Introduction

Shoulder external rotation (ER) exercises are key components of rehabilitation programs aimed at enhancing shoulder muscle stability and preventing subsequent injuries1,2,3. Scapular stabilization and activation of the surrounding musculature are important for improving dynamic shoulder function during activities of daily living. Numerous studies have reported that rotator cuff weakness, combined with overactivation of compensatory muscles, increases the risk of shoulder pathologies such as impingement, scapular dyskinesis, and rotator cuff tears. For example, excessive activation of the posterior deltoid, coupled with rotator cuff weakness, could contribute to shoulder instability and humeral head migration1,2. In particular, repetitive overhead activities in sports or occupational settings may lead to anterior and superior translation of the humeral head as a result of muscle imbalances4. Thus, it is of value to identify effective ER exercise methods that help restore shoulder muscle function and correct shoulder muscle imbalances.

Previous studies have investigated effective strategies for ER exercises aimed at strengthening the weakened rotator cuff while minimizing excessive activation of surrounding muscles. It has been suggested that the angle of humeral abduction (ABD) and elastic band resistance influence shoulder muscle activation patterns and could be critical for designing effective ER exercise protocols. For instance, upper trapezius and posterior deltoid muscle activity increased at 90° compared to 0° ABD during shoulder ER exercises2,3. Serratus anterior activity increased at 90° compared to 45° ABD, which may serve as a therapeutic target for restoring smooth shoulder function in patients with shoulder disorders3. In addition, different levels of ER exercise resistance appear to alter shoulder and rotator cuff muscle activity, suggesting that low to moderate resistance may be effective for targeting rotator cuff muscle activity5. These findings support the idea that shoulder ABD angles and exercise resistance could be key factors in designing ER exercise methods.

Balanced rotator cuff muscle strength is a primary goal for individuals with shoulder disorders, as it contributes to restoring glenohumeral stability and maintaining proper scapular mechanics6,7,8. Shoulder muscle co-contraction is an important neuromuscular control mechanism that improves joint stability and aids in controlling shoulder motion9,10. In addition to individual muscle activity, co-contraction indices (CCIs) among shoulder muscles, including deep rotator cuff muscles, may provide a useful measure of joint stability across different shoulder postures. Moreover, only one study has assessed rotator cuff activity using surface EMG at different shoulder ABD angles during ER exercises involving upper arm stretching11. Since it did not provide a comprehensive assessment of deep rotator cuff muscle due to the absence of a needle EMG approach, there is a great need for investigating the effects of shoulder ABD angles on rotator cuff activity and co-contraction during ER exercises.

Elastic band exercises are widely used in clinical and field settings to improve shoulder muscle function due to their portability and accessibility. Previous studies have investigated how elastic band resistance influences shoulder and rotator cuff muscle activity during various shoulder exercises12,13,14. Specifically, elastic band training has been reported to impose different muscle activation demands, which may benefit individuals with shoulder disorders characterized by excessive upper trapezius and deltoid activity12,13. It has also been shown that elastic band exercise likely reduces overactivation of the upper trapezius and middle deltoid muscles14. In addition, elastic resistance generates lower ER joint moments than constant-resistance loading, which may help reduce excessive stress on the shoulder joint15. However, the effects of elastic band resistance on rotator cuff and surrounding shoulder muscle activity during ER exercises remain poorly understood.

Therefore, the purpose of this study was to investigate the effects of different humeral ABD angles and levels of elastic band resistance on shoulder muscle activity during ER exercises. We hypothesized that: (1) the upper trapezius and serratus anterior activity would be increased at 90° of shoulder ABD compared to 0° and 45° during ER. (2) Rotator cuff muscle activity, as well as CCIs among the supraspinatus, infraspinatus, and teres minor would increase with higher levels of elastic band resistance (i.e., from low to moderate resistance). This study aims to advance understanding of effective ER exercise strategies to inform rehabilitation and shoulder injury prevention programs.

Materials and methods

Participants and ethical approval

A total of 13 healthy male participants with a mean (± SD) age of 24.08 (± 2.93) years, height of 176.77 (± 5.85) cm, and mass of 70.88 (± 9.14) kg were recruited for this study. All participants were right-handed, and testing was conducted on their dominant shoulders. Both shoulders were free of pain or pathology, and all participants were capable of performing shoulder ER exercises without restriction. The participants met international physical activity guidelines (≥ 150 min/week of moderate-intensity activity)16 and had not engaged in regular shoulder ER training within the past 12 months. Sample size estimation was performed using G*Power (version 3.1), following Cohen’s (1988) guidelines17 for a repeated-measures ANOVA. A large effect size (f = 0.4) was selected based on prior findings regarding SSP activity across different shoulder ABD angles1. With α = 0.05 and power (1–β) = 0.80, the required sample size was 12 participants. Prior to enrollment in the study, all participants received an explanation of the experimental protocol and signed an informed consent form approved by the university’s institutional review board (IRB ID: KHGIRB-23-489).

