, SA: 7.36% MVIC, LS: 25.62% MVIC) during the frontal shrug. The results indicate that the frontal shrug has potential benefits for subjects with SDRS by preferentially activating the UT muscles. The UT/LS muscle activity ratio significantly increased during the frontal shrug by 31.11%, compared to the preferred shrug exercise. The greater UT/LS muscle activity ratio suggests UT activation increased and LS activation decreased in this study, UT activity significantly increased during the frontal shrug and LS activity decreased in the frontal shrug compared to the preferred shrug and stabilization shrug, although there were no significant differences in LS activity among the three shrug exercises. These results support the research hypothesis. If scapula muscle weakness is present, then the scapula will go into a position that reflects that weakness, while highlighting any dominant strategies. For example, if the UT is weak and the LS is dominant, then the scapula will pull into a downward rotation. By following the EMG activity ratio of the UT to the LS during a shrug exercise with 30° shoulder abduction, one might assume that the frontal shrug is the appropriate exercise for UT activation, relative to LS activation among the three shrug exercises. No previous studies have examined the UT/LS muscle activity ratio during shrug exercises; thus, it is not possible to compare these results with other work. This study has several limitations. First, it was cross-sectional; thus, the long-term effects of the stabilization and frontal shrugs on the scapular upward rotation angle and UT activity cannot be determined. Second, crosstalk may have occurred between the UT and LS muscles, although the authors took all safety measures to ensure the reliability of the EMG signal. Third, it is also suggested that scapular dyskinesis is related to alterations in the timing of scapular muscle activations (Chester, Smith, Hooper, & Dixon, 2010; Phadke, Camargo, & Ludewig, 2009). Fourth, the findings of this study cannot be generalized to other patient groups because only young subjects with SDRS were recruited. Future research should investigate the long-term effects of the stabilization and frontal shrugs in patients with various pathologies. 5. Conclusions This study compared three different types of shrug exercises (preferred shrug vs. frontal shrug vs. stabilization shrug) with 30°shoulder abduction in terms of the amounts of SDR, scapular upward rotation angle, UT, LT, SA, and LS, and the UT/LS muscle activity ratio in SDRS. The results showed that the stabilization shrug demonstrated a significantly greater scapular upward rotation angle compared with the preferred and frontal shrugs. Moreover, UT muscle activity and the UT/LS muscle activity ratio were significantly greater in the frontal shrug than in the preferred shrug in SDRS. Therefore,
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4
Ratio of upper trapezius/ levator scapulae
*
3
2
1
0 Preferred shrug
Frontal shrug
Stabilization shrug
Fig. 8. Ratio of muscle activity in the upper trapezius vs. the levator scapulae across three shrug exercises (Mean ± SD normalized electromyographic signal amplitude). * indicates a significant difference by Bonferroni adjustment (P < .017).
the stabilization shrug is superior to the preferred shrug to increase the scapular upward rotation angle. Finally, the frontal shrug is beneficial for increasing UT activity, relative to LS activity, compared with the preferred shrug in SDRS. References Antony, N. T., & Keir, P. J. (2010). Effects of posture, movement and hand load on shoulder muscle activity. Journal of Electromyography and Kinesiology, 20, 191–198. http: dx.doi.org/ 10.1016/j.jelekin.2009.04.010. Epub 2009 May 26. Ayotte, N. W., Stetts, D. M., Keenan, G., & Greenway, E. H. (2007). Electromyographical analysis of selected lower extremity muscles during 5 unilateral weight-bearing exercises. Journal of Orthopaedic and Sports Physical Therapy, 37, 48–55. http://dx.doi.org/10.2519/jospt.2007.2354. Bagg, S. D., & Forrest, W. J. (1986). Electromyographic study of the scapular rotators during arm abduction in the scapular plane. American Journal of Physical Medicine and Rehabilitation, 65, 111–124. Behrsin, J. F., & Maguire, K. (1986). Levator scapulae action during shoulder movement: A possible mechanism for shoulder pain of cervical origin. Australian Journal of Physiotherapy, 32, 101–106. Burkhead, W. Z., Jr., & Rockwood, C. A. Jr., (1992). Treatment of instability of the shoulder with an exercise program. Journal of Bone and Joint Surgery, 74, 890–896. Chester, R., Smith, T. O., Hooper, L., & Dixon, J. (2010). The impact of subacromial impingement syndrome on muscle activity patterns of the shoulder complex: A systematic review of electromyographic studies. BMC Musculoskeletal Disorders, 11–45. http://dx.doi.org/10.1186/1471-2474-11-45. Cole, A. K., McGrath, M. L., Harrington, S. E., Padua, D. A., Rucinski, T. J., & Prentice, W. E. (2013). Scapular bracing and alteration of posture and muscle activity in overhead athletes with poor posture. Journal of Athletic Training, 48, 12–24. http://dx.doi.org/10.4085/1062-6050-48.1.13. Cools, A. M., Struyf, F., De Mey, K., Maenhout, A., Castelein, B., & Cagnie, B. (2013). Rehabilitation of scapular dyskinesis: From the office worker to the elite overhead athlete. British Journal of Sports Medicine, 48, 692–697. http://dx.doi.org/10.1136/bjsports-2013-092148. Epub 2013 May 18. Criswell, E. (2010). Cram’s introduction to surface electromyography (2nd ed.). Aspen Publishers. 