Effects of latent myofascial trigger points on muscle activation patterns during scapular plane elevation
Creators: name: Lucas, Karen
Publisher: name: RMIT University
Year: 2024

description: Despite a paucity of experimental evidence, clinical opinion remains that though LTrPs allow pain-free movement, they are primarily associated with deleterious motor effects and occur commonly in 'healthy' muscles. The primary aim of this study was to investigate the effects of LTrPs on the muscle activation patterns (MAPs) of key shoulder girdle muscles during scapular plane elevation of the arm in the unloaded, loaded and fatigued states. In connection with the main aim, a preliminary study was carried out to examine the frequency with which LTrPs occur in the scapular positioning muscles in a group of normal subjects.<br><br>After establishing intra-examiner reliability for the clinical examination process, 154 healthy subjects volunteered to be screened for normal shoulder girdle function, then undergo a physical examination for LTrPs in the trapezius, rhomboids, levator scapulae, serratus anterior and the pectoralis minor muscles bilaterally. Of these 'healthy' subjects, 89.8% had at least one LTrP in the scapular positioning muscles (mean=10.65 ± 6.8, range=1-27), with serratus anterior and upper trapezius harbouring the most LTrPs on average (2.46 ± 1.8 and 2.36 ± 1.3 respectively). Consistent with clinical opinion, this study found that LTrPs occur commonly in the scapular positioning muscles.<br><br>To investigate the motor effects of LTrPs, surface electromyography (sEMG) was used to measure the timing of muscle activation of the upper and lower trapezius and serratus anterior (upward scapular rotators), the infraspinatus (rotator cuff) and middle deltoid (arm abductor). These studies found that LTrPs housed in the scapular upward rotator muscles affected the timing of activation and increased the variability of the activation times of this muscle group and were also associated with altered timing of activation in the functionally related but LTrP-free infraspinatus and middle deltoid. Compared with the control group (all muscles LTrP-free), the MAPs of the LTrP group appeared to be sub-optimal, particularly in relation to preserving the subacromial space and the loading of the rotator cuff muscles. After the initial sEMG evaluations, the LTrP subjects were randomly assigned to one of two interventions: superficial dry needling (SDN) followed by post-isometric relaxation (PIR) stretching to remove LTrP s or sham ultrasound, to act as a placebo treatment where LTrPs remained. Where LTrPs were removed, a subsequent sEMG evaluation found MAPs to be similar to the control group in most of the experimental conditions investigated. Of particular note, when LTrPs had been treated and the subjects repeated the fatiguing protocol, the resultant MAP showed no significant difference with that of the control group in the rested state, suggesting treating LTrPs was associated with an improved response to fatigue induced by repetitive overhead movements.<br><br>In conclusion, the findings indicate that LTrPs commonly occur in scapular positioning muscles and have deleterious effects on MAPs employed to perform scapular plane elevation and thus affect motor control mechanisms. Treating LTrPs with SDN and PIR stretching increases pressure-pain thresholds, removes associated taut bands and at least transiently optimises the MAP during scapular plane elevation. Discussion includes possible neuromuscular pathophysiology that might explain these results.