Watch your step! Towards predicting osteoarthritis onset based on side-to-side imbalances

Osteoarthritis (OA) is a debilitating disease characterized by the erosion of articular cartilage at the extremity of bones. OA contributes to economic burdens, pain, and abnormal locomotion to accommodate for loss of protective cartilage. Since there is no cure for OA, mitigating disease onset can relieve the lives of millions of people who are at higher risk of OA such as females and overweight people.The progressive disappearance of protective cartilage leads to bone-on-bone contact at the joints, which is aggravated by higher-than-normal joint contact forces. Although OA can affect any joint, the primary weight-bearing joints of the lower body, i.e. hip, knee, and ankle, suffer the most impairment. Thus, investigating walking behavior can aid in detecting abnormal locomotion that may lead to OA.The objectives of this study were (1) to investigate a simple mechanical model’s ability to accurately reproduce measured gait kinetics and (2) to propose and evaluate novel parameters to supplement current noninvasive clinical tools for gait analysis.For a total of forty healthy subjects, kinematic and kinetic parameters were optimized for 300 consecutive steps to fit experimental vertical ground reaction force data measured during treadmill walking. Using an existing inverted spring-loaded pendulum with a spring-loaded ankle, we assessed the variations in leg and ankle stiffnesses during gait. We quantified bilateral lower limb symmetry, gait regularity, and gait variability based on the optimized stiffness values, which highlighted gait disparities between males and females, and between different body mass index categories.Our results confirmed that all subjects exhibited a certain amount of side-to-side asymmetry, irregularity, and variability in their leg and ankle stiffnesses during walking. Furthermore, large inter-subject variability indicated that our simple model could detect idiosyncratic gait patterns and therefore estimate potential imbalances in gait patterns. Future studies to test these walking assessments with accelerations as input parameters, which are easier to measure in a clinical setting, can improve current screenings for OA

Kinematic and dynamic analysis for biped robots design

En esta tesis un nuevo método para encontrar sistemas dinámicamente equivalentes es propuesto. El objetivo es el de crear una herramienta para el análisis de robots bípedos. La herramienta consiste en modelos simplificados obtenidos del principio de equivalencia dinámica, que dice que si dos sistemas poseen la misma masa, el mismo centro de masa y el mismo momento de inercia, entonces son dinámicamente equivalentes. Este concepto no es nuevo y es comúnmente utilizado en el diseño de máquinas alternativas, o para encontrar el sweet spot de objetos esbeltos tales como bates o espadas. Con la aplicación del principio de equivalencia dinámica se encuentra el centro de percusión. La aportación en esta tesis es la aplicación de este concepto al análisis de robots bípedos, y la extensión del centro de percusión a cadenas cinemáticas. La herramienta fundamental para la obtención de resultados de investigación en esta tesis hace uso del lenguaje de simulación Modelica®. Las simulaciones son altamente detalladas gracias a la librería estándar Multibody incluida en las especificaciones del mismo. Como consecuencia de los trabajos desarrollados se crearon nuevas clases para extender la capacidad de la librería y aplicarla a m´aquinas caminantes. El desarrollo de esta tesis está centrado en el desarrollo de dos modelos. El primero es un péndulo invertido equivalente, con la característica que posee las mismas propiedades dinámicas del robot que modela. Dichas propiedades son la masas total, el centro de masa y el momento de inercia. Este modelo es luego utilizado para generar el caminar de un bípedo simple. El bípedo es simulado con un volante de inercia como cuerpo, y pies de contacto puntual. Posee rodillas y está totalmente actuado. Los eslabones del robot poseen propiedades de sólido rígido y ninguna simplificación ha sido considerada. El segundo modelo tiene el objetivo de imitar la topología del bípedo que representa, por lo tanto tiene un grado mayor de complejidad que el anterior. Este modelo es construido al dividir al robot en tres grupos: Las dos piernas, y otro grupo compuesto por la cabeza, los brazos y el torso (Denominado HAT por sus siglas en inglés). Este modelo es denominado modelo de cuatro masas puntuales. Este modelo es posteriormente validado utilizándolo para desacoplar la dinámica del sistema, la única información utilizada para llevar a cabo esta tarea es proporcionada por dicho modelo. ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------In this thesis a method to find dynamically equivalent systems is proposed. The objective is to provide a tool to analyze biped robots by simplifying their dynamics to simpler models. The equivalent models are obtained with the concept of dynamic equivalence that states that if two systems share the same total mass, the same center of mass, and the same moment of inertia then they are considered to be dynamically equivalent. This concept is not new and it is used in the design of alternative machines, or to find the sweet spot of long object like swords or bats. The result of the application of the dynamic equivalence principle is the point known as the center of percussion. The novelty in this thesis is to apply this concept to the analysis of biped robots, and the extension of the center of percussion to kinematic chains. The work in this thesis developed with the help of the simulation language Modelica®. The simulations are very detailed by implementing elaborated rigid body dynamics provided by the multibody standard library included in the language specifications. New classes were created in order to be able to simulate walking machines. Those classes introduce contact objects at ground foot interactions and mechanical stops for knee joints. The development of this thesis is centered around the proposal of two models. The first model is an equivalent inverted pendulum with the characteristic that it has the same dynamic properties, i.e., total mass, center of mass and moment of inertia, of the biped that models. This model is later used to synthesize gait in a simple, but realistic biped. The biped is simulated with a flywheel body, and point feet. It has knees and it is fully actuated. Also all the links have complete rigid body properties and no simplifications were done. The second model has the objective to resemble the topology of the biped it represents, therefore it is slightly more complex than the equivalent inverted pendulum. This model is constructed by grouping the components of the robot in three groups: Two legs and the HAT group (HAT stands for head, arms and trunk). This model is denominated four point masses model. The model is later validated by decoupling the dynamics of the system only with the information provided by the four point masses model