Even though pressure on feet is generally unavoidable and undue pressure on some regions have been widely accepted as the major source of foot discomfort, pain and problems, there is little understanding of related human perceptions. The primary objective of this thesis is to determine the discomfort and pain pressure thresholds at various locations on the foot and investigate their relationships with the stimulus characteristics of stimulus area, A and indentation speed, V, location differences and mechanical properties of foot tissue. Psychophysical functions related to pressure stimuli are also established....[ Read more ]

Even though pressure on feet is generally unavoidable and undue pressure on some regions have been widely accepted as the major source of foot discomfort, pain and problems, there is little understanding of related human perceptions. The primary objective of this thesis is to determine the discomfort and pain pressure thresholds at various locations on the foot and investigate their relationships with the stimulus characteristics of stimulus area, A and indentation speed, V, location differences and mechanical properties of foot tissue. Psychophysical functions related to pressure stimuli are also established.

There are significant deformations in the foot when the load on the foot is changed from no body-weight (NWB) to half body-weight (HWB). However, further deformations from HWB to full body-weight (FWB) are not significant in 9 critical foot dimensions with a few exceptions, indicating that most foot deformations from NWB to FWB occur when half of the body weight acts on each foot.

In order to investigate the relationships among pressure thresholds, stimulus characteristics and mechanical properties of foot tissue, a cost-effective indentation apparatus, the Automated Tissue Tester (ATT) was developed. This apparatus is automatic, flexible, and reliable with several safety features. It has varying indentation speeds, indention probes and alignment directions. The ATT was used to conduct a study (4 indentation probes * 3 indentation speeds * 13 test locations *2 repetitions) on the pressure perception of the right feet of 20 participants (10 males, 10 females). The dependent variables were pressure discomfort threshold (PDT), pressure pain threshold (PPT), tissue properties parameters (stiffness K50 and energy dissipation ratio EDR) and subjective ratings of perceived intensity at five levels of pressure stimuli.

Measurement reliability tests (intra-class correlation) on both PDT and PPT demonstrate that the participants are not only capable of judging the pain threshold, but also reliably differentiate discomfort from pain. PDT is highly correlated to PPT and PDT accounts for slightly less than half of the PPT for both males (ratio of 0.41~0.43) and females (ratio of 0.45 ~0.49).

PPT and PDT depend on area, speed and location. A higher PPT corresponds to a smaller area and faster speed. This indicates that there are significant spatial summation effects on the human perception of pressure. A faster speed contributing to a higher PPT is the result of an increase in tissue stiffness (K50) as K50 is found to have a significant positive relationship with PPT for both the foot sole and dorsum. As for the inter-location difference, except for the medial plantar arch and foot center, which have less contact with the ground, all tested foot locations on the foot sole have higher PPT than those on the foot dorsum.

PPT can be mathematically modeled incorporating area and speed in the form of PPT = a + b / √A + c*v (R²>0.92). Subsequently, a more unified model of the form, PPT = t₀ + t₁*(K50 / √A) + t₂*v across 7 test locations on the foot sole is also established for both males and females.

A psychophysical model relating physical pressure magnitude I to perceived intensity S is constructed in the form of S=100*I^β (in a ratio relative to PPT). The fitted power exponents, β, are less than 1.0 and consistent across the 13 test locations (β has a mean of 0.82 with SD of 0.10, range: 0.67-0.98). These findings indicate that even though the intensity of the perceived sensation increases monotonically with the magnitude of physical pressure stimulus, it is negatively accelerated (0<β<1) on the foot. In other words, a doubling of pressure intensity results in less than a doubling in sensation.

The mathematical models of PPT relating to characteristics of stimulus, tissue properties of different test locations and the psychophysical model bridging psychological domain (perceived intensity) with physical domain (physical magnitude of pressure stimulus) can help researchers with an increased understanding of pressure perception on the foot. The pressure sensitivity map on foot, quantitative data of PDT, PPT and mechanical properties of foot tissue, such as stiffness, can help footwear designers distinguish sensitive foot locations under pressure and then design tissue-compliant footwear products.