Hyperelastic modelling of nonlinear running surfaces

Accurate, 3-D analyses of running impact require a constitutive model of the running surface that includes the material nonlinearity shown by many modern surfaces. This paper describes a hyperelastic continuum that mimics the experimentally measured response of a particular treadmill surface. The material model sacrifices a little accuracy to admit a robust, low-order hyperelastic strain-energy functional. This helps prevent the premature termination of finite element simulations, due to numerical or material instabilities, that can occur with higher-order functionals. With only two free constants, it is also a more practical design tool. The best fit to the quasi-static response of the treadmill was achieved with an initial shear modulus =2 MPa and a power-stiffening index =25. The paper outlines the method used to derive the material constants for the treadmill, a device that is not amenable to the usual materials laboratory tests and must be reverse-engineered. Finite element analyses were then performed to ensure that the treadmill model interacts with the other components of the multibody running system in a numerically stable and physically realistic manner. The model surface was struck by a rigid heel, cushioned by a hyperfoam material that represents a shoe midsole. The results show that, while the ground reaction force is similar to that obtained with a rigid surface, the maximum principal stress in the shoe is reduced by 15%. Such a reduction, particularly when endured over many load cycles, may have a significant effect on comfort and damage to nearby tissue

Energy Management Mechanisms Employed at the Human-Material Interface of Traditional and Minimalist Shod Running

From recreational to elite athletes, greater than 50% of runners sustain overuse injuries each year, prompting substantial research efforts to identify causes of—and solutions to—the high injury rate. Different shoe types and material property degradation have been related to injury. Two popular footwear types are traditional shoes with thick graded soles and minimalist running footwear with thinner foam and/or non-graded soles. Notwithstanding 45 years of significant modifications to shoe design features, gained knowledge in kinesiology, and advanced technologies in polymer science, runner injury rates have not decreased and EVA foam has remained the primary running shoe midsole material since the 1970s. The purpose of this dissertation is to improve comprehensive understanding of energy management during multi-scale degradation of EVA foam and biomechanical responses of human runners. The grand challenge of this research was to navigate, adapt, and weave together polymer and kinesiology techniques to: (i) quantify midsole foam macroscopic, microscopic, and molecular-level degradation, and (ii) characterize human responses to the dynamic material properties of contemporary footwear. In the pursuit of human-material interactions, we first investigated fundamental energy management mechanisms. In Chapter II, we determined that humans innately reduced their impact preparatory mechanisms when foam thickness was increased from 0 – 50 mm. In Chapter III, we compared and defined molecular-level EVA foam degradation by thermal, UV, and mechanical exposures. The latter three chapters substantiated (Chapter IV) and utilized (Chapter IV-VI) a biofidelic footwear midsole mechanical ageing protocol informed by human running input variables. We determined that: (i) the foam midsole managed 90% of the shoe’s energy and inaccurate sample geometries overestimated energy absorption by 20% (Chapter IV), (ii) traditional and minimalist shoe energy management differences were due to thickness, wherein 66% thicker foams absorbed ­­83% more energy but degraded at a 49% faster rate (Chapter V), and (iii) subject-specific biomechanics were altered by unique degradation patterns induced from wearing and mechanically ageing traditional and minimalist shoes (Chapter VI). Overall, this dissertation improved multidisciplinary protocols, contributed data informed by end-use conditions, and incorporated body and shoe variables simultaneously, which is critical to future studies correlating energy management to running injuries

