Plantar foot ulcers are a severe and common complication associated with diabetes that overwhelmingly lead to non-traumatic major amputations among diabetic individuals. There are several known factors that contribute to the development of these ulcers, however it is possible that stiffening of foot structures (i.e. muscles, tendons, ligaments) is another important factor that has yet to be fully investigated. Increased soft tissue stiffness on the plantar surface of the foot has been found in diabetic individuals, but stiffness of individual foot structures has yet to be investigated. It has been proposed in literature that stiffening of muscles and tendons in diabetic feet cause increased plantar pressures, which often precede development of ulcers. However, to date, no study has comprehensively examined stiffness of individual foot structures in diabetic individuals and the effect of stiffness on plantar pressures during gait. Therefore, the ultimate purpose of the following work was to investigate the relationship between foot structure stiffness and plantar pressures during gait in diabetic individuals. Firstly, it was hypothesized that stiffness of foot structures would be directly and linearly related to plantar pressures during gait. Secondly, it was hypothesized that diabetics would exhibit higher stiffness and higher plantar pressures than controls. There is also evidence of structural changes in the diabetic foot compared to controls, including thickening of the plantar fascia (PF) and Achilles tendon. Plantar fasciitis is a common musculoskeletal disorder that, like diabetes, is associated with thickening of the PF. To date, few studies have investigated material properties of the PF, and there are currently no studies that have assessed material properties of other arch supporting structures (i.e. muscles, tendons) . It is possible that, in addition to thickening of the PF, plantar fasciitis populations exhibit material property changes of the PF and other arch supporting structures that contribute to the plantar fasciitis injury mechanism. Investigating material properties of the PF and arch supporting structures and how these properties relate to plantar pressures in individuals with plantar fasciitis may help provide relevant information to injury development in the foot in plantar fasciitis and diabetic populations. Therefore, material properties of foot structures and plantar pressures during gait were also assessed in individuals with plantar fasciitis. First, it was hypothesized that individuals with active plantar fasciitis symptoms would exhibit altered stiffness of foot structures compared to controls and individuals with a history of plantar fasciitis who are currently asymptomatic. Secondly, it was hypothesized that stiffness of PF stiffness would inversely and linearly relate to plantar pressures during gait in individuals with plantar fasciitis. The studies herein provide evidence that: 1) relationships are present between individual foot structures and plantar pressures in diabetic individuals and; 2) individual foot structures exhibited higher stiffness in diabetic individuals for some, but not all examined foot structures compared to controls. Contrary to the primary hypothesis, the observed relationships were mostly negative, suggesting that lower stiffness of individual foot structures relates to higher pressure. There is evidence that individuals with plantar fasciitis exhibit structural property changes similar to those observed in diabetic individuals, thus material properties of foot structures and their relationships with plantar pressures were also assessed in this population. Interestingly, individuals with plantar fasciitis exhibited mostly positive relationships, which was also contrary to the hypothesis for that population. Although some differential relationships existed within these groups, the diabetic and plantar fasciitis population displayed similar values for proximal plantar fascia stiffness that was negatively correlated with peak pressure under the heel. Structurally, diabetic individuals and individuals with plantar fasciitis similarly displayed decreased thickness of muscles and tendons which is suggestive of weakening and/or damage occurring to these structures. Taken together, these results support the idea of foot structure stiffness relating to plantar pressures and more specifically, are suggestive of damage occurring to the plantar fascia that is directly influencing plantar pressure distributions and foot function in diabetic individuals and individuals with plantar fasciitis. Thus, stiffness may still be an important factor to consider in understanding alterations of foot function and potentially in the ulcer injury mechanism in diabetic individuals.
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