title: Describing the active region boundary of EMG-assisted biomechanical models of the low back.
abstract: BACKGROUND
  Electromyography-assisted (EMG-assisted) biomechanical models are used to characterize the muscle and joint reaction forces in the lumbar region. However, during a full-range trunk flexion, there is a transition of extension moment from the trunk extensor muscles to the passive tissues of the low back, indicating that the empirical EMG data used to drive these EMG-assisted models becomes less correlated with the extensor moment. The objectives of this study were to establish the trunk flexion angles at which the passive tissues generate substantial trunk extension moment and to document how these angles change with asymmetry.
  
  
  METHODS
  Participants performed controlled trunk flexion-extension motions in three asymmetric postures. The trunk kinematics data and the electromyographic activity from L3- and L4-level paraspinals and rectus abdominis were captured. The time-dependent net internal active moment (from an EMG-assisted model) and the net external moment were calculated. The trunk and lumbar angles at which the net internal active moment was less than 70% of the external moment were found.
  
  
  RESULTS
  The trunk flexion angle at which the net internal moment reaches the stated criteria varied as a function of asymmetry of trunk flexion motion with the sagittally symmetric case providing the deepest flexion angle of 38° (asymmetry 15°: 33°; asymmetry 30°: 26°).
  
  
  CONCLUSIONS
  These results indicate that EMG-assisted biomechanical models need to consider the role of passive tissues at trunk flexion angles significantly less than previously thought and these flexion angles vary as a function of the asymmetry and direction of motion.
authors.affiliation: The Ergonomics Laboratory, Department of Industrial and Manufacturing Systems Engineering, Iowa State University, Ames, IA 50011-2164, USA. xiaopeng.ning@mail.wvu.edu
authors.forename: Xiaopeng
authors.initials: X
authors.lastname: Ning
authors.forename: Sangeun
authors.initials: S
authors.lastname: Jin
authors.forename: Gary A
authors.initials: GA
authors.lastname: Mirka
mesh_terms: D000328:Adult; D001415:Back / Q000502:physiology; D003198:Computer Simulation; D004576:Electromyography / Q000379:methods*; D006801:Humans; D008159:Lumbar Vertebrae / Q000502:physiology*; D008297:Male; D008954:Models, Biological; D009068:Movement / Q000502:physiology; D009119:Muscle Contraction / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D004856:Postural Balance / Q000502:physiology*; D011187:Posture / Q000502:physiology*; D016059:Range of Motion, Articular / Q000502:physiology; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity