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

Abstract

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.

Keywords: Biomechanics; Expanded butterfly plot; Gait interpretation; Pedotti diagram; Plantar surface.

Figure 1

A Pedotti diagram, illustrating total vertical force vectors experienced by the support surface, as resulting from shifting center of pressure during a step. The vectors in red, blue, and green indicate reaction forces occurring near heel strike, midstance, and toe-off instances, respectively. Vectors originate on the left side of the plot where heel strike occurs, and proceed towards the right, which shows vectors related to the metatarsal region. The image below the plot indicates the locations of the centers of pressure on the foot-ground interface. The image of the footprint shows the distribution of all pressure readings during the entire footstep.

Figure 2

Expanded Butterfly Plot of Pressure vs. Antero-Posterior shear stress (τ) along the midline of the foot, at times near: (a) heel strike; (b) midstance; (c) toe-off; and (d) cumulatively, from heel strike through toe-off. The footprint images below each plot show the vertical pressure recorded during the corresponding time interval. The midline used to make the plot is indicated in white. Each vector color is representative of a time frame corresponding to one instance in time, in increments of 10% of the total foot-platform contact time. Refer to the legend for the vector color chart.

Figure 3

Expanded Butterfly Plot of the forefoot line throughout stance phase, indicating: (a) Pressure vs. Medio-Lateral shear stress. Note that since these vectors fan outwards, they represent the situation where the transverse arch is flattening – thereby causing force vectors on the lateral side (right) to have both vertical and lateral components, and vectors on the medial side (left) to have both vertical and medial components. The legend indicates the time frame related to each vector, in increments of 10% of foot-platform contact time.