THE EFFECTS OF PECTORALIS MINOR STRETCHES ON SCAPULAR KINEMATICS AND MUSCLE LENGTH IN HEALTHY ADULTS: A SYSTEMATIC REVIEW AND META-ANALYSIS

Y.-T. Chan^1,2, D.-J. Yuh^1,3, D.-R. Hong¹, W.-Y. Chen¹, Y.-L. Lin¹

Background: During arm elevation, pectoralis minor muscle is passively elongated to allow scapula to upwardly rotate, externally rotate, and posteriorly tilt. Shortened pectoralis minor has been associated with scapular dyskinesis, including decreased scapular upward rotation and posterior tilt, which is a risk factor of shoulder pain. Therefore, to restore normal length of pectoralis minor, stretch is a common approach in clinical practice. According to the recommendation of ACSM, there are three types of stretch, including proprioceptive neuromuscular facilitation (PNF), dynamic stretch, and static stretch. Studies have investigated whether pectoralis minor length would increase and scapular kinematics would change following these stretch exercises in healthy subjects.  A previous systematic review and meta-analysis has found that a short-term PNF protocol could increase the pectoralis minor length. However, the effects of pectoralis minor stretches on scapular kinematics were still inconclusive.

Purpose:  We aimed to conduct a systematic review and meta-analysis to examine the changes in the scapular kinematics and length of pectoralis minor following stretch exercises.

Methods: We searched for studies in Pubmed, Ovid medline, CINAHL, Cochrane, and google scholar until March 2020, with keywords related to pectoralis minor muscle, stretch exercises, scapular kinematics, and muscle length. We screened a total of  2484 articles and included 2 RCTs, and 5 studies with a pretest-posttest design for analysis. The primary outcomes were 3-dimensional scapular kinematics during arm elevation. The secondary outcomes were pectoralis minor length (PML), and pectoralis minor index (PMI), the index of PML normalized by body height. Data from both RCTs and pretest-posttest studies were pooled for meta-analysis, with only including pretest-posttest data of the experimental group in the RCTs.

Results: In total, 133 healthy subjects were included in the meta-analysis. Following stretch exercise, no changes were found in all parameters of scapular kinematics at 30, 60, 90, and 120 degrees of arm elevation, including scapular upper rotation, posterior tilt, and external rotation. One study even demonstrated increased anterior tilt. In addition, there was a significant increase in PMI with a mean difference of 0.594  (95% CI [0.135-1.054], p=0.011) but no change in PML following stretch exercises. A higher dropout rate in the static stretch group due to pain or discomfort was reported in one study.

Conclusion(s): Although there is an effect of pectoralis minor stretch on the muscle length, scapular kinematics did not change following stretch exercise in healthy subjects. PMI may be sensitive to detect the change of muscle length following treatment, because the normalization may reduce the variability resulting from body height. Future work may investigate the effects of stretch exercise in patients with shoulder pain or individuals with shortened pectoralis minor.  

Implications: Stretch exercise for pectoralis minor could increase the muscle length but may not change scapular kinematics. Therefore, to change scapular kinematics, other exercises, such as scapular strengthening exercises, may need to be included in the treatment protocol. Static stretch, such as unilateral corner stretch, may need to apply with caution, especially in patients with shoulder injuries or pain, since it may induce pain or impingement.

Funding, acknowledgements: This study is not funded by any institution.

Keywords: pectoralis minor, stretching exercise, scapular kinematics

Topic: Musculoskeletal: upper limb

Did this work require ethics approval? No

Institution: N/A

Committee: N/A

Reason: The study design is a systematic review and meta-analysis