<p> The human knee joint exhibits a spring-type behavior during the stance phase of walking at the preferred speed, which is both subject-specific and gait-specific. This observation led us to hypothesize that the human knee joint could partially adapt to an externally-applied tuned mechanical stiffness during the stance phase leading to reduced muscle involvement and energy expenditure. We also hypothesized that a spring, which is tuned to the body size and gait speed, in parallel with an impaired knee joint during the stance phase can partially restore the natural spring-type behavior of the knee joint. Three experimental and theoretical steps were taken to test these hypotheses.</p><p> First, a series of statistical models were developed that can closely characterize the moment-angle behavior of the knee joint using a set of measurable parameters including body weight and height, gait speed, and joint excursion. It is explained that these models can be used to tune the components of knee exoskeletons/orthoses and prostheses to the body size and gait speed of users, as well as general applications in understanding gait biomechanics. The statistical models of the knee joint were used in the next steps of this research to tune the stiffness of the experimental exoskeletal devices throughout the experimental sessions.</p><p> To experimentally test the first main hypothesis, a pair of quasi-passive knee exoskeletons was developed. When worn on a healthy subject, each exoskeleton implements an interchangeable spring in parallel with the knee joint during the stance and allows free rotation during the swing phase. The exoskeletons with a range of stiffness were used in a series of experiments on healthy individuals to study the mechanics and energetics of human gait in interaction with exoskeletal impedances in parallel with the knee joint. Healthy lower extremity joints showed substantial adaptation to the exoskeleton stiffness/assistance suggesting that replicating the natural behavior of a joint could be a viable method for the design of lower extremity exoskeletons to reduce muscle involvement and energy expenditure. It was also observed that a healthy knee joint can fully accommodate external assistance only to a certain level, above which the knee joint adaptation saturates and biarticular effects emerge.</p><p> To test the second hypothesis, a compliant stance control orthosis was developed that implements a spring in parallel with an impaired knee joint during the stance and allows free rotation during the swing phase. It was found that a compliant stance control orthosis can restore the natural spring-type behavior of an impaired knee joint during the stance phase. The compliant stance control orthosis showed higher gait speed and more natural kinematic patterns when compared with the state-of-the-art stance control orthoses that rigidly lock the knee during the stance phase. The findings of this research also showed that a friction-based latching mechanism can be a viable option in the design of lower extremity assistive devices that require engagement and disengagement of passive components.</p>

<p> Locomoting from one medium to another is crucial to the survival of many animals. Bullfrogs (<i>Lithobates catesbeianus</i>) leap from water into air to capture aerial prey or to escape water-filled depressions. Here, kinematics and kinetics of leaping on land and water are described and compared. High-speed videography was used to record both types of leaps and these videos were analyzed for their kinematics (joint extension, duration, and take-off velocity) and to calculate kinetic energy at takeoff. A custom digital particle image velocimetry (DPIV) instrument recorded the vortex ring shed from each frog foot. Joint extension sequences of both types of leaps and differences in take-off velocities were statistically identical. The kinetic energy contained in the two vortices shed from each foot was small in magnitude compared to the kinetic energy in the body of the frog. This suggests that the kinetic energy transfer from the movement of the frog is more to other types of waves, and less to the vortices. How these frogs are able to produce enough thrust to leave the water is likely due to the paddle-like shape of their feet, their elastic, energy storing tendons and muscle fascia, powerful muscle contractions resulting in a land leap applied in water.</p>

The study of the human motion as a discipline is ancient almost like the man. Early theories and observations on these topics can be found in Hyppocrates' and Galeno's work. More recently Duchenne de Boulogne (1867), Marey (1885), Braune and Fisher (1888), Sherrington (1933), Luria and finally Haken (1996) applied new techniques to the study of movement trying to understand and localise also the main areas of the brain involved during motion. Despite the richness of the literature produced, "man in motion" still represents a fascinating and partially unknown theme to deal with, particularly in the dynamic behaviour of the arms during the execution of specific tasks. Such movement, indeed individual expression of the complex interaction of biological subsystems (brain, muscles, skeleton, etc. ) against the surrounding environment, hides nowadays its features and very few data are available on its kinematic and dynamic response. This gap is largely due to the lack of knowledge on the dynamic movement of the "shoulder complex" and of the related muscles involved during motion. In fact, the large number of degrees of freedom to be measured and the high deformability of skin and soft tissues prevent the direct measurement of skeletal movements and contribute to increment the above described indetermination. Against this complex background, the rehabilitationist faces the pragmatic difficulties to decide which joints require attention as a priority or, in the case of biological damage, to assess the degree of impairment and subsequent recovery. As a result, clinical assessmentis performed by the use of relatively elementary test tasks, which can be monitored either by timing or by some indirect measurement of the success of the execution. The aim of the present research is then to provide new means of measurements to be used for gaining objective information on the motion particularly of "non visible" joints like the shoulder complex in order to characterise properly their motion and, in turn, the workspace of the arm.

