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

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

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 ± 5.59 ∘ versus 19.94 ± 5.14 ∘ , 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 ( 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.