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Phys. Ther. Korea 2022; 29(2): 106-116

Published online May 20, 2022

https://doi.org/10.12674/ptk.2022.29.2.106

© Korean Research Society of Physical Therapy

어깨 가쪽돌림 운동 시 운동방법과 아래팔의 자세에 따른 가시아래근과 뒤어깨세모근의 근활성도 비교

손명기1, 김선엽2

1대전대학교 대학원 물리치료학과, 2대전대학교 보건의료과학대학 물리치료학과

Comparison of Infraspinatus and Posterior Deltoid Muscle Activities According to Exercise Methods and Forearm Positions During Shoulder External Rotation Exercises

Myeong-gi Son1 , PT, BHSc, Suhn-yeop Kim2 , PT, PhD

1Department of Physical Therapy, The Graduate School, Daejeon University, 2Department of Physical Therapy, College of Health and Medical Science, Daejeon University, Daejeon, Korea

Correspondence to: Suhn-yeop Kim
E-mail: kimsy@dju.kr
https://orcid.org/0000-0002-0558-7125

Received: April 19, 2022; Revised: May 11, 2022; Accepted: May 12, 2022

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

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Background: Shoulder external rotation exercises are commonly used to improve the stabilizing ability of the infraspinatus. However, during exercise, excessive activation of the posterior deltoid compared to the infraspinatus causes the humeral head to move anteriorly in an abnormal position. Many researchers have emphasized selective activation of the infraspinatus during shoulder external rotation exercise. Objects: This study aims to delineate the optimal exercise method for selective activation of infraspinatus by investigating the muscle activities of the infraspinatus and posterior deltoid according to the four shoulder exercise methods and two forearm positions.
Methods: Thirty healthy individuals participated in this study. The participants were instructed to perform shoulder external rotation exercises following four exercise methods: sitting external rotation (SIER); standing external rotation at 90° abduction (STER); prone external rotation at 90° abduction (PRER); side-lying external rotation (SLER), and two forearm positions (neutral, supinated). The electromyography (EMG) signal amplitude was measured during each exercise. Surface EMG signals were recorded from the posterior deltoid, infraspinatus, and biceps brachii.
Results: EMG results of the infraspinatus and posterior deltoid in PRER, were significantly higher than that of the other exercises (p < 0.01). The EMG ratio (infraspinatus/posterior deltoid) in SIER was significantly higher than that of the other exercises. EMG activation of the posterior deltoid in SIER, PRER, and SLER was significantly higher in neutral than in supinated (p < 0.01). Furthermore, the EMG of the infraspinatus in SIER was significantly higher in neutral than in supinated (p < 0.01). The EMG ratio (infraspinatus/ posterior deltoid) in SIER was significantly higher in neutral than in supinated (p < 0.05.) Contrarily EMG ratios in PRER and SLER were significantly higher in supinated than in neutral (p < 0.05).
Conclusion: The results show that clinicians should consider these exercise methods and forearm positions when planning shoulder external rotation exercises for optimal shoulder rehabilitation.

Keywords: Forearm position, Infraspinatus, Muscle activity, Posterior deltoid, Shoulder external rotation exercise

Article

Original Article

Phys. Ther. Korea 2022; 29(2): 106-116

Published online May 20, 2022 https://doi.org/10.12674/ptk.2022.29.2.106

Copyright © Korean Research Society of Physical Therapy.

어깨 가쪽돌림 운동 시 운동방법과 아래팔의 자세에 따른 가시아래근과 뒤어깨세모근의 근활성도 비교

손명기1, 김선엽2

1대전대학교 대학원 물리치료학과, 2대전대학교 보건의료과학대학 물리치료학과

Received: April 19, 2022; Revised: May 11, 2022; Accepted: May 12, 2022

Comparison of Infraspinatus and Posterior Deltoid Muscle Activities According to Exercise Methods and Forearm Positions During Shoulder External Rotation Exercises

Myeong-gi Son1 , PT, BHSc, Suhn-yeop Kim2 , PT, PhD

1Department of Physical Therapy, The Graduate School, Daejeon University, 2Department of Physical Therapy, College of Health and Medical Science, Daejeon University, Daejeon, Korea

