Influence of scapular motion cues on trapezius muscle activity during Y exercise

Abstract

BACKGROUND:

The Y exercise is a therapeutic exercise facilitating lower trapezius muscle activity.

OBJECTIVE:

To identify the effects of scapular movement cues (posterior tilt vs. posterior tilt with adduction/depression) on trapezius muscle activity during Y exercise.

METHODS:

Fifteen healthy men without current shoulder pain performed general Y exercise; Y exercise with cues for scapular posterior tilt; and Y exercise with cues for scapular posterior tilt, adduction, and depression. Electromyography (EMG) data for the trapezius muscles were collected during Y exercise. The posterior tilt angle of the scapula was measured in the prone position with and without cues for scapular posterior tilt using an inclinometer application.

RESULTS:

The greatest lower trapezius muscle activity was observed during Y exercise with cues for scapular posterior tilt, while the greatest EMG activity of the upper trapezius was observed during Y exercise with cues for scapular posterior tilt, adduction, and depression ( $p<$ 0.05). Middle trapezius muscle activity did not significantly differ among the three Y exercise conditions ( $p=$ 0.175). Cues for scapular posterior tilt significantly increased the scapular posterior tilt angle in the prone shoulder abduction position ( $p=$ 0.007).

CONCLUSION:

Cues for scapular posterior tilt were most effective in facilitating lower trapezius muscle activity during Y exercise.

Keywords

Biofeedback electromyography exercise scapula superficial back muscles

1. Introduction

Upward rotation and posterior tilting of the scapula are crucial movements for correct arm elevation [1, 2]. Insufficient upward rotation and posterior tilt of the scapula can cause narrowing of the subacromial space, mechanical abrasion of the rotator cuff muscles, and altered glenohumeral joint alignment, leading to shoulder injuries such as subacromial impingement, rotator cuff tears, and superior labral injuries [3]. Indeed, several articles have reported that reduced scapular upward rotation and posterior tilt are related to shoulder injuries and scapular dyskinesia [1, 4, 5, 6].

The upper and lower trapezius muscles produce scapular upward rotation movements via force-coupling with serratus anterior [2]. The lower trapezius contributes not only to the scapular upward rotation but also to scapular adduction, depression, and posterior tilt [2, 3]. However, abnormal muscle activation patterns, including increased upper trapezius activity and decreased lower trapezius activity, have often been observed in patients with shoulder injuries [7, 8]. Although the upper trapezius contributes to scapular upward rotation, excessive upper trapezius muscle activation, in turn, leads to excessive clavicle elevation and scapular anterior tilt, which interrupts the correct kinematics of the shoulder complex required for arm elevation [1, 9]. Thus, therapeutic exercises facilitating selective activation of the lower trapezius have been emphasized for shoulder rehabilitation [6, 10, 11, 12, 13].

The Y exercise is frequently utilized as a therapeutic exercise in clinical settings. It effectively facilitates lower trapezius muscle activity as the shoulder is positioned parallel to the direction of lower trapezius muscle fibers [10, 11, 14]. Cues that induce functional actions (e.g., scapular adduction, depression, and posterior tilt) of the lower trapezius have been proposed as alternative strategies for facilitating lower trapezius muscle activity [15, 16, 17]. However, a previous study reported no differences in lower trapezius muscle activity between Y exercise with and without verbal and tactile cues for scapular adduction and depression [15]. Others [10, 18] have reported that Y exercise with cues for scapular posterior tilt resulted in greater lower trapezius muscle activity than general Y exercise or the backward rocking diagonal arm lifting exercise, which is an effective exercise for strengthening of the lower trapezius [19].

Based on previous findings [6, 10, 18], researchers have inferred that additional scapular posterior tilt may be effective for increasing lower trapezius muscle activity. However, most previous studies emphasized on the cues for scapular adduction or depression only [15, 16, 17]. To identify the effects of exercises combined with functional lower trapezius actions on muscle activation, exercises that include cues for scapular posterior tilt as well as scapular adduction and depression should be examined as well. Moreover, considering that the upper and middle trapezius muscles act in conjunction with the lower trapezius to determine the alignment and motion of the scapula during shoulder movements [1, 2, 20], the influence of exercises combined with functional lower trapezius actions on activity in all three areas of the trapezius should be identified. Therefore, the present study aimed to examine how additional cues for scapular posterior tilt influence electromyography (EMG) activity of the trapezius muscles during Y exercise with and without scapular adduction and depression.

