The Frictional Properties of the Thoracic Skin-Fascia Interface

The frictional properties of the thoracic skin-fascia interface: implications in spine manipulation
Bereznick DE, Ross JK, and McGill SM. Clinical Biomechanics: 2002

  • Several manual therapy texts describe certain techniques that involve attempting to mobilize or manipulate the verteba in an inferior-superior direction or one that is consistent with facet orientation. The assumption may be that there is sufficient friction between the skin and the underlying vertebra to transmit these forces.

  • The purpose of this study was to determine the frictional properties that exist between the skin and underlying tissue interface. The authors hypothesized that the skin-fascia interface is frictionless. If true, the clinical implication would be that certain applied forces would in fact be ineffective at generating a shear force to the underlying vertebra.

  • Three male subjects, one for each body type (ectomorph, mesomorph, and endomorph) were selected. Subjects lay prone on a table with skin over thoracic spine exposed. A clear plexiglass plate w/ a load cell was placed over the skin at the apex of the thoracic spine. An infrared emitting diode (IRED) was placed on top of this acting as a marker for the tracking system

  • To determine if it was possible to "hook" onto a TP or SP of a vertabra, the same protocol was used using two different modeled hands of a chiropractor and similar loads were applied. To avoid the argument that "real hands" weren't used, nine chiropractic faculty with not less than 5 yrs experience were recruited to see if contact b/w TPs and SPs could be maintained during thrust manipulation

  • In subjects where the skin-fascia interface was used for normal applied loads, the amount of friction present appeared to be negligible.

  • Measurements of the real chiropractic hands demonstrated that the avg displacement while trying to maintain contact on the TP was 38.75 mm and when attempting to hook the SP, the average displacement was 33.25 mm.

  • Forces applied to the underlying thoracic vertebra are dominated by the perpendicular component. Attempts to apply any oblique force may be wasted effort. For example, applying a force at a 45 degree angle reduces the resultant magnitude to 70%