Background:​ One in five adults in the USA has a disability, with serious walking disabilities being most prevalent. These disabilities often include impairment of the ankle’s power generation capabilities. During late stance, the ankle generates a large amount of power, used to propel the body. During this same period, the distal foot deforms substantially and absorbs power, leading to an overall lower ankle-foot complex power. Researchers have found that, by adding stiffness to the foot, the power absorbed by the distal foot decreases. When this happens, the ankle power also decreases, indicating an interplay in the energetics of the two systems. Ankle-foot orthoses (AFOs) are often prescribed to treat walking impairments. However, these devices do not take advantage of the recent energetics research in the design of the footplate. This previous research indicates that it may be possible to apply these findings in the design of a foot orthosis that stores and returns energy. Unfortunately, there is not a good understanding of the force-deformation characteristics of the anklefoot complex during stance.

Purpose:​ This thesis aimed to characterize the force-deformation patterns of the distal foot in healthy individuals across a range of walking speeds in order to determine how stiffness about the metatarsophalangeal (MTP) joint changes. Then, these data were applied to design an in-shoe, customized deformable foot orthosis that aimed to modulate the energetics of the ankle-foot system in healthy individuals.

Methods:​ For Aim 1, eighteen healthy individuals were recruited to walk barefoot at four, height controlled, speeds in an overground motion capture laboratory with in-ground forceplates. Data collected from these trials were used to calculate stiffness about the MTP joint at each speed and a two-way repeated measures ANOVA was run to determine if stiffness about the MTP joint varied based on dominant/non-dominant limb or by walking speed.

For Aim 2, nine healthy individuals were recruited to walk in the same overground motion capture laboratory over two visits. In the first visit, subjects walked barefoot (BAREFOOT) at 0.8 statures/second. From these data, stiffness about the MTP joint for the dominant and non-dominant limbs were calculated, along with the power generation during stance of the distal foot, ankle/midfoot and ankle-foot complex. These stiffnesses were then used as an approximate stiffness level in designing a deformable foot orthoses, customized to the subject’s shoe size and metatarsal axis location delineating the deformable region, out of carbon fiber. The subjects then returned for a second visit where they walked in flexible, minimalist shoes with (ORTHOSIS) and without (SHOE ONLY) the foot orthosis inserted. These data were then analyzed to determine the power profiles during stance of the distal foot, ankle/midfoot and ankle-foot complex. A 2x3 dependent measures MANOVA was then run with dominant/non-dominant limbs and footwear conditions as factors on the distal foot positive work, net ankle/midfoot work and net ankle-foot complex work as dependent variables.

Results:​ For Aim 1, the two-way dependent ANOVA indicated that stiffness about the MTP joint did not vary by dominant/non-dominant limb. Additionally, the ANOVA indicated that at each speed the stiffness about the MTP joint was significantly different across speeds. For Aim 2, the MANOVA indicated that, for positive distal foot work, net ankle/midfoot work and net ankle-foot complex work, the dominant/non-dominant limb was not significantly different. Additionally, it showed that there was no difference between BAREFOOT and SHOE ONLY for any variable and that the ORTHOSIS condition, compared with the other two conditions, showed significantly higher positive distal foot work, lower net ankle/midfoot work and unchanged net ankle-foot complex work.

Significance:​ These results showed that the stiffness about the MTP joint of the individual dependent on the walking speed and that, for all measures, the dominant and non-dominant limbs behaved similarly. Additionally, it showed that a foot orthosis can modulate the energetics of the ankle-foot system to store and return energy for healthy individuals.