2. Materials and methods

2.1 Study design

This cross-sectional study examined the effects of scapular motion cues (scapular posterior tilt or a combination of scapular posterior tilt, adduction, and depression) on EMG of trapezius muscles during Y exercise. The study was approved by the Catholic University of Pusan Institutional Review Board in accordance with The Code of Ethics of the World Medical Association (Declaration of Helsinki). The study was registered in the Clinical Research Information Service (registration number KCT0005311).

2.2 Participants

Fifteen healthy men (mean age $=$ 23.53 $\pm$ 1.96 years; mean height $=$ 175.33 $\pm$ 4.82 cm; mean body weight $=$ 72.93 $\pm$ 12.39 kg) without current shoulder pain and scapular dyskinesia were recruited for this study. Those with a history of shoulder injuries, including impingement, adhesive capsulitis, or tendinitis, were excluded from the study. Written informed consent was obtained from all participants prior to testing. The sample size was calculated based on a large effect size using partial eta squared ( $\eta^{2}=$ 0.14) [21], with a significance level of 0.05 and 90% power.

2.3 Procedures

2.3.1 Measures of muscle activity

The Noraxon surface EMG system (wireless miniDTS and MyoResearch 3 software; Noraxon, Inc., Scottsdale, USA) was used to collect muscle activity data from the upper, middle, and lower trapezius muscles on the side of the dominant arm. EMG electrodes were attached to the landmark of each muscle fiber [22] (Fig. 1). Raw data were collected at 1,500 Hz with a common-mode rejection ratio of $>$ 100 dB, noise of $<$ 1 $\mu$ V, and bandwidth of 10–450 Hz. All data were full-wave rectified using the root-mean-square with a 125-ms window.

Figure 1.

Placement of electromyography electrodes.

All participants performed two maximum voluntary isometric contraction (MVIC) trials against manual resistance by an examiner for 5 s each, and manual muscle testing maneuvers were used to normalize the EMG data collected during exercises [23, 24]. For the upper trapezius, participants performed an action combining shoulder elevation, cervical rotation to the opposite side, and cervical bending to the same side. Participants performed Y exercise with 90 ${}^{\circ}$ shoulder abduction and 90 ${}^{\circ}$ elbow flexion for the middle trapezius and Y exercise with 125 ${}^{\circ}$ shoulder abduction combined with external rotation and elbow extension for the lower trapezius. The EMG data collected during Y exercises were normalized based on the mean value of the middle two 3-s sections of the MVIC for each muscle.

2.3.2 Measures of scapular posterior tilt

Scapular posterior tilt angles were measured in the prone shoulder abduction positions with and without cues for scapular posterior tilt using smartphone support to verify whether posterior scapular movement occurs when providing cues for scapular posterior tilt, as suggested by Scibek and Carcia [25]. In their study, Scibek and Carcia [25] reported moderate to good correlation of scapular posterior tilt angles during shoulder movement between measurements obtained using an inclinometer with smartphone support and an electromagnetic tracking system. The smartphone support system consists of a wooden plate, an acrylic board perpendicular to the wooden plate, and two movable feet embedded in the wooden plate. In this study, an examiner adjusted the arrangements of the two feet such that they were placed on the root of the scapular spine and scapular inferior angle, respectively [25], when participants took the position of prone shoulder abduction at 125 ${}^{\circ}$ with shoulder external rotation and elbow extension. An examiner placed a smartphone (Galaxy S9; Samsung, Suwon, Republic of Korea) on the wooden plate, and vertical alignment to the plate was maintained using the acrylic board. The anterior-posterior tilt angle of the scapula was then measured using a smartphone inclinometer application (Clinometer level and slope finder; Plaincode Software Solutions, Stephanskirchen, Germany) (Fig. 2). In this study, positive angles were used to represent the scapular posterior tilt.

Figure 2.

Measurement of the scapular posterior tilt angle in the prone position.

Scapular posterior tilt angles were measured with the participant performing prone shoulder abduction at 125 ${}^{\circ}$ without cues of scapular posterior tilt before scapular posterior tilt training and with cues of scapular posterior tilt after training. All measurements were repeated thrice under each condition, and the mean value was calculated.

