Muscle activation patterns in the scapular positioning muscles during loaded scapular plane elevation: the effects of Latent Myofascial Trigger Points.

BACKGROUND: Latent Myofascial Trigger Points are pain-free neuromuscular lesions that have been found to affect muscle activation patterns in the unloaded state. The aim was to extend these observations to loaded motion by investigating muscle activation patterns in upward scapular rotator muscles (upper and lower trapezius and serratus anterior) hosting Latent Myofascial Trigger Points simultaneously with lesion-free synergists for shoulder abduction (infraspinatus and middle deltoid). This approach allowed examination of the effects of these lesions on both their hosts and their lesion-free synergists in order to understand their effects on the performance of shoulder abduction. METHODS: Surface electromyography was employed to measure the timing of onset of muscle activation of the upper and lower trapezius and serratus anterior (upward scapular rotators), infraspinatus (rotator cuff) and middle deltoid (abductor of the arm) initially without load and then with light (1-4 kg) dumbbells. Comparisons were made between control (no Latent Trigger Points; n=14) and Latent Trigger Point (n=28) groups. FINDINGS: The control group displayed a relatively stable sequence of muscle activation that was significantly different in timing and variability to that of the Latent Trigger Point group in all muscles except middle deltoid (all P<0.05). The Latent Trigger Point group muscle activation pattern under load was inconsistent, with the only common feature being the early activation of the infraspinatus. INTERPRETATION: The presence of Latent Trigger Points in upward scapular rotators alters the muscle activation pattern during scapular plane elevation, potentially predisposing to overuse conditions including impingement syndrome, rotator cuff pathology and myofascial pain.

The impact of latent trigger points on regional muscle function.

To date, most investigation of latent myofascial trigger points (LTrPs) has occurred in pain populations. Many have thought that LTrPs are clinically relevant as -potential precursors to developing active myofascial trigger points and spontaneous pain. Nociceptive substances have been found in greater concentrations at LTrP sites compared with non-TrP sites, indicating the potential for group III and IV afferent fibers to provide input to the central nervous system from affected peripheral sites. Fatigue and neurophysiologic studies provide evidence as to the pathways via which group III and IV afferents can alter activity of the motoneuron pool and therefore affect muscle activation and performance. This article offers suggestions as to the mechanisms via which LTrP-related pathophysiology may explain the clinical examination findings associated with LTrP-containing and functionally related muscles.