443 Maryland, 345–456. Ekstrom, R. A., Soderberg, G. L., & Donatelli, R. A. (2005). Normalization procedures using maximum voluntary isometric contractions for the serratus anterior and trapezius muscles during surface EMG analysis. Journal of Electromyography and Kinesiology, 15, 418–428. Finley, M. A., & Lee, R. Y. (2003). Effect of sitting posture on 3-dimensional scapular kinematics measured by skin-mounted electromagnetic tracking sensors. Archives of Physical Medicine and Rehabilitation, 84, 563–568. Hellwig, E. V., & Perrin, D. H. (1991). A comparison of two positions for assessing shoulder rotator peak torque: The traditional frontal plane versus the plane of the scapula. Isokinetics and Exercise Science, 4, 202–206. Hermens, H. J., Freriks, B., Disselhorst-Klug, C., & Rau, G. (2000). Development of recommendations for SEMG sensors and sensor placement procedures. Journal of Electromyography and Kinesiology, 10, 361–374. Hintermeister, R. A., Lange, G. W., Schultheis, J. M., Bey, M. J., & Hawkins, R. J. (1998). Electromyographic activity and applied load during shoulder rehabilitation exercises using elastic resistance. The American Journal of Sports Medicine, 26, 210–220. Holtermann, A., & Roeleveld, K. (2006). EMG amplitude distribution changes over the upper trapezius muscle are similar in sustained and ramp contractions. Acta Physiologica (Oxford, England), 186, 159–168.
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Human Movement Science journal homepage: www.elsevier.com/locate/humov
Various shrug exercises can change scapular kinematics and scapular rotator muscle activities in subjects with scapular downward rotation syndrome Ji-Hyun Lee a, Heon-Seock Cynn a,⇑, Woo-Jeong Choi a, Hyo-Jung Jeong a, Tae-Lim Yoon b a b
Applied Kinesiology and Ergonomic Technology Laboratory, Department of Physical Therapy, The Graduate School, Yonsei University, South Korea Department of Physical Therapy, College of Health Science, Cheongju University, South Korea
a r t i c l e
i n f o
Article history: Received 26 February 2015 Revised 20 November 2015 Accepted 20 November 2015
Keywords: Frontal plane Spinal stabilization Upper trapezius
a b s t r a c t Scapular dyskinesis, characterized by scapular downward rotation syndrome (SDRS) affects scapula-humeral rhythm and results in shoulder dysfunction. Previous study has led to the recommendation of standard shrug exercise to contend with SDRS and strengthen the upper trapezius (UT) muscle. However, few researchers have examined which shrug exercise is most effective. The aim of this research was to compare scapular kinematic changes and scapular rotator muscles activity across three different shrug exercises in SDRS. The amounts of scapular downward rotation were measured by a caliper and the scapular upward rotation angle was measured using two digital inclinometers. Surface electromyography was used to measure EMG amplitude from the UT, lower trapezius (LT), serratus anterior (SA), and levator scapula (LS). Seventeen subjects with SDRS were recruited for this study. The subjects performed three shrug exercises with 30° shoulder abduction (preferred shrug, frontal shrug, and stabilization shrug). The stabilization shrug showed a significantly greater scapular upward rotation angle compared with the preferred shrug (P = 0.004) and frontal shrug (P = 0.006). The UT activity was significantly greater in the frontal shrug than in the preferred shrug (P = 0.002). The UT/LS muscle activity ratio was also significantly greater in the frontal shrug than in the preferred shrug (P = 0.004). The stabilization shrug should be preferred to enhance the upward rotation angle. In addition, the frontal shrug can be used as an effective method to increase UT activity and to decrease LS activity in SDRS. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction The upper trapezius (UT) muscle is the key contributor to normal scapular motion and control and it activates the scapular upward rotators in normal shoulder functions (Moraes, Faria, & Teixeira-Salmela, 2008; Wadsworth & Bullock-Saxton, 1997). Muscle imbalance between the elongated UT muscle and the shortened levator scapulae (LS) may cause scapular dyskinesis, characterized by a drooping scapula and increased downward rotation. This type of dyskinesis is defined as
⇑ Corresponding author at: Department of Physical Therapy, The Graduate School, Yonsei University, 1 Yonseidae-gil, Wonju, Kangwon-do 220-710, South Korea. E-mail address: [email protected] (H.-S. Cynn). http://dx.doi.org/10.1016/j.humov.2015.11.016 0167-9457/Ó 2015 Elsevier B.V. All rights reserved.