Experimental procedure

The participants performed standing ER exercises without any arm support under all six conditions, which consisted of two levels of band resistance (low and moderate) and three shoulder ABD angles (0°, 45°, and 90°; Fig. 1). During the warm-up stage, participants performed repeated shoulder elevation and ER motions without resistance to ensure comfort and muscle readiness. Following the warm-up, the participants performed five repetitions of ER in the scapular plane at a velocity controlled by a metronome set at 50 beats per minute. All ER exercises were performed using a commercial tubing band (1200 mm in length × 9 mm in diameter; BASEBALLCENTER, Korea) commonly used for shoulder strengthening exercises. Each participant completed both a familiarization session for the ER exercise and the main trials, which consisted of five repetitions under each of the six conditions in a randomized order. Throughout the experimental trials, verbal guidance and instructions were provided to ensure proper execution of ER exercises under each condition.

Fig. 1
Fig. 1
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(A) External and internal rotation in scapular humeral ABD at 90° (B) External and internal rotation in scapular humeral ABD at 0° (C) External and internal rotation with scapular humeral ABD at 45°.

The two elastic band resistance conditions (low and moderate) were set at 1.5% and 3% of body mass, respectively (Mean ± SD: low resistance, 1.06 ± 0.14 kg; moderate resistance, 2.13 ± 0.27 kg). Normalizing resistance to a percentage of body mass is a commonly applied, field-appropriate method in shoulder rehabilitation exercise programs18,19,20,21,22. Previous research has recommended low to moderate resistance (0.5–2 kg) to reduce posterior deltoid activation and enhance infraspinatus isolation14,23,24. Consistent with previous findings and our pilot trials, the use of 1.5% and 3% of body mass was deemed appropriate for low to moderate resistance22.

The point at which the elastic band was individually elongated to achieve each normalized resistance was measured in kilograms using a digital tension meter (portable digital electronic scale, TXY, China; Fig. 2D) and marked on the experimental board beside the participant (Fig. 2C). Each participant was given the rest as needed between conditions, with a minimum break of two minutes required. All tests were conducted by three researchers, including one expert proficient in handling the equipment for needle insertion and two with experience operating hardware and software.

Electromyography

Shoulder ABD angles were measured using an IMU-based system (Ultium myoMOTION Research Pro, Noraxon, USA). Two sensors were placed along the spinal column at C7 and T12, and another was positioned midway along the lateral humeral shaft. Trunk kinematic data from IMU sensors were monitored to control for trunk rotation during ER exercises, as trunk movement could be a significant confounding factor in this study. After static calibration in the upright standing position, kinematic data of shoulder ABD and trunk rotation angles were recorded.

Surface and intramuscular needle EMG (Ultium EMG, myoMUSCLE Research Pro, Noraxon, USA; 2000 Hz) were used in this study. According to standard SENIAM guidelines, surface electrodes were placed parallel to the muscle fibers of four muscles, including the posterior deltoid (PD), upper trapezius (UT), lower trapezius (LT), and serratus anterior (SA) by the same tester (JHS) to ensure consistency. Any excess hair was shaved, and an abrasive paste (Idopharm, Ido Skin Swab, Korea) was used to prepare the skin. Intramuscular needle EMG was used to assess activity in the three rotator cuff muscles: the supraspinatus (SSP), infraspinatus (ISP), and teres minor (TM). Paired hook fine-wire stainless-steel electrodes (diameter 50.8 μm; AM SYSTEMS, LLC, Washington, USA) were inserted into the muscle belly using single-use 23-gauge sterile hypodermic needle (length: 6 cm; KOVAX-NEEDLE, Korea) as cannulas (Fig. 2A-B). Needle insertion was performed by an experienced orthopedic surgeon (DL) under real-time ultrasound guidance to ensure accuracy. The electrodes were secured to the skin with tape, and their placement was verified during all testing sessions. After electrode placement, three repetitions of maximal voluntary isometric contractions (MVIC) were recorded for each muscle using manual muscle resistance testing25,26 (Table 1).

Fig. 2
Fig. 2
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(A) single-use needle, (B) intramuscular EMG, (C) elastic band setup, and (D) portable digital electronic scale.

