The stresses can be evaluated from formula (

11) as follows:

where

*W* is the strain energy function.

We used the form of the strain energy function for soft tissues described by Demiray.

^{13} This function is given by

where

*C*_{1} and

*C*_{2} are mechanical parameters to be determined. These parameters are analogous to the elastic parameters in the small deformation theory of elasticity. Their values are determined by curve fitting as explained in the Evaluation of Mechanical Parameters section.

After applying the equilibrium equations and the boundary conditions, we arrived at the following equations for evaluating the normal and tangential stresses, N and T, respectively. The details are provided in Chaudhry et al.^{8}

Using equations (

13) and (

14), we have

Then the normal pressure (N) is given by

The tangential stress (T) along CD in

*Figure 1* becomes

We assumed that the forces applied to the surface of the skin are transmitted entirely to the underlying fat and fascia, independent of the thickness of these tissues. We tested this hypothesis by placing various materials as proxies for skin, fat, and fascia on a pressure mat and applying forces of 10 kg, which was within the range of forces applied in manual therapies.

^{8} The forces were applied through these layers to the pressure mat in several configurations, and in all cases the forces were transmitted entirely through the layers, with measurements varying from 5% to 7%, which is within the sensitivity range of this device. This hypothesis does not conflict with the results in authoritative articles by others who have studied the transmission of forces through tissue.

^{14-16} Bereznick et al

^{14} studied frictional properties at the thoracic skin-fascia interface during spinal manipulative therapy. In their introduction, the authors stated that in the absence of friction, the normal component of the applied forces is directly transmitted to the underlying vertebrae, which is what we assumed. Kawchuk and Perle

^{15} studied the relationship between the application angle of spinal manipulative therapy and the resultant vertical acceleration produced. According to their findings (presented in

Table 1 of their article), the experimentally measured transmitted force can be slightly more or less than the predicted force, and the average difference between the predicted force (based on the applied force) and the transmitted force to the underlying vertebral layer is −0.59%, with an SD of more than 6%. This finding indicates that any difference between the applied force and the measured transmitted force is well within the measurement capabilities of the experiment. That is, within experimental measurement error, the applied force is totally transmitted to the underlying layer. It should be noted that Perle and Kawchuk

^{16} studied the distribution of pressure generated on a single rigid surface during hand manipulation and its relationship with the location and magnitude of hand configurations; thus, their findings are not directly relevant to the work in the current article.