2015 ISAKOS Biennial Congress ePoster #2405

Rotator Cuff Activation Varies with Core Stability – An EMG Analysis

Wataru Morita, MD, DPhil, Tokyo UNITED KINGDOM
David Lewis Wain, BSc, London UNITED KINGDOM
Anju Jaggi, MSc, MCSP, London UNITED KINGDOM
Susan Alexander, FRCS, PhD, MSc, London UNITED KINGDOM

Institute of Orthopaedics and Musculoskeletal Science, University College London, London, UNITED KINGDOM

FDA Status Not Applicable

Summary: Muscle activity of the shoulder external rotators was monitored using surface EMG in 9 different postures, which showed that the peak muscle activity tended to increase in more stable postures from sitting, standing, side-lying and then to prone, implying the importance of core stability and consideration to posture when designing a shoulder rehabilitation program.




Physiotherapy is a well-recognized form of treatment for shoulder pathology, and aims to recruit and strengthen muscles that control the shoulder in a coordinated manner. Exercises focused on activating the rotator cuff (RC) muscles have been established as a conservative treatment measure in conditions such as instability and RC pathologies, and are also applied in overhead athletes’ conditioning programs for performance enhancement and injury prevention. The kinetic chain model suggests the linkage of the core muscles that stabilize the posture in coordinating the desired function at the upper limb. Although previous studies have implied the direct effect of posture upon the shoulder muscles, the ideal posture in which to perform shoulder exercises is unknown, and no study has focused on the RC. The aim of this study is to investigate the effect of posture on recruitment of RC in order to accelerate and optimize shoulder therapy program. The hypothesis is that destabilizing overall posture will increase RC activity. Methods: Twenty healthy adult volunteers performed isokinetic shoulder external rotation movement in 9 different postures; (1) bipedal standing; (2) standing on contralateral leg; (3) standing on ipsilateral leg; (4) standing on tiptoes; (5) standing on a wobble board; (6) sitting on a stool; (7) side-lying; (8) prone position, all with 0° shoulder abduction; and (9) prone position, 90° shoulder abduction. Muscle activity of the external rotators was monitored by surface electromyography (EMG) as a reference of RC muscles. Sampled EMG data were normalized to the reference EMG, which was the peak muscle activity obtained by maximum voluntary isometric contraction in 0° shoulder abduction, neutral rotation (%MVIC). Differences of normalized EMG between postures were assessed statistically using two-way repeated-measures analysis of variance with posture and hand-dominance as factors. Tukey test was used for post-hoc analysis. Intra-class correlation coefficient (ICC) was calculated to assess the reliability of EMG. Statistical significance was set at p<0.05. Results: Peak muscle activity in external rotator cuff muscles was higher in the order of prone, side-lying, standing and then sitting. Bipedal standing tended to show higher muscle activity than destabilized standing postures. External rotation whilst lying prone with 90° abduction demonstrated the highest muscle activity, (p< 0.05 compared to standing postures). Overall ICC was 0.81, 95% CI = (0.78, 0.84). Conclusions: Muscle activation of shoulder external rotators increased with more stable postures, contrary to the hypothesis. It is vital to consider this when designing shoulder rehabilitation protocols in order to gradually recruit or strengthen muscles at different stages of recuperation. Inappropriate loading, too early may cause damage, and initial work to improve core stability and altering posture may protect the cuff muscles and improve outcome.