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scapular downward rotation syndrome (SDRS) (Sahrmann, 2002). Increased UT muscle length in SDRS does not allow for the transfer of weight from an upper extremity load to the sternoclavicular joint (Johnson, Bogduk, Nowitzke, & House, 1994). Furthermore, increased stiffness of the LS muscle may contribute to an increased compressive load and shear force on the cervical spine during active neck movement (Szeto, Straker, & Raine, 2002). Previous investigations show that LS can be activated simultaneously with UT during shoulder rehabilitation exercises, such as rowing, shoulder abduction, and shrug exercises (Moseley, Jobe, Pink, Perry, & Tibone, 1992). Thus, the UT muscle is the focus of therapeutic exercise protocols for rehabilitation from SDRS. Traditionally, the standard shrug exercise has been prescribed in shoulder rehabilitation programs to strengthen the UT muscle (Burkhead & Rockwood, 1992; Hintermeister, Lange, Schultheis, Bey, & Hawkins, 1998). However, previous studies reported that the shrug, with a 30° shoulder abduction exercise (i.e., completing the exercise with 30° of shoulder abduction, rather than with the arm by the side), generates greater activity in the UT muscle, as well as the serratus anterior (SA) and lower trapezius (LT), than the standard shrug (Pizzari, Wickham, Balster, Ganderton, & Watson, 2014; Watson, Balster, Finch, & Dalziel, 2005; Watson, Pizzari, & Balster, 2010). For this reason, the shrug with a 30° shoulder abduction exercise has been recommended for increasing the activation of the scapular upward rotators. Although the shrug with 30° abduction was proven to be superior to the traditional shrug, other shrug exercises could potentially produce even greater benefits for training the UT and upward rotators. Previous investigators reported that UT muscle activity was higher during arm abduction in the frontal plane (abduction plane) compared to the sagittal (flexion) plane (Antony & Keir, 2010). This is likely due to the relatively increased constant moment arm in the frontal plane compared with the flexion plane (Hughes & An, 1996). In addition, Hellwig and Perrin (1991) demonstrated that abduction within the frontal plane requires significant scapular upward rotation. Previous studies also revealed that the trapezius increases its activity as the plane of humeral elevation moves from the sagittal plane to the frontal plane (Bagg & Forrest, 1986; Inman, Saunders, & Abbott, 1944). These findings indicate that the exercise plane, where the shrug exercise is performed, is another critical factor that should be taken into consideration because the UT muscle may activate differently depending on the exercise plane during the shrug exercise. Moreover, previous researchers have focused on altered craniocervical and thoracic postures during scapular movement because spinal alignment is thought to influence scapular position and movement (Cole et al., 2013; Kibler, 1991). Others have reported that scapular upward rotation significantly decreased in flexed-head positions (Ludewig & Cook, 1996) and decreased with greater thoracic kyphosis (Finley & Lee, 2003; Kebaetse, 1999). Cervical stability may require synergistic muscle actions for active shoulder functions (Behrsin & Maguire, 1986). The LS can be particularly activated as a cervical stabilizer (extensor), rather than a scapular elevator, during arm movements because it directly connects the cervical spine to the scapula (Kendall, McCreary, & Provance, 2007). However, no study has determined the influence of the exercise plane and craniocervicothoracic stability during shrugs with 30° shoulder abduction exercises on scapular kinematic changes and scapular rotator muscle activity, even though arm elevation on various planes, and craniocervical and thoracic stabilization have been shown to affect scapular kinematics and scapular rotator muscle activity during shrugs with 30° shoulder abduction exercise. The purpose of this research was to compare the amount of scapular downward rotation (SDR), scapular upward rotation angle, UT, LT, SA, and LS electromyography (EMG) activity, and the EMG activity ratio of UT/LS during three different shrug exercises in SDRS, including the preferred shrug, frontal shrug, and stabilization shrug. The hypothesis is that the amounts of SDR, scapular upward rotation angle, UT, LT, SA, LS EMG activity, and the EMG activity of the UT/LS ratio would differ during three different shrug exercises in SDRS. The results of current study could provide valuable evidence to shoulder rehabilitation strategies for individuals with SDRS. 2. Methods 2.1. Subjects G-power software was used for the power analyses. The necessary sample size of seven subjects was calculated using data obtained from a pilot study of 10 subjects to achieve a power of 0.80 and an effect