Adaptive Controllers for Assistive Robotic Devices

Lower extremity assistive robotic devices, such as exoskeletons and prostheses, have the potential to improve mobility for millions of individuals, both healthy and disabled. These devices are designed to work in conjunction with the user to enhance or replace lost functionality of a limb. Given the large variability in walking dynamics from person to person, it is still an open research question of how to optimally control such devices to maximize their benefit for each individual user. In this context, it is becoming more and more evident that there exists no "one size fits all" solution, but that each device needs to be tuned on a subject-specific basis to best account for each user's unique gait characteristics. However, the controllers that run in the background of these devices to dictate when and what type of actuation to deliver often have up to a hundred different parameters that can be tuned on a subject-specific basis. To hand tune each parameter can be an extremely tedious and time consuming process. Additionally, current tuning practices often rely on subjective measures to inform the fitting process. To address the current obstacles associated with device control and tuning, I have developed novel tools that overcome some of these issues through the design of control architectures that autonomously adapt to the user based upon real-time physiological measures. This approach frames the tuning process of a device as a real-time optimization and allows for the device to co-adapt with the wearer during use. As an outcome of these approaches, I have been able to investigate what qualities of a device controller are beneficial to users through the analysis of whole body kinematics, dynamics, and energetics. The framework of my research has been broken down into four major projects. First, I investigated how current standards of processing and analyzing physiological measures could be improved upon. Specifically, I focused on how to analyze non-steady-state measures of metabolic work rate in real time and how the noise content of theses measures can inform confidence analyses. Second, I applied the techniques I developed for analyzing non-steady-state measures of metabolic work rate to conduct a real-time optimization of powered bilateral ankle exoskeletons. For this study I employed a gradient descent optimization to tune and optimize an actuation timing parameter of these simple exoskeletons on a subject-specific basis. Third, I investigated how users may use an adaptive controller where they had a more direct impact on the adaptation via their own muscle recruitment. In this study, I designed and tested an adaptive gain proportional myoelectric controller with healthy subjects walking in bilateral ankle exoskeletons. Through this work I showed that subjects adapted to using increased levels of total ankle power compared to unpowered walking in the devices. As a result, subjects decreased power at their hip and were able to achieve large decreases in their metabolic work rate compared to unpowered walking. Fourth, I compared how subjects may use a controller driven by neural signals differently than one driven by mechanically intrinsic signals. The results of this project suggest that control based on neural signals may be better suited for therapeutic rehabilitation than control based on mechanically intrinsic signals. Together, these four projects have drastically improved upon subject-specific control of assistive devices in both a research and clinical setting.PHDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/144029/1/jrkoller_1.pd

The design, construction and evaluation of sprint footwear to investigate increased sprint shoe bending stiffness on sprint performance and dynamics

The design, construction and evaluation of sprint footwear to investigate increased sprint shoe bending stiffness on sprint performance and dynamic

Finite element analysis of pin positioning in Lapidus procedure for treating Hallux Valgus

A finite element analysis is carried out to find the optimum position of the pin placement of mini fixator in Lapidus procedure in the treatment of Hallux Valgus. Various parameters are considered for analysis like diameter of the pin, positioning of the pin from the fracture site, number of pins effecting the stability of the fixation device and fusion site, rail distance from the fusion site, effect of width and length of the rail, effect of fusion angle and effect of pin angle positioning in fusion the joint by using FEMLAB 2.3 for both modeling and analysis. A 2D model is constructed with the bone joint consisting of first metatarsal and cuneiform along with the fixation device. The dimensions of the model are taken similar to a prototype model of the foot. Static analysis was done to find the displacement between the first metatarsal and cuneiform with the application of the mini fixator

Age-related determinants of the walk-to-run transition in youth : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Sport and Exercise Science, Massey University, Wellington, New Zealand

There is a lack of consensus regarding when mature or adult-like gait is achieved in youth. The ability to adjust gait during the walk-to-run transition (WRT) may be a good indicator of whether gait has matured. Specifically, age-related differences in the preferred transition speed (PTS) and determinants of WRT can provide insight into self-organising behaviours and how effectively gait patterns are regulated in youth. This thesis therefore assessed WRT in 49 youth (10-17-year-olds) and 13 young adults (19-29-year-olds) to: 1) investigate how effectively youth can adjust to increasing gait speed; and 2) explore age-related differences in determinants of PTS. Participants completed a WRT treadmill protocol that started at a self-selected walking speed and increased by 0.06 m∙s⁻¹ every 30 s to determine PTS. Participants also walked and ran on a treadmill at speeds near PTS (PTS, PTS±0.14 m·s⁻¹, PTS±0.28 m·s⁻¹). During these tests, muscle activity (rectus femoris, biceps femoris, tibialis anterior, medial gastrocnemius), oxygen consumption, heart rate and perceived exertion were assessed for their role in determining PTS. There were no age-related differences in PTS despite there being anthropometric differences. However, 10-12-year-olds exhibited more exploratory behaviour when determining PTS, while adults and 15-17-year-olds generally used a single transition to determine PTS. Age-related differences in PTS determinants were observed. Specifically, the biceps femoris and medial gastrocnemius were additional weak links among 10-12-year-olds and 10-17-year-olds, respectively, suggesting these muscles continue developing through childhood and adolescence. Because youth transition to minimise the demands of more muscles than adults, they may have more conflicting sources of feedback arising from the musculature when adjusting their gait. The 10-14-year-olds also exhibited greater difficulties distinguishing differences in perceived exertion between walking and running at speeds near PTS. The inability to anticipate increases in effort as gait speed increased could explain the indecisiveness in determining PTS among 10-12-year-olds. Overall, this thesis improves our understanding about rate-limiting factors of gait maturation. It seems that 10-12-year-olds have more conflicting sensory cues involved in regulating gait, which can cause difficulties determining how to optimise their gait. As the musculoskeletal system matures through adolescence, so does the ability to adapt gait effectively