A series of experiments are described which explore the relationship between biomechanical properties and the control of jaw movement in speech. This relationship is documented using kinematic analyses in conjunction with a mathematical model of jaw motion and direct measures of jaw stiffness. / In the first experiment, empirical and modeling studies were carried out to examine whether the nervous system compensates for naturally occurring forces acting on the jaw during speech. As subjects walk or run, loads to the jaw vary with the direction and magnitude of head acceleration. While these loads are large enough to produce a measurable effect on jaw kinematics, variation in jaw position during locomotion is shown to be substantially reduced when locomotion is combined with speech. This reduction in jaw motion is consistent with the idea that in speech, the control of jaw movement is adjusted to offset the effects of head acceleration. Results of simulation studies using a physiologically realistic model of the jaw provide further evidence that subjects compensate for the effects of self-generated loads by adjusting neural control signals. / A second experiment explores the idea that a principle mechanical property of the jaw---its spring-like behavior, or stiffness---might influence patterns of kinematic variation in speech movements. A robotic device was used to deliver mechanical perturbations to the jaw in order to quantify stiffness in the mid-sagittal plane. The observed stiffness patterns were non-uniform, with higher stiffness in the protrusion-retraction direction. Consistent with the idea that kinematic patterns reflect directional asymmetries in stiffness, a detailed relationship between jaw kinematic variability and stiffness was observed---kinematic variability was consistently higher under conditions in which jaw stiffness was low. Modeling studies suggested that the pattern of jaw stiffness is significantly determined by jaw geometrical properties and muscle force generating abilities. / A third experiment examines the extent to which subjects are able to alter the three-dimensional pattern of jaw stiffness in a task-dependent manner. Destabilizing loads were applied to the jaw in order to disrupt the ability of subjects to maintain a static jaw posture. Subjects adapted by increasing jaw stiffness in a manner that depended on the magnitude and, to a more limited extent, direction of the destabilizing load. The results support the idea that stiffness properties can be controlled in the jaw, and thus may play a role in regulating mechanical interactions in the orofacial system.

External loads, such as those due to the orientation of body segments relative to gravity, affect the extent to which control signals result in the achievement of desired goals. The degree to which subjects adjust control signals to compensate for loads provides a measure of what the nervous system knows about forces affecting motion and gives an indication of the complexity of control signals needed for voluntary movement. In the present study, we have explored the hypothesis that subjects take no account of the orientation of the head relative to gravity when making jaw movements during speech. We used a simulation model of the jaw to predict the kinematic effect of using a single set of motor commands (which take no account of the relative direction of the gravitational force) to produce speech-like movements while the body was in three different orientations: upright, prone and supine. The simulations predict a systematic change in jaw pitch angle and horizontal translation resulting from the change in body orientation. Empirical results for five subjects tested under the same conditions as those explored in the simulations are for the most part consistent with the pattern predicted by the model. This suggests that in the case of jaw movements during speech, control signals are not adjusted to account for changes in head and body orientation relative to gravity.

<p> The goal of this research is to study tissue growth via developing mathematical formulations and computational modeling. Tissue growth modeling has many applications --- including tumor growth, wound healing, bone remodeling, epithelial tissue remodeling, and other problems in developmental biology. Key to this study is incorporating the results of the analysis of non-destructive medical images that augment the models. Quantitative image analysis for the purpose of providing input parameters for and validation of tumor growth models (TGMs) is discussed. Two types of computational TGMs are studied in detail: one is based on the logistic equation, the other is based on the theory of porous media, or mixture theory. For the mixture-based model, we developed an algorithm that couples a level set method to track tumor boundaries while the tissues themselves are treated as a perfused mixture. After the mathematical foundation of each of the TGMs is formulated, we discuss implementation aspects, along with computational results. Finally, we validate the computational results with experimental observations of tumor volume versus time via imaging data acquired from animal models. The RMS deviation between predicted and observed values is as close as 11\% of the time-averaged volume.</p>

The act of raising the heel up is an essential portion of the gait cycle in humans, comprising the third rocker in the gait cycle. This act further demands specific motions from the hindfoot, and the surrounding structures. These motions have been previously studied and are reasonably well understood. End-stage osteoarthritis of the ankle (or ESOA) has been theorized to affect not only the ankle joint, but the same joints required for heel rise. (i.e. - hindfoot, lower leg, and foot) In the present research, the powerful effect that ESOA has on the lower leg, hindfoot and forefoot biomechanical relationship was demonstrated as significantly different from that of healthy age and sex-matched controls. This has implications not only for further research, but potentially treatment as well.