size of 0.59 (calculated from the partial g2 of 0.26 from the pilot study), with an a level of 0.05. Finally, 17 subjects with SDRS participated including 10 participants of the pilot study (age = 19.82 ± 1.60 years, height = 167.77 ± 6.92 cm, weight = 56.93 ± 6.38 kg, BMI = 20.19 ± 1.52, and amount of SDR before the exercises = 1.05 ± 0.55 cm). The SDR determination was confirmed using a caliper. This SDR measurement was modified using Kibler’s method to measure scapular alignment (Kibler, 1991; Watson et al., 2010). Both sides were measured to determine the amount of SDR in the subject. Next, the side that had greater scapular downward rotation was used in the data collection process (one subject had SDR in the right side and 16 Subjects had SDR in the left side). The exclusion criteria were a history or clinical exam revealing pain or dysfunction that substantially limited shoulder motion or resulted in gross instability of the shoulder during daily activities, signs and symptoms of cervical pain, adhesive capsulitis, thoracic outlet syndrome, or a current complaint of numbness or tingling in the upper extremity, forward head posture, and scoliosis. Prior to participation, the subjects provided written informed consent. The investigation was approved by Yonsei University Wonju Institutional Review Board.
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2.2. Instrumentation Surface EMG data were collected using a Noraxon TeleMyo-DTS (Noraxon, Inc., Scottsdale, AZ, USA) and analyzed using the Noraxon MyoResearch 1.06 XP software. The EMG signals were amplified, band pass-filtered (10 and 450 Hz), and notchfiltered (60 Hz, 120 Hz) before being recorded digitally at 1000 Hz and processed into root-mean-square data. Input impedance was 1 megaOhm. The common mode rejection rate ratio was 92 dB at 60 Hz. The data were collected from the UT, LT, SA, and LS on the tested side. After shaving and rubbing the skin with alcohol, disposable Ag/AgCl surface electrodes were placed on each muscle at standardized sites (Criswell, 2010). The UT electrodes were placed at one-half the distance between the cervical spine at C7 and the acromion (Hermens, Freriks, Disselhorst-Klug, & Rau, 2000). The LT electrodes were placed at an oblique vertical angle with one superior and one inferior electrode at a point 5 cm inferomedial from the root of the spine of the scapula. The SA (lower part) electrodes were attached just below the axillary area at the level of the inferior tip of the scapula and medial to the latissimus dorsi. The LS electrodes were placed between the posterior margin of the sternocleidomastoid and the anterior margin of the UT (Ludewig & Cook, 1996). Two electrodes were placed approximately 20 mm apart in the direction of the muscle fibers. The electrode pairs were placed parallel to the target muscle fibers. Inter-electrode distance was 10 mm (Holtermann & Roeleveld, 2006). Correct electrode placement was confirmed by visual inspection of the EMG signals on a computer screen during specific muscle testing. EMG data were collected for 5 s during the isometric phase and calculated using the middle 3 s of each exercise to avoid any connecting element of the skin electrode, as well as possible starting or ending effects (Ayotte, Stetts, Keenan, & Greenway, 2007). 2.3. Testing procedure First, the SDR determination was confirmed using a caliper. The subjects were standing with their arms relaxed by their sides. With a pen, the examiner marked three anatomical landmarks (Lewis, Green, Reichard, & Wright, 2002), including (1) the root of the scapular spine, (2) inferior angle of the scapula, and (3) the second and seventh thoracic spinous processes. The amounts of SDR were calculated using the following equation: distance between the second thoracic spinous process and spine of the scapula minus the distance between the seventh thoracic spinous process and inferior angle of the scapula. Positive values indicate downward rotation (Fig. 1). This study measured inter- and intra-rater reliabilities. Inter- and intrarater reliabilities of SDR determination using a caliper was examined by two raters who were experienced physical therapists with 1 and 2 years of clinical experience. Before data collection, the raters underwent a 2-h training session to ensure standardization of the caliper and accurate location of reference points by the principal investigator (JHL), who had 10 years of clinical experience. To examine intra-rater reliability, rater 1 measured two sessions of SDR determination. To avoid measurement recall, 1 h was allowed between sessions. The interclass correlation coefficient (ICC) of the inter-rater reliability was 0.85 (95% confidence interval [CI]: 0.63–0.94, standard error of measurement [SEM]: 0.04 cm). The ICCs of the intrarater reliabilities were 0.88–0.96 (95% CI: 0.70–0.98, SEM: 0.02–0.05 cm).
