Evaluation of Electromyographic Activity of Upper Trapezius Muscle Fatigue in Patients with Latent Myofascial Trigger Point: A Randomized Control Trial

Latent Myofascial Trigger Points (LMTrPs) are defined as certain pain-free hyperirritable spots in a taut band of muscle, which lead to alternation in muscle activation pattern in both loaded and unloaded conditions. Fatigue can alter the stability of the cervical spine because of transferring loads to the passive connective tissues and also increasing muscle activity. Few studies have investigated the effects of fatigue on the parameters of the upper trapezius muscle as a more common muscle to MTrPs. This study attempts to examine the electromyographic activity of the upper trapezius muscle fatigue during shoulder elevation.

Thirty-six right-handed subjects without upper extremity disorders took part in this study. The highest measured force level was assumed to be the Maximal Voluntary Contraction (MVC) of the trapezius. A sustained submaximal contraction of the trapezius was performed. The subjects were asked to sustain a unilateral (80%) MVC isometric shoulder elevation until the force gauge monitor showed (50%) of MVC in at least three minutes.

The Root Mean Square (RMS norm) of the sustained trapezius contractions showed differences between the groups. The Myocardial Depressant Factor (MDF) parameters of the left and right sides of both healthy subjects and patients were significantly different (P<0.001).

The increase in RMS is related to the recruitment of additional motor units and also an increased firing rate. These are necessary to compensate for the loss of force. This accumulation also inhibits the excitability of the muscle membrane, thereby causing a decrease in the firing rate and, consequently, a decrease in Median Frequency (MF).