Walker, Stephanie Julie2025-12-032025https://mro.massey.ac.nz/handle/10179/73897Background: Biomechanical research and footwear engineering have facilitated specific running shoe designs, with a particular focus on the shoe’s midsole. The primary aim of the midsole is to facilitate energy absorption (cushioning) from the initial impact of each step incurred during running, while recovering the maximum amount of energy. However, relatively few research articles report the pre-production foam properties or the postproduction capability of the shoes’ midsole in accordance with established in vitro industry standard testing. Moreover, parameters of running gait are often assessed in environments that do not replicate real-world overground running, whereby a limited number of steps are sampled, along with a lack of consideration for different speeds or changes in terrain, creating variation within in vivo test results. This highlights the nuanced understanding of how midsole cushioning interacts with the biomechanics of running. Aims: 1. Assess the in vitro mechanical properties of aliphatic thermoplastic polyether polyethylene (ATPU) foam midsoles with varying densities. 2. Evaluate the in vivo effects of midsole foam density, within geometrically identical running shoes, on parameters of gait (spatiotemporal, kinetic, and joint kinematics) across different running speeds, with a particular focus on vertical peak impact force and average loading during real-world overground running. Hypotheses: 1. In vitro testing will demonstrate the lower-density ATPU foam midsole to have greater energy absorption and energy recovery. 2. During in vivo experimental trials, parameters of running gait related to ground reaction force, specifically vertical peak impact force and average loading rate, will be reduced in the lower-density midsole. It is also expected that increases in running speed will influence spatiotemporal, kinetic, and joint kinematic parameters of gait. Methods: In vitro testing was performed using a modified industry standard test (ISO 20344:2021 (5.17)). The midsole was compressed with 2.2 kN of force, in the vertical direction at a deformation rate of 100 mm·min-¹. Following conditioning, five continuous cycles were performed while recording deformation (mm) and load (kN), from which the final cycle was extracted for analysis. In vivo trials consisted of 16 recreational to nationally competitive endurance runners. The experimental protocol consisted of shoes classified by the density of the foam’s midsole (high-ρ (0.17 g·cm-³) and mid-ρ (0.14 g·cm-³)) and three running speeds (12, 14, and 16 km·h-¹). Participants ran 360 m on tarmacadam, at each running speed, paced by a cyclist. LoadSol® insoles were used to collect spatiotemporal and kinetic parameters of running gait, while four AHRS-IMU’s attached to the sacrum and right lower limb (shank, femur, and foot) simultaneously recorded joint kinematics. Results: In vitro results demonstrated that the mid-ρ midsole absorbed significantly more energy than the high-ρ midsole (t (30) = 6.412, p < 0.0001), as well as recovering significantly more energy (t (30) = 9.052, p < 0.0001). In vivo trials showed that increases in running speed significantly increased vertical peak impact force (F ₂, ₃₀) = 32.24, p < 0.0001), average loading rate (F (₂, ₃₀) = 38.70, p < 0.0001), stance phase hip extension (F (₂, ₃₀) = 100.7, p < 0.0001), and swing phase hip flexion (F (₂, ₃₀) = 197.3, p < 0.0001), while decreasing stride duration (F (₂, ₃₀) = 34.95, p < 0.0001), ground contact time (F (₂, ₃₀) = 233.6, p<0.001), and impulse (F (₂, ₃₀) = 19.64, p<0.0001). There was no significant main effect of midsole density for vertical peak impact force (F (₁, ₁₅) = 0.01175, p = 0.915), average loading rate (F (₁, ₁₅) = 0.5649, p = 0.464), or any other parameter of running gait, along with no significant interactions. Conclusion: The ATPU foam materials differed in density and produced significant differences during in vitro testing when manufactured into midsoles; however, these differences were not substantial enough to elicit significant changes during the in vivo overground running trials. This suggests that a greater level of material difference may be required to produce observable changes in parameters of running gait. It is also plausible that runners adapt their movement patterns in response to variations of midsole cushioning. The practical implications of this study indicate that individuals may not experience changes in running performance solely due to subtle differences in midsole foam properties.enThe authorThesis