Muscle activation patterns in patients with recurrent shoulder instability.
Instability of the glenohumeral joint (GHJ) often presents as a cryptogenic pain and/ or a feeling of, or even actual, displacement. Physiotherapy, consisting of exercises aiming to strengthen the rotator cuff muscles, is the first treatment of choice in patients with recurrent atraumatic unidirectional or multidirectional instability. To date, it remains unclear whether inappropriate activation levels or timing of activation of other, more superficially located muscles, could actually play a destabilizing role with regard to the GHJ.
A large cohort of patients with shoulder instability was followed and dynamic electromyography (DEMG) data were collected on the pectoralis major (PM), latissimus dorsi (LD), anterior deltoid (AD) and infraspinatus (IS) muscles during shoulder flexion, abduction, extension, external rotation and cross-body horizontal adduction. The acquired activation patterns were then compared with normal activation patterns based on established literature and biomechanical actions of muscles.
In cases presenting with anterior instability, PM and LD showed distinct abnormal activation patterns. The most important muscles with deviating activation patterns in patients with posterior instability were the LD and IS – the latter being remarkably inactive. In multidirectional instability, LD was the most dominant muscle involved.
Given the fact that the LD is a possible contributor to both anterior as well as posterior instability, the most plausible mechanism is for LD to cause an inferior translation of the GHJ and, depending on the plane of arm elevation either additional anterior or posterior translation > From Jaggi et al., In J Shoulder Surg 6 (2012) 101-107. All rights reserved to the International Journal of Shoulder Surgery.
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