Fig. 1. Scapular downward rotation measurement. Scapular downward rotation index was calculated using the equation: distance between spinous process and spine of the scapula – distance between spinous process and inferior angle of scapula. Positive values mean downward rotation and negative values refer to upward rotation.
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Second, verification of EMG signal quality was completed for each muscle by having the subject performs maximal voluntary isometric contractions (MVICs) recommended by previous studies (Ekstrom, Soderberg, & Donatelli, 2005; Zanca, Oliveira, Ansanello, Barros, & Mattiello, 2014). MVICs were collected to normalize the EMG data from the UT, LT, SA, and LS. To determine the MVIC value for UT, each subject was tested in the sitting position with no back support. The subject elevated shoulder abduction at 90° with simultaneous resistance to the head after the neck was first side-bent to the same side, rotated to the opposite side, and then extended (Ekstrom et al., 2005; Zanca et al., 2014). To determine the MVIC value for LT, each subject was tested in the prone position. The subject’s arm was placed diagonally overhead, in line with the lower fibers of the trapezius muscle during external rotation while resistance was applied distal to the elbow (Ekstrom et al., 2005). To obtain the MVIC value for SA, the subject was seated on a treatment table with no back support. The subject’s shoulder was rotated internally and abducted to 125° in the scapular plane, while resistance was applied proximal to the subject’s elbow by the investigator. To obtain the MVIC value for LS, each subject was tested in the prone position. The subject adducted and elevated the scapula with medial rotation of the inferior angle with the elbow flexed. The examiner applied pressure with one hand against the patient’s arm in the direction of the abducting scapula, while rotating the inferior angle laterally and against the patient’s shoulder with the other hand in the direction of the depression (Kendall et al., 2007). Each contraction was held for 5 s, with maximal effort against manual resistance, and a 2 min rest was given between trials to minimize muscle fatigue (Vera-Garcia, Moreside, & McGill, 2010). The mean MVIC value of the two trials was calculated. The first and last seconds of the EMG data from each MVIC trial were discarded, and the remaining 3 s of the data were used. The gathered EMG amplitudes for UT, LT, SA, and LS during the exercises were expressed as percentages of the mean MVIC (% MVIC). Third, the subjects were uniformly instructed by a primary investigator (JHL) on the standardized position of the three exercises and how to perform each of the exercises. The subjects were then allowed to familiarize themselves with the exercises for approximately 10 min until the proper motion and timing were achieved. A 5 min resting period was allowed after the familiarization period and before the data collection began. Fourth, each subject first performed the preferred shrug exercise before completing the other shrug exercises. The frontal shrug and stabilization shrug were performed in randomization using the random number generator in Microsoft Excel (Microsoft Corp., Redmond, WA, USA). Each exercise was maintained for 5 s before slowly returning to the starting position. The subjects performed two trials of each exercise with a 1 min resting period between trials. A 10 min resting period was allowed between the three conditions to avoid fatigue. To ensure that each subject performed the exercises at a standard speed, a metronome was set to one beat per second (Nyland, Kuzemchek, Parks, & Caboru, 2004). For the data collection, the mean value of the two trials for each exercise was used for the data analysis. The amount of SDR and scapular upward rotation angle was measured immediately following the three shrug exercises. 2.3.1. Scapular upward rotation angle measurement after three shrug exercises The subjects were in a standing position, while the scapular upward rotation was measured after three shrug exercises using two inclinometers. One inclinometer was used to measure the shoulder abduction angle, and a second inclinometer was used to measure the upward rotation of the scapula. For measuring 30° shoulder abduction, the first inclinometer was placed on the shaft of the humerus, just above the lateral humeral epicondyle. At 30° shoulder abduction position, the degree of upward rotation of the scapula was measured using a second inclinometer. This was achieved by manually aligning the base of the second inclinometer along the spine of the scapula. The degree of upward rotation of the scapula was confirmed by measuring between the horizontal plane and the spine of the scapula. Previous study reported the intra-rater reliabilities at total shoulder abduction range. The overall ICC was 0.88 (Watson et al., 2005). 