2.3.3 Scapular posterior tilt training

Before performing Y exercise with scapular movement cues, participants practiced the scapular posterior tilt strategy. Participants were asked to pull the coracoid process backward while pulling the scapular inferior angle forward without scapular adduction or downward rotation as if rolling the scapula when sitting, to induce scapular posterior tilt movements [10, 18, 26]. For effective training, visual biofeedback for scapular posterior tilting was performed using a Smart KEMA motion sensor with a sampling rate of 25 Hz and Smart KEMA software (Korea Tech Co, Ltd, Seoul, Korea). The Smart KEMA motion sensor includes a tri-axillar gyroscope, and information from the sensor is transmitted to a tablet PC with Smart KEMA software installed. The software calculates the rotation angle of the sensor in real-time [27]. In this study, the examiner attached the Smart KEMA motion sensor on the midpoint between the root of the spine and the inferior angle of the scapula. During scapular posterior tilt training, participants were asked to monitor the tablet PC screen for visual biofeedback of scapular posterior tilt movements. When correct active scapular posterior tilting was possible in a sitting position, participants practiced correct active scapular posterior tilting in a prone position with 125 ${}^{\circ}$ shoulder abduction, shoulder external rotation, and elbow extension.

Figure 3.

The Y exercise.

2.4 Y exercises

Participants performed three types of Y exercise: general Y exercise; Y exercise with cues for scapular posterior tilt; and Y exercise with cues for scapular posterior tilt, adduction, and depression. In this study, all Y exercises were performed in a 125 ${}^{\circ}$ shoulderabduction position, because this shoulderabduction angle is similar to the angle used in previous studies [17, 18, 28], and a previous study has shown that a 125 ${}^{\circ}$ shoulderabduction position was more effective in increasing LT muscle activity than a 160 ${}^{\circ}$ shoulderabduction position [11]. In addition, the target bar was placed at the 125 ${}^{\circ}$ shoulderabduction position of participants, and the participants were instructed to raise their arm while touching the target bar. For general Y exercise, participants performed shoulder abduction at 125 ${}^{\circ}$ with shoulder external rotation and elbow extension in the prone position while supporting the testing arm on the table. An examiner asked the participants to raise the testing arm as high as possible without trunk rotation (Fig. 3). For Y exercise with cues for scapular posterior tilting, participants took the same position as during general Y exercise, following which the examiner asked them to perform scapular posterior tilting. The examiner then instructed the participants to raise the testing arm while maintaining as much scapular posterior tilt force as possible [10]. For Y exercise with cues for scapular posterior tilting, adduction, and depression, participants took the position and were asked to perform scapular posterior tilting as during Y exercise with cues for scapular posterior tilt alone before raising the testing arm. They were then instructed to raise the testing arm with maximum scapular adduction and depression while maintaining scapular posterior tilt force. If participants had difficulty in recognizing scapular adduction and depression movements, an examiner guided these respective movements before the testing trials. During all Y exercises, participants were asked to maintain the end range of arm-raising for 5 s. The mean EMG value of the middle 3 s of isometric arm-raising was collected for data analysis. Three test trials were repeated under each Y exercise condition with 1 min of rest between test trials, and a 5 min rest period was provided between different Y exercise conditions.

2.5 Experimental procedures

In the present study, the scapular posterior tilt was first measured in the position of prone shoulder abduction, and then general Y exercise was performed. After scapular posterior tilt training, the scapular posterior tilt angle was measured in the prone shoulder-abduction position during active scapular posterior tilting. Y exercise with cues for scapular posterior tilt and Y exercise with cues for scapular posterior tilting, adduction, and depression were performed in a randomized order.

2.6 Statistical analyses

SPSS version 26.0 (IBM Corp., Armonk, USA) was used for statistical analysis. The Shapiro-Wilk test was used to verify the normality of the distribution for all outcome variables ( $p>$ 0.05). One-way repeated-measures analyses of variance (ANOVAs) were used to verify the influences of scapular movement cues on EMG activity in the upper, middle, and lower trapezius muscles during Y exercises. If main effects were observed, post hoc analysis was performed using Bonferroni correction to identify differences in EMG activity of the trapezius muscles among the Y exercise conditions. Paired $t$ -tests were used to identify the effects of scapular posterior tilt cues on scapular posterior tilting in the prone shoulder abduction position. For all analyses, the alpha level was set to 0.05.

