title: The frictional properties at the thoracic skin-fascia interface: implications in spine manipulation.
abstract: OBJECTIVE
  To assess the friction at the thoracic skin-fascia interface to determine the potential reaction force vectors during thoracic manual therapy.
  
  
  METHODS
  A basic in vivo study of human subjects, documenting the frictional properties at the interface between the thoracic skin and underlying fascia.
  
  
  BACKGROUND
  Chiropractors, and other spine manipulative therapists, during thoracic manipulation have been attempting to apply force vectors to spine tissues in specific directions in addition to those applied normal to the skin. For obliquely applied forces to be directly transmitted to the underlying vertebrae, either friction is required at the skin-fascia interface or the applied force must "hook" on a bony process.
  
  
  METHODS
  Subjects were placed in the prone position with the thoracic skin exposed. The posterior thoracic region was loaded with normal forces, incrementally from 125.3 to 392.9 N. The interface between the load and the skin was either a plexiglass plate or modelled hands. A force was then applied to either apparatus in the cephalad direction. The applied forces and corresponding displacements were measured using a load cell and an optoelectronic camera system, respectively. Chiropractors then performed actual thoracic manipulation to determine if they could maintain their location of contacts (spinous process/transverse process) on the underlying vertebra.
  
  
  RESULTS
  Each of the subjects exhibited negligible friction between the thoracic skin and underlying fascia for both the plexiglass and modelled hand contacts. Furthermore, in each case, the apparatus travelled a distance greater than that between two transverse or spinous processes without showing an abrupt change in the slope of the force-displacement curves. The hands of chiropractors performing thoracic manipulation travelled a similar distance during the dynamic thrust.
  
  
  CONCLUSIONS
  The skin-fascia interface over the thoracic spine exhibits negligible friction. Therefore, the reaction force from a thoracic vertebra will be normal to the overlying skin. Furthermore, the data show that the ability to "hook" either a thoracic transverse or spinous process in the superior-inferior direction during a manipulative thrust may be greatly over-rated.
  
  
  CONCLUSIONS
  During thoracic spinal manipulation, one cannot direct a force vector to a thoracic vertebra at a given angle by simply directing their thrust in that direction.
authors.affiliation: Faculty of Applied Health Sciences, Department of Kinesiology, Spine Biomechanics Laboratory, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada.
authors.forename: David E
authors.initials: DE
authors.lastname: Bereznick
authors.forename: J Kim
authors.initials: JK
authors.lastname: Ross
authors.forename: Stuart M
authors.initials: SM
authors.lastname: McGill
mesh_terms: D000328:Adult; D001696:Biomechanical Phenomena; D005205:Fascia / Q000502:physiology*; D017276:Friction; D006801:Humans; D008297:Male; D020393:Manipulation, Spinal / Q000379:methods*; D011312:Pressure; D011446:Prospective Studies; D012016:Reference Values; D012680:Sensitivity and Specificity; D012879:Skin Physiological Phenomena; D013904:Thoracic Vertebrae / Q000502:physiology*