Ann Rehabil Med.  2016 Apr;40(2):318-325. 10.5535/arm.2016.40.2.318.

Changes in Activation of Serratus Anterior, Trapezius and Latissimus Dorsi With Slouched Posture

• Lee ST
• Moon J
• Lee SH
• Cho KH
• Im SH
• Kim M
• Min K

• KMID: 2309932
• DOI: http://doi.org/10.5535/arm.2016.40.2.318

Abstract

OBJECTIVE
To compare quantitative muscle activation between erect and slouched sitting postures in the muscles around the scapula, and to investigate the correlation between the angle of thoracic kyphosis and the alteration of muscle activity depending on two different sitting postures.
METHODS
Ten healthy males participated in the study. Unilateral surface electromyography (SEMG) was performed for serratus anterior, middle trapezius (MT), and lower trapezius (LT), which are scapular stabilizer muscles, as well as latissimus dorsi. Participants elevated their shoulders for 3 seconds up to 90° abduction in the scapular plane, tilting 30° anterior in the coronal plane. They were told to hold the position for 10 seconds and voluntary isometric contractions were recorded by SEMG. These movement procedures were conducted for three times each for erect and slouched sitting postures and data were averaged.
RESULTS
Activities of MT and LT increased significantly more in the slouched sitting posture than in the erect one. There was no significant correlation between kyphotic angle and the area under curve of each muscle.
CONCLUSION
Because MT and LT are known as prime movers of scapular rotation, the findings of this study support the notion that slouched sitting posture affects scapular movement. Such scapular dyskinesis during arm elevation leads to scapular stabilizers becoming overactive, and is relevant to muscle fatigue. Thus, slouched sitting posture could be one of the risk factors involved in musculoskeletal pain around scapulae.

Keyword

Posture; Kyphosis; Shoulder; Electromyography

MeSH Terms

Area Under Curve
Arm
Electromyography
Humans
Isometric Contraction
Kyphosis
Male
Muscle Fatigue
Muscles
Musculoskeletal Pain
Posture*
Risk Factors
Scapula
Shoulder
Superficial Back Muscles*

Figure

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  Fig. 1 The measurement of Shoulder Pain and Disability Index. (A) Numeric rating scale, (B) Shoulder Pain and Disability Index.

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  Fig. 2 Two clinical tests for the assessment of scapular positioning: (A) the distance between the posterior border of the acromion and the table in the supine position, (B) the distance between the medial border and the fourth thoracic spinous process.

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  Fig. 3 Upright sitting and slouched sitting position: (A) resting upright sitting position, (B) upright sitting position with shoulder abduction, (C) resting slouched sitting position, and (D) slouched sitting position with shoulder abduction.

• 

  Fig. 4 Recording sites of surface electromyography (EMG) and measurement timing of muscle activation. (A) Serratus anterior (SA), middle trapezius (MT), lower trapezius (LT) and latissimus dorsi (LD). (B) Area under curve (mV·ms) was measured for 6 seconds.

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  Fig. 5 Measurement sites of thoracic kyphosis. (A) The level of 1st and 2nd thoracic spine. (B) The level of 12th thoracic spine and 1st lumbar spine. Thoracic kyphosis=(A) angle+(B) angle.

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  Fig. 6 Differences of muscle activities between upright and slouched sitting positions (n=10). Values are presented as mean±standard deviation. SA, serratus anterior; MT, middle trapezius; LT, lower trapezius; LD, latissimus dorsi. *p<0.05 by Wilcoxon signed-rank test.

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