Table 1
Muscle activity during each exercise

Measure (%MVIC)	General Y exercise	Y exercise with cues for scapular posterior tilt	Y exercise with cues for scapular posterior tilt, adduction, and depression	$P$ -value
Upper trapezius	25.42 $\pm$ 11.75	27.09 $\pm$ 12.89	32.97 $\pm$ 13.68 ${}^{{\dagger}}$	0.001 ${}^{\ast}$
Middle trapezius	62.57 $\pm$ 11.33	65.17 $\pm$ 13.92	58.54 $\pm$ 13.05	0.175
Lower trapezius	61.27 $\pm$ 14.60	72.15 $\pm$ 15.99 ${}^{{\dagger}}$	57.74 $\pm$ 11.63	0.009 ${}^{\ast}$

${}^{\ast}$ Significant differences among exercises, $p<$ 0.05. ${}^{{\dagger}}$ significant difference compared to the other two Y exercises, $p<$ 0.05. Abbreviations: MVIC, maximal voluntary isometric contraction.

Figure 4.

Electromyographic measurement of upper and lower trapezius activity during Y exercise.

3. Results

Significant main effects of EMG activity in the lower ( $p=$ 0.009) and upper trapezius ( $p=$ 0.001) were observed for Y exercises (Table 1). The post hoc analysis indicated that lower trapezius activity was significantly increased during Y exercise with cues for scapular posterior tilting when compared to that during general Y exercise ( $p=$ 0.023, effect size [ES] $=$ 0.71; Fig. 4) and Y exercise with cues for scapular posterior tilt, adduction, and depression ( $p=$ 0.024, ES $=$ 1.01; Fig. 4). However, there was no significant difference in lower trapezius activity between Y exercise with and without cues for scapular posterior tilt, adduction, and depression ( $p=$ 1.000; Fig. 4). In addition, upper trapezius muscle activity was greater during Y exercise with cues for scapular posterior tilt, adduction, and depression than during general Y exercise ( $p=$ 0.011, ES $=$ 0.59; Fig. 4) and Y exercise with cues for scapular posterior tilt ( $p=$ 0.020, ES $=$ 0.44; Fig. 4). However, upper trapezius activity did not significantly differ between Y exercise with and without cues for scapular posterior tilt ( $p=$ 0.630; Fig. 4). EMG activity of the middle trapezius did not significantly differ among the three Y exercise conditions ( $p=$ 0.175).

The posterior tilt angle of the scapula in the prone shoulder abduction position was significantly greater when scapular posterior tilt cues were provided than when they were not (23.38 $\pm$ 5.59 ${}^{\circ}$ versus 19.94 $\pm$ 5.14 ${}^{\circ}$ , $p=$ 0.007, ES $=$ 0.64).

4. Discussion

In this study, we examined the influence of scapular movement cues on trapezius muscle activity during Y exercise. Our findings indicated that Y exercise with cues for scapular posterior tilt could effectively facilitate lower trapezius muscle activity without an excessive increase in upper trapezius muscle activity during Y exercises.

In the current study, lower trapezius muscle activity ( $p=$ 0.023) was significantly greater during Y exercise with cues for scapular posterior tilt than during Y exercise without such cues. Furthermore, an increased scapular posterior tilt angle was observed in the prone shoulder abduction position when scapular posterior tilt cues were provided than when they were not ( $p=$ 0.007). Consistent with the findings of a previous study by Kim et al. [18], the findings of the present study indicate that increase in the scapular tilt angle may contribute to increased lower trapezius activity during Y exercise with scapular posterior tilt cues. Our inference is supported by the findings of another study by Kim et al. [6], who reported that lower trapezius muscle activity increases in resting and elevated arm positions during a scapular setting exercise that includes scapular posterior tilt movement. Although these findings suggest that additional cues for scapular posterior tilt are effective in facilitating lower trapezius muscle activity during Y exercise, another previous study reported no significant change in lower trapezius muscle activity between Y exercise with and without cues for scapular posterior tilt and external rotation [28]. However, we could not directly compare our results with those of the previous study as we measured muscle activity during the isometric phase of arm-raising, whereas the previous study measured muscle activity during the concentric and eccentric phases. In addition, it is possible that the weighted resistance applied during Y exercise in the previous study by Staker et al. [28] influenced the level of lower trapezius activation, leading to conflicting results between the previous and present studies.