Table 1 Location of EMG electrodes and details of maximum voluntary isometric contraction trials.
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Raw EMG signals were evaluated for corrupted data using visual amplitude inspection and power spectral analysis27. The signals were then band-pass filtered with a fourth-order Butterworth filter (10 and 450 Hz) to reduce movement artifact contamination. Additionally, a 60 Hz notch filter implemented with a fourth-order Butterworth filter, was applied to remove interference from nearby electronic sources. To obtain the EMG linear envelope, the signals were rectified and subsequently low-pass filtered at 10 Hz. EMG amplitudes for each muscle were calculated as the average value of the linear envelope during the ER exercises and normalized to the peak amplitude recorded during MVIC trials. CCIs were calculated using the equation below28:

$$CC{1_{m1:m2}}=ave\left\{ {\sum\limits_{{i=initial}}^{{i=final}} {\frac{{\hbox{min} \left\{ {EM{G_{m1}}\left( i \right),EM{G_{m2}}\left( i \right)} \right\}}}{{\hbox{max} \left\{ {EM{G_{m1}}\left( i \right),EM{G_{m2}}\left( i \right)} \right\}}}\left( {EM{G_{m1}}\left( i \right)+EM{G_{m2}}\left( i \right)} \right)} } \right\}$$

In this equation, m1/m2 represents the two muscles being analyzed, initial/final were set to 1–100% of ER movement, min represents the EMG linear envelope values from the less active muscle group, and max represents the EMG linear envelope values of the more active muscle group at each time point. CCIs were calculated for (m1/m2): ISP/PD, SA/UT, LT/UT, SSP/ISP, SSP/TM, and ISP/TM. All data analyses were conducted using custom MATLAB scripts.

Statistical analysis

All statistical analysis was performed using R version 4.1.3 (Rstudio, Boston, MA, USA) with a significance level of 0.05. The effects of different shoulder ABD angles and band resistance levels on shoulder muscle EMG during ER exercises were tested with repeated measures analyses of variance (ANOVA). Specifically, 2 × 3 repeated measures ANOVAs were used to test the effect of the three shoulder ABD angles (0° vs. 45° vs. 90°) and the two resistance levels (low vs. moderate). Sphericity was examined using Mauchly’s test, and Greenhouse–Geisser corrections were applied when the sphericity assumption was violated. Effect sizes for repeated-measures ANOVA were calculated as partial eta squared (η²) and interpreted as small (0.01), medium (0.06), or large (0.14)17. When a significant main effect was observed, Bonferroni-adjusted post hoc tests were conducted to account for multiple comparisons among the ABD angles for each dependent variable (e.g., muscle activity and CCI).

The normality was assessed using the Shapiro–Wilk test. When the assumption of normality was violated, nonparametric analysis of longitudinal data was performed using the nparLD package in R, which implements the rank-based Brunner–Langer approach29. When a significant main effect of the ABD angles was identified in the nonparametric model, pairwise comparisons were conducted using Wilcoxon signed-rank tests with Bonferroni correction. Effect sizes were not reported for the nonparametric analyses because the rank-based test does not provide a standardized effect size metric30.

Results

EMG activity amplitudes

The nonparametric tests revealed significant main effects of shoulder ABD angles on the activity amplitudes of the UT and SA (both p < 0.001; Table 2). Further post-hoc tests showed that both UT and SA activity significantly increased at 90° compared to 0° (both p < 0.001) and 45° ABD (both p < 0.001). The increase in SA activity (from 5.7% to 20.6% MVIC) was more substantial than that of the UT (from 13.4% to 27.6% MVIC) when comparing 0° to 90° ABD. UT and SA activity also increased at 45° compared to 0° ABD (p < 0.001, p = 0.027). SSP activity increased at both 45° and 90° ABD compared to 0° ABD (p = 0.007, p = 0.015).

In addition, significant main effects of band resistance were found for the PD (p < 0.001), UT (p < 0.001), LT (p < 0.004), SSP (p = 0.003), ISP (p = 0.028), and TM (p < 0.001) activity (Table 2). For all six muscles, mean activity was greater for moderate compared to low band resistance. The magnitude of increases in PD, UT, and LT ranged from 10.6% to 22.8% MVIC when applying band resistance from low to moderate, whereas SSP, ISP, and TM exhibited substantially larger increases, ranging from 18.6% to 38.3% MVIC, respectively. There was no interaction effect between ABD angles and band resistance. Based on descriptive values and without statistical comparisons, ISP showed the highest muscle activity at 45° ABD and under moderate load (44.1% and 38.3% of MVIC, respectively).

