Expanded butterfly plots: A new method to analyze simultaneous pressure and shear on the plantar skin surface during gait

The current method of visualizing pressure and shear data under a subject's foot during gait is the Pedotti, or "butterfly" diagram. This method of force platform data visualization was introduced in the 1970s to display the projection of the ground reaction force vector in the sagittal plane. The purpose of the current study was to examine individual sub-components of the vectors displayed in Pedotti diagrams, in order to better understand the relationship between one foot region and another. For this, new instrumentation was used that allows multiple Pedotti diagrams to be constructed at any instant during the gait cycle. The custom built shear-and-pressure-evaluating camera system (SPECS) allows for simultaneous recordings of pressure and both components of the horizontal force vector (medio-lateral and antero-posterior) at distinctive regions under one's foot during gait. Data analysis of such recordings affirms three conclusions: (i) pressure and shear values on individual sites on the plantar surface of the foot are not associated in a linear manner, (ii) force vectors in the heel and forefoot regions exhibit horizontal force components that oppose one another, and similarly, (iii) force vectors in the frontal plane transecting the forefoot region also exhibit medial-lateral shear components that counteract one another. This approach sheds light on individual vectors that collectively sum to each vector displayed in a Pedotti diagram. The results indicate that shearing between the foot and the ground is not simply a passive event. The structures of the arches and/or muscular activities are major contributors to the observed interfacial stresses.