2.3.2. Preferred shrug with 30° shoulder abduction (preferred shrug) The subjects stood with their feet positioned shoulder-width apart. Two target bars were positioned at the subjects’ sides bilaterally to control the height of the shrug (subject’s maximal shrug height). The inclinometer was used to maintain the 30° shoulder abduction angle. During the exercise period, the subjects were instructed to move both of their shoulders until their acromion touched the target bars. Although only the SDRS side was being measured, both shoulders performed the shrug exercise simultaneously in order to avoid the tendency to lean to the other side. The subjects were instructed to shrug both of their shoulders in their preferred way with 30° shoulder abduction (Fig. 2). 2.3.3. Shrug with 30° shoulder abduction in the frontal plane (frontal shrug) The subjects stood with their feet positioned shoulder-width apart. The height of target bars was the same as ‘preferred shrug’. They also performed in the same way as in the preferred shrug exercise except the frontal plane was added. Two plastic guides were used to maintain the frontal plane. The subjects were instructed to touch slightly the plastic guides by the radial borders of their wrists to maintain the frontal plane (Fig. 3). 2.3.4. Shrug with 30° shoulder abduction in the frontal plane with craniocervicothoracic stabilization (stabilization shrug) They performed in the same way as in the frontal shrug, excluding the craniocervicothoracic stabilization. The principal investigator (JHL) pushed subject’s chin and most prominent spinous process of thoracic to maintain craniocervicothoracic stabilization, the investigator applied all the craniocervicothoracic stabilization to control the pushing pressure (Fig. 4).
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Fig. 2. Preferred shrug with 30° shoulder abduction (preferred shrug).
2.4. Statistical analysis PASW Statistics 18 software (SPSS, Chicago, IL, USA) was used to perform all of the statistical analyses. A one-way, repeated-measures ANOVA was used to assess the statistical significance of the amounts of SDR, scapular upward rotation angle, UT, LT, SA, LS muscle activity, and the UT/LS muscle activity ratio during the three shrug exercises (preferred shrug vs. frontal shrug vs. stabilization shrug). The level of significance was set at 0.05. A Bonferroni adjustment was performed if a significant difference was found (with a = 0.05/3 = 0.017). 3. Results 3.1. Amounts of SDR and scapular upward rotation angles There were no significant differences in the amounts of SDR during the three shrug exercises (F2, 15 = 2.578, P = .109) (Fig. 5). However, there were significant differences in the scapular upward rotation angles (F2, 15 = 8.677, P = 0.003) among three shrug exercises. The stabilization shrug (3.50 ± 11.23°) showed significantly greater scapular upward rotation angle than the preferred shrug ( 2.04 ± 10.58°, P = 0.004). The stabilization shrug also showed a significantly greater scapular upward rotation angle than the frontal shrug ( 1.82 ± 9.96°, P = 0.006) (Fig. 6). 3.2. UT, LT, SA, and LS EMG activity There was a significant difference in UT muscle activity (F2, 15 = 13.978, P = 0.000) during the three shrug exercises. The frontal shrug (35.46 ± 20.10%MVIC) showed significantly greater UT muscle activity than the preferred shrug (22.89 ± 9.46% MVIC, P = 0.002) (Fig. 7). However, there were no significant differences between the LT (F2, 15 = 2.184, P = 0.081), SA (F2, 15 = 3.350, P = 0.063), and LS muscle activity (F2, 15 = 3.634, P = .052) during the three shrug exercises.
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Fig. 3. Shrug with 30° shoulder abduction in the frontal plane (frontal shrug).
Fig. 4. Shrug with 30° shoulder abduction in the frontal plane with craniocervicothoracic stabilization (stabilization shrug).
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Amounts of Scapular Downward Rotation (cm)
3
2
1
0 Preferred shrug
Frontal shrug
Stabilization shrug
Fig. 5. Scapular downward rotation across three shrug exercises (Mean ± SD). Amounts of scapular downward rotation were calculated using the equation: distance between spinous process and spine of the scapula – distance between spinous process and inferior angle of scapula. None of the depicted differences was significant after Bonferroni adjustment (P < .017).
