title: Active trunk stiffness during voluntary isometric flexion and extension exertions.
abstract: OBJECTIVE
  Compare muscle activity and trunk stiffness during isometric trunk flexion and extension exertions.
  
  
  BACKGROUND
  Elastic stiffness of the torso musculature is considered the primary stabilizing mechanism of the spine. Therefore, stiffness of the trunk during voluntary exertions provides insight into the stabilizing control of pushing and pulling tasks.
  
  
  METHODS
  Twelve participants maintained an upright posture against external flexion and extension loads applied to the trunk. Trunk stiffness, damping, and mass were determined from the dynamic relation between pseudorandom force disturbances and subsequent small-amplitude trunk movements recorded during the voluntary exertions. Muscle activity was recorded from rectus abdominus, external oblique, lumbar paraspinal, and internal oblique muscle groups.
  
  
  RESULTS
  Normalized electromyographic activity indicated greater antagonistic muscle recruitment during flexion exertions than during extension. Trunk stiffness was significantly greater during flexion exertions than during extension exertions despite similar levels of applied force. Trunk stiffness increased with exertion effort.
  
  
  CONCLUSIONS
  Theoretical and empirical analyses reveal that greater antagonistic cocontraction is required to maintain spinal stability during trunk flexion exertions than during extension exertions. Measured differences in active trunk stiffness were attributed to antagonistic activity during flexion exertions with possible contributions from spinal kinematics and muscle lines of action.
  
  
  CONCLUSIONS
  When compared with trunk extension exertions, trunk flexion exertions such as pushing tasks require unique neuromuscular control that is not simply explained by differences in exertion direction. Biomechanical analyses of flexion tasks must consider the stabilizing muscle recruitment patterns when evaluating spinal compression and shear loads.
authors.affiliation: Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA.
authors.forename: Patrick J
authors.initials: PJ
authors.lastname: Lee
authors.forename: Kevin P
authors.initials: KP
authors.lastname: Granata
authors.forename: Kevin M
authors.initials: KM
authors.lastname: Moorhouse
mesh_terms: D000704:Analysis of Variance; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D008954:Models, Biological; D018482:Muscle, Skeletal / Q000502:physiology*; D005082:Physical Exertion; D017568:Rectus Abdominis / Q000502:physiology*; D013131:Spine / Q000502:physiology* / Q000503:physiopathology*