Latent Myofascial Trigger Points Are Associated With an Increased Intramuscular Electromyographic Activity During Synergistic Muscle Activation

The aim of this observational study was to measure both intramuscular and surface electromyographic (EMG) activity of latent neuro-trigger points (LNTP) compared to non-LNTP in the upper trapezius (UT) during shoulder abduction in asymptomatic subjects. The investigators recorded intramuscular EMG activity of the UT and surface EMG recordings for the middle deltoid, and upper, middle, and lower trapezius at rest and during 25% maximum voluntary contraction (MVC) with the shoulder positioned at 90° abduction. In addition to the EMG recording, the investigators recorded visual analogue scale (VAS) pain with needle insertion and during 25% MVC.

Finally, they recorded any pain referral that was experienced by the subject during the needle insertion. The results showed an increase in spontaneous electrical activity (SEA) for LNTP compared to non-LNTP while the extremity was at rest. The intramuscular EMG activity was also higher in the upper trapezius with a LNTP than a non-LTNP. Surface EMG activity did not show significant muscle activity during synergistic muscle contraction. Subjects also reported a significantly higher VAS score when the needle was inserted into a LNTP vs. the non-LNTP. Finally, no subjects reported referred pain from needle insertion when the non-LNTP was needled. However, 7 of the 15 subjects did report symptom referral when a needle was inserted into a LNTP of the UT.

Clinical Relevance/Commentary: The upper trapezius is a synergistic muscle for active shoulder abduction. The deltoid is the prime mover for shoulder abduction and the upper trapezius acts to assist the deltoid. This study supports the idea that the presence of LNTP in synergistic muscles impairs normal muscle activation patterns (Lucas 11’, Ibarra 11’). LNTP of synergistic muscles results in those muscles working harder in order to produce the same amount of force. This increased load may result in further inflammation to the neuro-muscular system and the progression of a LNTP into an active neuro-trigger point (ANTP). Furthermore, the increased SEA produced by the nerve creates contracted muscle fibers resulting in ischemia of the tissue. This ischemia perpetuates the loss of microcirculation to the tissue, which is one proposed mechanism for the pain experienced from trigger points (Simons/Travell 81’). This was demonstrated in this study by the VAS scores reported by the subjects with LNTP vs. non-LNTP. Inhibiting the neurological activity by reducing nerve inflammation using dry needling helps to reduce this ischemic phenomenon and improve circulation. Finally, it should be understood that the investigators only assessed the intramuscular activity of a single LNTP. It should be acknowledged that several LNTP might be present in a single muscle. It is likely that other synergistic muscles that assist with shoulder abduction share the same neuromuscular abnormalities as the upper trapezius. It is for this reason that we advocate a global assessment and treatment model. We believe a focus on the peripheral nervous system is required to optimize the benefits of dry needling and maximize patient and athletic performance.
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