Co-contraction indices

Significant main effects of shoulder ABD angle were found for the SA/UT (p < 0.001), LT/UT (p < 0.001), and SSP/ISP (p < 0.001; Table 3). Post-hoc analyses revealed that the SA/UT and LT/UT CCIs increased at 90° compared to 0° (both p < 0.001) and 45° ABD (p < 0.001, p = 0.005). The change in magnitude of SA/UT CCIs was greater than that of LT/UT CCIs when comparing 0° to 45° ABD and 90° ABD (SA/UT: 3.5% vs. 5.3% vs. 11.6% of MVIC; LT/UT: 5.7% vs. 8.3% vs. 10.4% of MVIC). SA/UT, LT/UT, and SSP/ISP CCIs also increased at 45° compared to 0° (p = 0.036, p < 0.001, p < 0.001).

Significant main effects of band resistance were also observed for the ISP/PD (p = 0.005), LT/UT (p < 0.001), SSP/ISP (p < 0.001), SSP/TM (p = 0.012), and ISP/TM (p < 0.001) CCIs, all of which significantly increased for moderate compared to low resistance. The CCIs of the rotator cuff muscles exhibited greater increases (SSP/ISP: 8.6% vs. 15.8% of MVIC; SSP/TM: 5.6% vs. 8.2% of MVIC; ISP/TM: 4.4% vs. 7.4% of MVIC) compared with the ISP/PD and LT/UT CCIs (ISP/PD: 6.1% vs. 8.0% of MVIC; LT/UT: 7.2% vs. 9.1% of MVIC). No interaction effects between ABD angle and band resistance were observed. Among all CCIs, the highest values across ABD angles and resistance levels were observed for SSP/ISP at 45° ABD and under moderate resistance(16.7% and 15.8% of MVIC).

Table 2 Mean and standard deviations for EMG amplitudes (% MVIC) during ER exercises and statistical results from ANOVAs.
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Table 3 Mean and standard deviations for co-contraction indices (% MVIC) during ER exercises and statistical results from ANOVAs.
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Discussions

The purpose of this study was to investigate the effects of different humeral ABD angles and band resistance levels on shoulder and rotator cuff EMG activity during ER exercises. Evaluation of the deep rotator cuff muscles using intramuscular needle EMG provides a more accurate understanding of the muscle activation patterns than surface EMG alone.

Effects of shoulder ABD angle

The first hypothesis that UT and SA activity would be increased at 90° of shoulder ABD compared to 0° and 45° was supported. We found that increasing the shoulder ABD angle led to greater UT and SA muscle activity. These findings are consistent with previous studies reporting that greater shoulder ABD angles result in increased UT and SA activation during ER exercises, which may facilitate scapular upward rotation and elevation2,3,31,32. Proper coordination between the UT and SA could generate balanced force production and promote efficient scapular upward rotation31,33,34. Conversely, weakness in the SA appears to compromise scapula stability, leading to compensatory UT overactivity during various shoulder movements18,34,35. This imbalance could alter shoulder kinematics and increase the risk of shoulder impingement and rotator cuff injuries18,33,34. Thus, appropriate levels of SA activity are essential for stabilizing the scapula and maintaining optimal subacromial space, both of which are critical for smooth shoulder mechanics. Our results suggest that performing ER exercise at 90° shoulder ABD may serve as a targeted intervention by promoting SA activity and balanced SA-UT activity, potentially preventing shoulder injuries and enhancing scapulohumeral function.

SSP activity was also sensitive to changes in shoulder ABD angles, as increased SSP activity was found at 45° and 90° ABD compared to 0°. Previous studies reported that the SSP could be more actively involved in ER at higher ABD angles (e.g., > 45°)1,36,37. A cadaver study reported that the moment arm of the SSP was highly variable among the rotator cuff muscles at 10° of ABD36, whereas it consistently shortened at 60° of ABD, which allows the SSP to contribute more effectively to ER. In addition, the weight of the upper arm and the less stable position of the GH joint at higher ABD angles may increase the demand on SSP involvement1,37. If moderate stress on the SSP should be avoided in patients with acute inflammation in the early postoperative period, positions near 0° ABD should be considered. Accordingly, performing ER exercises at 0° of ABD may represent a safe and appropriate strategy during the early phase of rehabilitation, when minimizing stress on the SSP is critical. In contrast, ER above 45° of ABD may provide a more effective approach for selectively targeting and strengthening the SSP during the later stages of rehabilitation, when higher levels of muscle activation are desired.