Correspondence to:Suhn-yeop Kim
E-mail: kimsy@dju.kr
https://orcid.org/0000-0002-0558-7125

Received: April 19, 2022; Revised: May 11, 2022; Accepted: May 12, 2022

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Background: Shoulder external rotation exercises are commonly used to improve the stabilizing ability of the infraspinatus. However, during exercise, excessive activation of the posterior deltoid compared to the infraspinatus causes the humeral head to move anteriorly in an abnormal position. Many researchers have emphasized selective activation of the infraspinatus during shoulder external rotation exercise. Objects: This study aims to delineate the optimal exercise method for selective activation of infraspinatus by investigating the muscle activities of the infraspinatus and posterior deltoid according to the four shoulder exercise methods and two forearm positions.
Methods: Thirty healthy individuals participated in this study. The participants were instructed to perform shoulder external rotation exercises following four exercise methods: sitting external rotation (SIER); standing external rotation at 90° abduction (STER); prone external rotation at 90° abduction (PRER); side-lying external rotation (SLER), and two forearm positions (neutral, supinated). The electromyography (EMG) signal amplitude was measured during each exercise. Surface EMG signals were recorded from the posterior deltoid, infraspinatus, and biceps brachii.
Results: EMG results of the infraspinatus and posterior deltoid in PRER, were significantly higher than that of the other exercises (p < 0.01). The EMG ratio (infraspinatus/posterior deltoid) in SIER was significantly higher than that of the other exercises. EMG activation of the posterior deltoid in SIER, PRER, and SLER was significantly higher in neutral than in supinated (p < 0.01). Furthermore, the EMG of the infraspinatus in SIER was significantly higher in neutral than in supinated (p < 0.01). The EMG ratio (infraspinatus/ posterior deltoid) in SIER was significantly higher in neutral than in supinated (p < 0.05.) Contrarily EMG ratios in PRER and SLER were significantly higher in supinated than in neutral (p < 0.05).
Conclusion: The results show that clinicians should consider these exercise methods and forearm positions when planning shoulder external rotation exercises for optimal shoulder rehabilitation.

Keywords: Forearm position, Infraspinatus, Muscle activity, Posterior deltoid, Shoulder external rotation exercise

Fig 1.

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Figure 1.Study design. MVIC, maximal voluntary isometric contraction; SIER, sitting external rotation at 0° abduction; STER, standing external rotation at 90° abduction; PRER, prone external rotation at 90° of abduction; SLER, side-lying external rotation at 0° abduction; Neu, neutral; Sup, supinated.
Physical Therapy Korea 2022; 29: 106-116https://doi.org/10.12674/ptk.2022.29.2.106

Fig 2.

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Figure 2.Electrode placement. (A) posterior deltoid, (B) infraspinatus, (C) biceps brachii.
Physical Therapy Korea 2022; 29: 106-116https://doi.org/10.12674/ptk.2022.29.2.106

Fig 3.

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Figure 3.Shoulder external rotation exercises in forearm neutral position. (A) sitting external rotation at 0° abduction, (B) standing external rotation at 90° abduction, (C) prone external rotation at 90° of abduction, (D) side-lying external rotation.
Physical Therapy Korea 2022; 29: 106-116https://doi.org/10.12674/ptk.2022.29.2.106

Fig 4.

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Figure 4.Shoulder external rotation exercises in forearm supination position. (A) sitting external rotation at 0° abduction, (B) standing external rotation at 90° abduction, (C) prone external rotation at 90° of abduction, (D) side-lying external rotation.
Physical Therapy Korea 2022; 29: 106-116https://doi.org/10.12674/ptk.2022.29.2.106

Fig 5.

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Figure 5.Muscle activity of each exercise and forearm position. SIER, sitting external rotation at 0° abduction; STER, standing external rotation at 90° abduction; PRER, prone external rotation at 90° of abduction; SLER, side-lying external rotation at 0° abduction; %MVIC, %maximal voluntary isometric contraction; PD, posterior deltoid; IF, infraspinatus; BB, biceps brachii; Neu, neutral; Sup, supinated. *p < 0.05. **p < 0.01.
Physical Therapy Korea 2022; 29: 106-116https://doi.org/10.12674/ptk.2022.29.2.106

Fig 6.

