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Pressure related spatial summation effects

Authors Rodrigo, Weliwitage Don Asanka Sanjeewa
Issue Date 2010
Summary Interaction between humans and different types of equipment is unavoidable and thus human interfaces, such as feet, hands and buttocks, always experience various types of forces during such interactions. When the force per unit area, which is known as pressure, is excessive, people will experience pain or discomfort (Gonzalez et al., 1999). Therefore, the aim of this research was to determine a loading arrangement that would minimize potential pain and discomfort. Thus, the characteristics of pressure perception with different loading arrangements, namely (1) concentric probes (2) equal load distribution and (3) unequal load distribution have been tested and analyzed. The plantar foot tissue deformation was modeled using a two-element spring model. The tissue stiffness parameters on the plantar foot differ with location, indentation speed and probe area. The pressure perception characteristics were evaluated using pressure discomfort threshold (PDT) and pressure pain threshold (PPT) and the reliability analysis showed that both variables have very high test-retest reliability. It was found that the stiffness of the second deformation region is positively correlated with the PPT (r = 0.75, p < 0.001). According to the dimensional analysis, the PPT can be mathematically modeled in the form of PPT=c[(V.t)2/A]α or PPT=c[d2/A]α,(R2 = 0.98) where V is the indentation speed, t is the time of indentation, A is the probe area and d is the indentation depth. The coefficient, c, can characterize the tissue property and the exponent, α is possibly a representation of the spatial summation effect of pressure related pain. It is found that pressure sensation follows Stevens‘ power law (1957) and the exponent β of the power law shows a linear relationship with the probe area (A). Furthermore, the perceived pressure sensation can be mathematically modeled in the form of S = 0.12A0.6P (R2 = 0.99) where P is the pressure of the probe. Among the three locations tested on the plantar foot, the heel had the highest PPT and PDT values, followed by the area under the third metatarsal phalangial joint. The lowest PPT and PDT were under the medial arch. The indentation speeds significantly affected the pressure thresholds, and the PPT and PDT increased when the indentation speed is increased. However, the probe gap or probe inner diameter of concentric indentors did not affect the pain thresholds. With dual probes, probe separations of less than 21 mm have no effect on PPT and PDT. However, uneven distribution of load between the two probes can significantly change the resultant pressure thresholds. Distributing the load in a 1:2 ratio between the two probes gave the highest PPT and PDT values, which is around 15.6% higher than that of even load distribution. A load arrangement that minimizes pressure discomfort and pain by suppressing the effect of spatial summation of pressure pain was determined. It is found that one way of suppressing the effects of spatial summation is unequal loading among probes. The findings of this research have important implications in the design of load bearing devices and accessories. The results of the research can also be useful in explaining why some footwear designs are more or less comfortable than other designs.
Note Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2010
Language English
Format Thesis
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