Another key finding was the increased CCIs in the SA/UT and LT/UT pairs as shoulder ABD angles increased during ER exercises. Specifically, both SA/UT and LT/UT CCIs were increased at 90° ABD compared to 0° and 45°, which may reflect the higher scapular stability demands at the 90° ABD position. In the upwardly rotated scapula position, SA may indirectly assist ER movement by contributing to posterior tilt as the ABD angle increases38,39. Similarly, we found substantial increases in SA and UT activity at 90°ABD, which may be necessary to facilitate scapula upward rotation and posterior tilt. Thus, performing ER at 90° ABD requires coordinated activation of both scapular stabilizers (LT, UT, SA) and the rotator cuff (SSP) to meet additional stability demand and improve dynamic shoulder function.

Taken together, we recommend initiating ER exercises at 0° of shoulder ABD to ensure safe SSP involvement, then gradually progressing to 45° as rehabilitation advances to further strengthen the rotator cuff muscles. Because shoulder muscle activation strategies vary widely during ER—particularly as ABD increases—it is important to assess each patient’s individual recruitment pattern31. To maximize SA involvement, performing ER at 90° of shoulder ABD may be considered, although this position is accompanied by some UT activity that remains within a non-excessive range. These findings can help clinicians determine whether a patient is relying on compensatory strategies or exhibiting insufficient activation of the rotator cuff and surrounding musculature.

Effects of band resistance

Our second hypothesis that rotator cuff muscle activity, as well as CCIs among the SSP, ISP, and TM, would increase with higher levels of elastic band resistance was mostly supported. Greater activation of the SSP and ISP was observed under moderate resistance compared to low resistance. Both muscles showed approximately 40% higher activation with moderate resistance, suggesting that elastic band ER exercises may be effective for strengthening the rotator cuff. Consistent with the increases in SSP and ISP activity, multiple CCIs pairings (e.g., SSP/ISP, SSP/TM, ISP/TM) also increased as elastic band resistance increased. Accordingly, elastic band exercises appear to impose greater demands on joint stability and neuromuscular control, thereby increasing CCIs of the rotator cuff muscles. Unlike constant-resistance devices (e.g., pulley systems, dumbbells), elastic bands produce variable resistance throughout the entire ER movement, which is linked to greater CCIs to maintain joint stability15,40,41. Therefore, greater elastic band resistance may increase rotator cuff CCIs, making elastic band ER exercises particularly suitable for the later stages of rehabilitation when higher resistance is typically prescribed.

Although PD and UT activity also significantly increased from low to moderate resistance, their activation levels remained relatively low, at approximately 13% and 23% of MVIC, respectively. For comparison, a previous study on ER exercises using a 2-kg dumbbell reported PD activity exceeding 50% MVIC and UT activity exceeding 70% MVIC, both substantially higher than the levels observed in our study1,42. Considering the activation levels of shoulder muscles, elastic band exercises may be more appropriate for rehabilitation purposes. In addition, weak rotator cuff muscles, combined with overactivation of PD and UT, may increase the risk of injuries such as shoulder impingement and scapular dysfunction1,2,3. Previous studies also suggested that performing ER with elastic resistance can reduce UT involvement by minimizing gravitational influence14,15,43. In this context, elastic bands may preferentially strengthen rotator cuff while minimizing unwanted activation of PD and UT.

This study has several limitations that suggest directions for future research. First, we did not assess additional muscles that influence scapular and glenohumeral motion, such as the latissimus dorsi and pectoralis major. Future studies should therefore include these surrounding muscles. Second, comparisons with previous research are challenging because of heterogeneity in experimental protocols (e.g., body posture, muscle-contraction type). Given the widespread clinical use of elastic-band exercises, additional studies employing standardized methods are warranted, including further investigation of how ABD angles and band resistance levels affect rotator-cuff activation. Third, the relatively small sample size (n = 13) limits the generalizability of our findings. Finally, studies involving patients with shoulder disorders are needed to determine whether the present findings extend beyond healthy individuals.

Conclusions

Our results suggest strategies for designing band exercises that consider shoulder ABD angles and resistance levels for various rehabilitation goals. During the early rehabilitation phase, performing ER at 0° ABD may help maintain safe SSP activity. As rehabilitation progresses, gradually increasing the ABD angle during ER exercises above 45° may help strengthen the SSP and other shoulder girdle muscles (e.g., UT and SA). In addition, applying elastic band resistance may effectively increase rotator muscle activity and co-contraction among rotator cuff muscles, which could improve joint stability. These findings may serve as a foundation for future research and help optimize shoulder external rotator exercise strategies in rehabilitation programs.