title: Small vertical changes in jaw relation affect motor unit recruitment in the masseter.
abstract: Strategies for recruitment of masseter muscle motor units (MUs), provoked by constant bite force, for different vertical jaw relations have not previously been investigated. The objective of this study was to analyse the effect of small changes in vertical jaw relation on MU recruitment behaviour in different regions of the masseter during feedback-controlled submaximum biting tasks. Twenty healthy subjects (mean age: 24·6 ± 2·4 years) were involved in the investigation. Intra-muscular electromyographic (EMG) activity of the right masseter was recorded in different regions of the muscle. MUs were identified by the use of decomposition software, and root-mean-square (RMS) values were calculated for each experimental condition. Six hundred and eleven decomposed MUs with significantly (P < 0·001) different jaw relation-specific recruitment behaviour were organised into localised MU task groups. MUs with different task specificity in seven examined tasks were observed. The RMS EMG values obtained from the different recording sites were also significantly (P < 0·01) different between tasks. Overall MU recruitment was significantly (P < 0·05) greater in the deep masseter than in the superficial muscle. The number of recruited MUs and the RMS EMG values decreased significantly (P < 0·01) with increasing jaw separation. This investigation revealed differential MU recruitment behaviour in discrete subvolumes of the masseter in response to small changes in vertical jaw relations. These fine-motor skills might be responsible for its excellent functional adaptability and might also explain the successful management of temporomandibular disorder patients by somatic intervention, in particular by the use of oral splints.
authors.affiliation: Department of Prosthodontics, Dental School, University of Heidelberg, Heidelberg, Germany.
authors.forename: S
authors.initials: S
authors.lastname: Terebesi
authors.affiliation: Department of Prosthodontics, Dental School, University of Heidelberg, Heidelberg, Germany.
authors.forename: N N
authors.initials: NN
authors.lastname: Giannakopoulos
authors.affiliation: Department of Prosthodontics, Dental School, University of Heidelberg, Heidelberg, Germany.
authors.forename: F
authors.initials: F
authors.lastname: Brüstle
authors.affiliation: Department of Prosthodontics, Dental School, University of Heidelberg, Heidelberg, Germany.
authors.forename: D
authors.initials: D
authors.lastname: Hellmann
authors.affiliation: Department of Reconstructive Dentistry and Temporomandibular Disorders, Dental School, University of Basel, Basel, Switzerland.
authors.forename: J C
authors.initials: JC
authors.lastname: Türp
authors.affiliation: Research Group Biomechanics, Institute for Mechanics, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany.
authors.forename: H J
authors.initials: HJ
authors.lastname: Schindler
mesh_terms: D000704:Analysis of Variance; D001732:Bite Force; D003766:Dental Occlusion; D004576:Electromyography; D005260:Female; D064368:Healthy Volunteers; D006801:Humans; D008297:Male; D008406:Masseter Muscle / Q000000981:diagnostic imaging / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology*; D011999:Recruitment, Neurophysiological / Q000502:physiology*; D055815:Young Adult
keywords: craniomandibular disorders; electromyography; masseter muscle; neurophysiological recruitment; occlusal splints; vertical dimension