Objective: To assess the friction at the thoracic skin-fascia interface to determine the potential reaction force vectors during thoracic manual therapy. Design: 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.