Shoulder joint functional alteration during arm elevation with impingement syndrome & latent trigger points

Shoulder elevation mechanics in impingement syndrome:

Both glenohumeral and scapulothoracic kinematics are altered during impingement syndrome. Normal & altered mechanics during shoulder elevation is discussed below.

Normal mechanics:

Functions of Trapezius muscle: Upper trapezius produces clavicular elevation and retraction. The middle trapezius is primarily a medial stabilizer of the scapula. The lower trapezius assists in medial stabilization and upward rotation of the scapula.

Functions of serratus anterior muscles: The middle and lower serratus anterior muscles produce scapular upward rotation, posterior tilting, and external rotation.

Pectoralis minor: The pectoralis minor is aligned to resist normal rotations of the scapula during arm elevation.

Rotator cuff: The rotator cuff is critical to stabilization and prevention of excess superior translation of the humeral head, as well as production of glenohumeral external rotation during arm elevation.

Alterations of shoulder muscle functions in impingement syndrome:

Alterations in activation amplitude or timing have been identified across various investigations of subjects with shoulder impingement. These include
1. Decreased activation of the middle or lower serratus anterior and rotator cuff,
2. Delayed activation of middle and lower trapezius
3. Increased activation of the upper trapezius and middle deltoid.

* Subjects with tight pectoralis minor exhibit altered scapular kinematic patterns similar to those found in persons with shoulder impingement.

Scapular tipping and serratus anterior muscle function are important to consider in the rehabilitation of patients with symptoms of shoulder impingement (3).

Impact of Latent myofascial trigger point in upward scapular rotator muscles (upper and lower trapezius and serratus anterior).

What are latent myofascial trigger points ?: These lesions are not pain producers but present as nodules in the muscle bulk. Commonly in shoulder upper and lower trapezius and serratus anterior hosting Latent Myofascial Trigger Points.

Lucas et al found presence of Latent Trigger Points in upward scapular rotators alters the muscle activation pattern during scapular plane elevation. This potentially predisposes to overuse conditions including impingement syndrome, rotator cuff pathology and myofascial pain.

Lucas et al found
1. In normal condition there is a relatively stable sequence of muscle activation.
2. The sequence is significantly different in timing and variability in the subjects with Latent Trigger Point group in all muscles except middle deltoid.
3. The Latent Trigger Point group muscle activation pattern under load was inconsistent, with the only common feature being the early activation of the infraspinatus.

Hence latent trigger points should be searched for treatment when it is even not producing non-segmental pain as it is known to produce.

References:

1. Phadke V et al; Rev Bras Fisioter. 2009 Feb 1;13(1):1-9. (Scapular and rotator cuff muscle activity during arm elevation: A review of normal function and alterations with shoulder impingement.)
2. Lucas KR et al; Clin Biomech (Bristol, Avon). 2010 Oct;25(8):765-70. Epub 2010 Jul 27. (Muscle activation patterns in the scapular positioning muscles during loaded scapular plane elevation: the effects of Latent Myofascial Trigger Points.)
3. Ludewig PM et al;Phys Ther. 2000 Mar;80(3):276-91. (Alterations in shoulder kinematics and associated muscle activity in people with symptoms of shoulder impingement.)