Calculating Traction Force
Traction pulls are opposed by friction forces. In order for traction forces to effect spinal segments, traction force has to exceed friction force. There is a mathematical relationship between body weight and the amount of friction force from the treatment surface. this is called the "coefficient of static friction".
The relationship of friction between a person and a mat table/treatment table has been found to equal 0.5 or 50% of body weight undergoing lumbar traction. Thoracic belts used in lumbar traction can decrease the amount of friction force, resulting in more net traction pull provided by the machine.
Patient body weight * 0.25 = minimum applied traction force to lumbar spine
In the cervical spine, traction forces must be 0.62 or 62% of the weight of the head in order to overcome the friction force from the treatment table.
The mathematical relationships of traction, friction, and the role of straps is reinforced in the linked traction lecture from Miami-Dade PTA Program