3.3. UT/LS muscle activity ratio There was a significant difference in the UT/LS muscle activity ratio (F2, 15 = 5.406, P = 0.016) during the three shrug exercises. The frontal shrug (1.80 ± 1.52) showed a significantly greater UT/LS muscle activity ratio, compared to the preferred shrug (1.24 ± 1.15, P = 0.004) (Fig. 8). 4. Discussion The purpose of this study was to compare the amounts of SDR, scapular upward rotation angle, UT, LT, SA, and LS muscle activities, and the muscle activity ratio of UT/LS during three different shrug exercises (preferred shrug vs. frontal shrug vs. stabilization shrug) in SDRS. The findings indicate that the stabilization shrug demonstrated a significantly greater scapular upward rotation angle compared with the preferred and frontal shrugs. The UT muscle activity and the UT/LS muscle activity ratios were significantly greater in the frontal shrug than in the preferred shrug in subjects with SDRS. To the best of our knowledge, this is the first study to compare these three shrugs with 30° shoulder abduction exercises in terms of the amounts of SDR, scapular upward rotation angle, UT, LT, SA, and LS, and UT/LS muscle activity ratios in subjects with SDRS. Although the amount of SDR measured at the resting position decreased by 0.46 cm (39.66%) and 0.31 cm (26.72%) following the frontal and stabilization shrugs, compared with the preferred shrug, the decrement of SDR was not significant. However, the degree of the scapular upward rotation angle was significantly greater (increasing by 5.54°and 5.32°, respectively) in the stabilization shrug compared with the preferred and frontal shrugs. These findings partially support our research hypothesis. The mechanism behind the changes in the scapular upward rotation angle observed in this study is likely the proximal stabilization of the trunk, which occurred during the stabilization shrug. Craniocervicothoracic stabilization used during the stabilization shrug in this study may have activated the deep segmental stabilizing muscle of the craniocervicothoracic spine, which provided a stable base when the UT initiated and maintained the scapular upward rotation. Given that the UT is an axioscapular muscle, the stable spine would enhance scapular movement rather than unwanted spine movement. A previous study stated that distal mobility requires a stable base (proximal stability) for shoulder rehabilitation (Oliver, Sola, Dougherty, & Huddleston, 2013). Cools et al. (2013) demonstrated that it is important to integrate scapular
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Fig. 6. Scapular upward rotation across three shrug exercises (Mean ± SD). Positive values refer to scapular upward rotation and negative values to scapular downward rotation. * indicates a significant difference by Bonferroni adjustment (P < .017).
orientation with spinal posture correction, especially in patients who have a forward head posture with an increasing thoracic kyphosis and protraction of the shoulder girdle. Kibler (1991) also demonstrated that altered craniocervical and thoracic posture influence scapular position and movement. Additionally, Ludewig and Cook (1996) reported that scapular upward rotation significantly decreased in flexed head positions. Other studies revealed that scapular upward rotation decreased with increased thoracic kyphosis (Finley & Lee, 2003; Kebaetse, 1999). Thus, the stabilization shrug may be preferred to enhance the upward rotation angle. The UT activity was significantly greater during the frontal shrug than in the preferred shrug by 35.45%. However, there were no significant differences in LT, SA, and LS activity among the three shrug exercises, although there were strong trends towards significance with P-values of 0.081, 0.063, and 0.052, respectively. The LT increased by 13.66% and 28.95% in the stabilization shrug compared to the preferred shrug and frontal shrug. The SA increased by 31.02% and 39.70% in the stabilization shrug compared to the preferred shrug and frontal shrug. Especially, the LS decreased by 17.81% and 24.80% in the frontal shrug compared to the preferred shrug and stabilization shrug. These findings partially support our research hypothesis that UT, LT, SA, and LS activity would be different among the three shrug exercises. There are several possible explanations for the increased UT activity in the frontal shrug compared to the preferred shrug and the lack of significant differences in the LT, SA, and LS. First, this may be due to the direction of the UT muscle contraction in the frontal plane. In other words, the line of pull in UT muscle (as a fusiform muscle) coincides with the frontal plane to optimize the activation of the muscle. Other researchers have verified that muscle activity increases when the direction of the muscle contraction is parallel with the muscle fiber orientation (Kang, Jeon, Kwon, Cynn, & Choi, 2013; Smidt & Rogers, 1982; Soderberg, 1983). Second, the shrug exercise and initial scapular upward rotation motion focus on improving UT muscle functioning in order