Interestingly, additional cues for scapular adduction and depression during Y exercise with scapular posterior tilt cues decreased lower trapezius muscle activity ( $p=$ 0.024), even though these scapular motions are functional actions of the lower trapezius [2, 24]. It is believed that Y exercise naturally accompanies scapular adduction and depression [2, 29], which may facilitate lower trapezius muscle activation [11, 14, 24]. However, in the present study, participants were asked to perform “maximum” scapular adduction and depression during Y exercise with the three types of cues. Because Y exercise requires high muscle activation demands of the lower trapezius even without additional scapular movements [15, 22], it is possible that maximum scapular adduction and depression movements facilitate the contribution of other synergists (e.g., latissimus dorsi, levator scapulae, and rhomboid muscles) rather than the lower trapezius. In particular the levator scapulae and rhomboid muscles act as scapular adductors and downward rotators [2]. Therefore, a greater contribution of these muscles may lead not only to scapular adduction but also to scapular downward rotation. Considering that the lower trapezius is a force-coupling muscle for scapular upward rotation, it is reasonable to assume that scapular downward rotation may disturb the facilitation of the lower trapezius. The present findings indicate that activity of the middle trapezius, an agonist of scapular adduction, increased by 7% MVIC when providing additional cues for scapular adduction and depression during Y exercise with scapular posterior tilt, although the change was not significant. Therefore, we believe that excessive scapular adduction and depression lead to greater contributions of synergists for these scapular movements, which may have influenced findings for the lower and middle trapezius muscles. However, we did not measure muscle activation in synergists for scapular adduction and depression or the downward rotation angle of the scapula during Y exercise. Thus, further studies measuring these variables are required to verify our hypothesis.

In our study, upper trapezius muscle activity was greatest during Y exercise with cues for scapular posterior tilt, adduction, and depression ( $p<$ 0.05), suggesting that scapular adduction and depression movements increase upper trapezius muscle activity. Based on a kinesiological model, clavicular retraction movements should be accompanied at the sternoclavicular joint for scapular adduction [2]. Considering the anatomical line of action of the upper trapezius, contraction of the upper trapezius results in clavicular retraction movement [30, 31]. Therefore, we believe that the upper trapezius contracted more strongly for the clavicular retraction required for maximum scapular adduction, which results in greater upper trapezius muscle activity during Y exercise with cues for scapular posterior tilt, adduction, and depression.

There are several limitations to this study. First, we did not measure the EMG activity of other synergists for scapular adduction and depression. Second, we only measured the scapular posterior tilt angle in prone shoulder abduction positions with and without cues of scapular posterior tilt but not during active Y exercises, as it is difficult to find accurate landmarks of the scapula due to muscle belly movement during active Y exercise. Thus, future studies should measure scapular kinematics using an electromagnetic tracking system during active Y exercise. Third, we only recruited healthy men and had a relatively small number of participants; thus, future studies should include a larger number of patients with shoulder injuries or scapular dyskinesia to extend the clinical implication of our findings. Lastly, because the armraising range was not accurately measured in this study, it is possible that differences in the armraising range during repeated trials influence our results.

Our findings showed that cues for scapular posterior tilt contribute to significantly lower trapezius muscle activity without increasing the upper trapezius muscle activity. However, incorporating cues for scapular posterior tilting, adduction, and posterior tilting increased upper trapezius muscle activity during Y exercise. Considering that increasing lower trapezius muscle activity without increasing upper trapezius muscle activity is the goal of rehabilitation and aids in preventing shoulder injuries, Y exercise with cues of scapular posterior tilt may aid in facilitating selective activation of the lower trapezius during shoulder rehabilitation.

5. Conclusion

The present study demonstrates that scapular posterior tilt cues are ideal for facilitating activation of the lower trapezius muscle without increasing activation of the upper trapezius muscle when incorporating functional actions of the lower trapezius during Y exercise.

Footnotes

Conflict of interest

The authors declare that they have no conflict of interest.

Funding

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2021R1F1A1047329) and a research fund offered by the Catholic University of Pusan.

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Influence of scapular motion cues on trapezius muscle activity during Y exercise

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSION:

Keywords

1. Introduction

2. Materials and methods

2.1 Study design

2.2 Participants

2.3 Procedures

2.3.1 Measures of muscle activity

2.5 Experimental procedures

2.6 Statistical analyses

Table 1 Muscle activity during each exercise

4. Discussion

5. Conclusion

Footnotes

Conflict of interest

Funding

References

Table 1
Muscle activity during each exercise