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Figure 6.Muscle activity rankings per exercise of posterior deltoid and infraspinatus. PD, posterior deltoid; IF, infraspinatus; %MVIC, %maximal voluntary isometric contraction; Neu, neutral; Sup, supinated; PRER, prone external rotation at 90° of abduction; SLER, side-lying external rotation at 0° abduction; STER, standing external rotation at 90° abduction; SIER, sitting external rotation at 0° abduction.
Physical Therapy Korea 2022; 29: 106-116https://doi.org/10.12674/ptk.2022.29.2.106

Table 1 . General characteristics of subjects.

VariableData
Age (y)21.97 ± 1.59
Sex (Male/Female)14/16
Dominant side (Rt/Lt)29/1
Height (cm)167.13 ± 7.54
Weight (kg)62.03 ± 9.82
BMI (kg/m2)22.13 ± 2.46

Values are presented as mean ± standard deviation or number only. Rt, right; Lt, left; BMI, body mass index..

Table 2 . Comparison of muscle activity of each muscle by exercises and forearm positions.

VariableSIERSTERPRERSLERF
Posterior deltoidNeutral2.65 ± 1.3515.69 ± 5.51a52.38 ± 13.21a,b17.12 ± 6.98a,c152.400**
Supinated2.35 ± 1.1115.77 ± 6.62a47.95 ± 10.81a,b15.51 ± 6.90a,c171.749**
Difference0.30 ± 0.52–0.08 ± 2.884.43 ± 7.61a,b1.61 ± 2.47a5.994**
t3.164**–0.1503.184**3.580**
InfraspinatusNeutral26.77 ± 9.0145.01 ± 13.74a80.39 ± 18.67a,b54.66 ± 11.51a,b,c180.628**
Supinated23.09 ± 8.8743.99 ± 13.42a77.08 ± 16.54a,b52.95 ± 11.13a,b,c215.421**
Difference3.68 ± 3.901.02 ± 4.703.31 ± 9.371.71 ± 5.671.633
t5.168**1.1881.9381.651
Biceps brachiiNeutral9.23 ± 4.5013.49 ± 9.51a9.73 ± 11.01b3.30 ± 2.51a,b,c43.155**
Supinated8.03 ± 4.2216.69 ± 9.85a11.78 ± 14.10b4.28 ± 3.08a,b,c29.706**
Difference1.20 ± 3.34–3.20 ± 3.38a–2.05 ± 5.22a–0.98 ± 1.70a,b9.574**
t1.967–5.185**–2.153*–3.162**

Values are presented as mean ± standard deviation. SIER, sitting external rotation at 0° abduction; STER, standing external rotation at 90° abduction; PRER, prone external rotation at 90° of abduction; SLER, side-lying external rotation at 0° abduction. aThere is a significant difference from SIER (p < 0.05), bThere is a significant difference from STER (p < 0.05), cThere is a significant difference from PRER (p < 0.05). *p < 0.05, **p < 0.01..

Table 3 . Comparison of IF/PD muscle activity ratio by exercises and forearm positions.

VariableSIERSTERPRERSLERF
IF/PD ratioNeutral11.33 ± 4.063.05 ± 1.03a1.59 ± 0.40a,b3.75 ± 1.95a,c79.067**
Supinated10.64 ± 3.613.06 ± 1.13a1.66 ± 0.39a,b4.08 ± 2.16a,c65.323**
Difference0.69 ± 1.61–0.02 ± 0.49–0.07 ± 0.16–0.33 ± 0.832.531
t2.356*–0.191–2.212*–2.210*

Values are presented as mean ± standard deviation. SIER, sitting external rotation at 0°abduction; STER, standing external rotation at 90°abduction; PRER, prone external rotation at 90°of abduction; SLER, side-lying external rotation at 0°abduction; IF, infraspinatus; PD, posterior deltoid. aThere is a significant difference from SIER (p < 0.05), bThere is a significant difference from STER (p < 0.05), cThere is a significant difference from PRER (p < 0.05). *p < 0.05, **p < 0.01..