Regional peak plantar pressures are highly sensitive to region boundary definitions

Traditional pedobarographic analyses subsample pressure data over a number of discrete anatomical regions of interest (ROIs). To our knowledge, the sensitivity of these data to ROI boundary definitions has not been previously addressed. Eight subjects each performed 20 trials of self-paced walking; commercial software was used to define 10 ROIs for each of the 160 total peak pressure images, and regional peak pressures (RPPs) were extracted for each image (total: 1600 values). We then asked three specific questions regarding RPP sensitivity to ROI boundary definition: (1) Is the ROI centroid representative of the RPP location? (2) How frequently do RPPs lie on the ROI boundary? and (3) By how much do RPP values change if the ROI boundary is changed by one pixel (resolution: 5.08 x 7.62 mm)? We found that the RPP locations differed from the ROI centroid in 80% of the cases and that the RPPs lay on the ROI boundary with a probability of 65%. We also found that a single-pixel change in the ROI boundary caused a mean RPP change of 10.8%. The most sensitive region was the midfoot for which a single-pixel ROI change yielded a median 29.4% change in RPP. These results indicate that RPP data are biased by regionalization schemes, which delineate pressure fields based on anatomy rather than on the field's geometric properties, and ultimately that regionalization may constitute a poor method of quantifying complex pressure fields. RPP sensitivity should be considered when making statistical inferences regarding foot function.