title: Latent myofascial trigger points are associated with an increased intramuscular electromyographic activity during synergistic muscle activation.
abstract: The aim of this study was to evaluate intramuscular muscle activity from a latent myofascial trigger point (MTP) in a synergistic muscle during isometric muscle contraction. Intramuscular activity was recorded with an intramuscular electromyographic (EMG) needle inserted into a latent MTP or a non-MTP in the upper trapezius at rest and during isometric shoulder abduction at 90° performed at 25% of maximum voluntary contraction in 15 healthy subjects. Surface EMG activities were recorded from the middle deltoid muscle and the upper, middle, and lower parts of the trapezius muscle. Maximal pain intensity and referred pain induced by EMG needle insertion and maximal pain intensity during contraction were recorded on a visual analog scale. The results showed that higher visual analog scale scores were observed following needle insertion and during muscle contraction for latent MTPs than non-MTPs (P < .01). The intramuscular EMG activity in the upper trapezius muscle was significantly higher at rest and during shoulder abduction at latent MTPs compared with non-MTPs (P < .001). This study provides evidence that latent MTPs are associated with increased intramuscular, but not surface, EMG amplitude of synergist activation. The increased amplitude of synergistic muscle activation may result in incoherent muscle activation pattern of synergists inducing spatial development of new MTPs and the progress to active MTPs.
  
  
  CONCLUSIONS
  This article presents evidence of increased intramuscular, but not surface, muscle activity of latent MTPs during synergistic muscle activation. This incoherent muscle activation pattern may overload muscle fibers in synergists during muscle contraction and may contribute to spatial pain propagation.
authors.affiliation: Laboratory for Musculoskeletal Pain and Motor Control, Center for Sensory-Motor Interaction, Department of Health Science and Technology, Aalborg University, Aalborg, Denmark. Electronic address: ghy@hst.aau.dk.
authors.forename: Hong-You
authors.initials: HY
authors.lastname: Ge
authors.affiliation: Rovira i Virgili University, Medicine and Surgery Department, REUS, Tarragona, Spain.
authors.forename: Sonia
authors.initials: S
authors.lastname: Monterde
authors.affiliation: Laboratory for Musculoskeletal Pain and Motor Control, Center for Sensory-Motor Interaction, Department of Health Science and Technology, Aalborg University, Aalborg, Denmark.
authors.forename: Thomas
authors.initials: T
authors.lastname: Graven-Nielsen
authors.affiliation: Laboratory for Musculoskeletal Pain and Motor Control, Center for Sensory-Motor Interaction, Department of Health Science and Technology, Aalborg University, Aalborg, Denmark.
authors.forename: Lars
authors.initials: L
authors.lastname: Arendt-Nielsen
mesh_terms: D000328:Adult; D057645:Deltoid Muscle / Q000503:physiopathology; D004576:Electromyography / Q000379:methods*; D005260:Female; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D008297:Male; D009068:Movement / Q000502:physiology; D009209:Myofascial Pain Syndromes / Q000503:physiopathology*; D010146:Pain / Q000503:physiopathology; D010147:Pain Measurement; D053591:Referred Pain / Q000503:physiopathology; D012146:Rest / Q000502:physiology; D012782:Shoulder / Q000503:physiopathology; D020069:Shoulder Pain / Q000503:physiopathology; D064171:Superficial Back Muscles / Q000503:physiopathology*; D061028:Trigger Points / Q000503:physiopathology*
keywords: Intramuscular electromyography; isometric contraction; motor control; muscle synergy; myofascial trigger points