to correct SDR (Watson et al., 2010). Third, performing a shrug with 30° shoulder abduction might have placed the UT at the optimal length and position, rather than the preferred plane. These results are in accordance with previous findings, which reported greater UT activity in the frontal plane. Previous investigators have reported that trapezius activity increases as the plane of humeral elevation moves from the sagittal plane to the frontal plane (Bagg & Forrest, 1986; Inman et al., 1944). Other studies have revealed that UT muscle activity is higher during arm abduction in the frontal plane than in the flexion plane (Antony & Keir, 2010). In addition, Hellwig et al. (1991) demonstrated that abduction within the frontal plane requires significant scapular upward rotation. Previous researchers have explained that the moment arm of UT may be decreased in the flexion plane compared with the frontal plane (Antony & Keir, 2010; Hughes & An, 1996). Pizzari et al. (2014) also demonstrated that UT activity was significantly greater in shrug exercises with 30° shoulder abduction than in standard shrug exercises for subjects with multi-directional instability; however, SA muscle activity was not
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100
% Maximal Voluntary Isometric Contraction
80
*
60
40
20
0 Preferred shrug
Frontal shrug
Stabilization shrug
Fig. 7. Muscle activity in the upper trapezius across three shrug exercises (Mean ± SD normalized electromyographic signal amplitude). * indicates a significant difference by Bonferroni adjustment (P < .017).
statistically different between the two exercises. The current findings indicate that the UT muscles (35.46% MVIC) were more active than the other muscles (LT: 3.42% MVIC, SA: 7.36% MVIC, LS: 25.62% MVIC) during the frontal shrug. The results indicate that the frontal shrug has potential benefits for subjects with SDRS by preferentially activating the UT muscles. The UT/LS muscle activity ratio significantly increased during the frontal shrug by 31.11%, compared to the preferred shrug exercise. The greater UT/LS muscle activity ratio suggests UT activation increased and LS activation decreased in this study, UT activity significantly increased during the frontal shrug and LS activity decreased in the frontal shrug compared to the preferred shrug and stabilization shrug, although there were no significant differences in LS activity among the three shrug exercises. These results support the research hypothesis. If scapula muscle weakness is present, then the scapula will go into a position that reflects that weakness, while highlighting any dominant strategies. For example, if the UT is weak and the LS is dominant, then the scapula will pull into a downward rotation. By following the EMG activity ratio of the UT to the LS during a shrug exercise with 30° shoulder abduction, one might assume that the frontal shrug is the appropriate exercise for UT activation, relative to LS activation among the three shrug exercises. No previous studies have examined the UT/LS muscle activity ratio during shrug exercises; thus, it is not possible to compare these results with other work. This study has several limitations. First, it was cross-sectional; thus, the long-term effects of the stabilization and frontal shrugs on the scapular upward rotation angle and UT activity cannot be determined. Second, crosstalk may have occurred between the UT and LS muscles, although the authors took all safety measures to ensure the reliability of the EMG signal. Third, it is also suggested that scapular dyskinesis is related to alterations in the timing of scapular muscle activations (Chester, Smith, Hooper, & Dixon, 2010; Phadke, Camargo, & Ludewig, 2009). Fourth, the findings of this study cannot be generalized to other patient groups because only young subjects with SDRS were recruited. Future research should investigate the long-term effects of the stabilization and frontal shrugs in patients with various pathologies. 5. Conclusions This study compared three different types of shrug exercises (preferred shrug vs. frontal shrug vs. stabilization shrug) with 30°shoulder abduction in terms of the amounts of SDR, scapular upward rotation angle, UT, LT, SA, and LS, and the UT/LS muscle activity ratio in SDRS. The results showed that the stabilization shrug demonstrated a significantly greater scapular upward rotation angle compared with the preferred and frontal shrugs. Moreover, UT muscle activity and the UT/LS muscle activity ratio were significantly greater in the frontal shrug than in the preferred shrug in SDRS. Therefore,
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4
Ratio of upper trapezius/ levator scapulae
*
3
2
1
0 Preferred shrug
Frontal shrug
Stabilization shrug
Fig. 8. Ratio of muscle activity in the upper trapezius vs. the levator scapulae across three shrug exercises (Mean ± SD normalized electromyographic signal amplitude). * indicates a significant difference by Bonferroni adjustment (P < .017).
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