The act of raising the heel up is an essential portion of the gait cycle in humans, comprising the third rocker in the gait cycle. This act further demands specific motions from the hindfoot, and the surrounding structures. These motions have been previously studied and are reasonably well understood. End-stage osteoarthritis of the ankle (or ESOA) has been theorized to affect not only the ankle joint, but the same joints required for heel rise. (i.e. - hindfoot, lower leg, and foot) In the present research, the powerful effect that ESOA has on the lower leg, hindfoot and forefoot biomechanical relationship was demonstrated as significantly different from that of healthy age and sex-matched controls. This has implications not only for further research, but potentially treatment as well.

Maximum voluntary forces and psychophysically acceptable forces are often used to set force guidelines for exertions as a means to protect against overexertion injuries in the workplace. The focus of this dissertation was the exploration of the roles of whole body balance, shoe-floor friction and joint strength in limiting the capacity of a person to produce maximum voluntary hand forces and psychophysically acceptable hand forces. The underlying goal was to advance knowledge regarding how physical exertion capacity is biomechanically governed, then to use this information to develop models to predict capability based on these governing principles. The hypothesis underscoring this work was that maximum voluntary hand force capability is governed by whole body balance, shoe-floor friction and joint strength; and consequently, psychophysically acceptable forces would be chosen proportionally to this maximum voluntary force capability, where the magnitude of the proportionality was dependent on the limiting factor, or ‘weakest link’. To investigate this hypothesis, both experimental and mathematical modeling paradigms were used. Initially, an experimental study was used to investigate how biomechanical factors governed maximum hand force capability across a range of exertions. It revealed that each governing factor differentially limited maximum force capability. Moreover, this study identified how foot placement, handle height, distance from the handle, friction, and body posture all influence the underlying biomechanical weakest link, and ultimately force producing capability. Data gathered in the experimental study was next used to evaluate a mathematical model that was developed to predict maximum force capability, given information on posture and direction of force application. In addition, the model also predicted population variability in maximum capacity based on the inclusion of a novel approach to probabilistically represent population variability. The evaluation demonstrated that the model underestimated maximum hand force capability compared to measured hand forces by approximately 18, 26, and 41% during medial, pulling and downward exertions respectively. However, it appeared that the ‘weakest link’ principle for predicting maximum force capacity was plausible, as evidenced by significant rank ordered correlations between the measured and predicted hand forces. Further research investigated if psychophysically acceptable forces were selected as a proportion of task specific maximum voluntary force capability, where the proportionality was related to the biomechanical weakest link. Using an experimental design, psychophysically acceptable forces and corresponding maximum forces were measured. Participants chose psychophysically acceptable forces that were 4/5ths of their task specific maximum voluntary force capability when capability was limited by balance. Additionally, they choose psychophysically acceptable forces that were 2/3rds of their maximum voluntary force capability when capability was limited by joint strength. The identification and confirmation of a weakest link proportionality principle represents an important contribution to the field of occupational biomechanics. The weakest link proportionality principle was integrated into the model to allow prediction of: maximum voluntary hand force capability, the limiting factor, and psychophysically acceptable hand force capability. The updated model underestimated empirically measured psychophysically acceptable forces by 24% and 43% during downward and pulling exertions respectively. However, the original model underestimated the maximum hand force capacity by 23% and 34% during the same exertions, without the proportional relationships. This underestimation may be a result of the underlying assumption that joint strength is independent, resulting in an underestimation of maximum joint strength capacity and a corresponding underestimation of maximum hand force capacity. The underestimation may also be due to differences in strength capacities between the participants tested during this thesis compared to those tested in past research used to determine the maximum strength indices reported in the literature. This body of work supported the hypothesis that psychophysically acceptable forces are selected as a proportion of the maximum voluntary hand force, where the proportionality depends on the underlying biomechanical weakest link. The model is a promising first step towards predicting maximum and psychophysically acceptable occupational force threshold limits.

One of the most poorly understood co-morbidities associated with obesity is the pathway to osteoarthritis of the knee. To implement appropriate preventative strategies, it is important to explore how obesity is a causal factor for osteoarthritis. The present research compared the kinematics and kinetics of a group of young obese, but otherwise healthy, adults to a group of young, healthy-weight adults, in an attempt to identify mechanical abnormalities at the knee during walking that may predispose the obese to osteoarthritis of the knee. Optotrak motion capture (Northern Digital Inc. Waterloo, Ontario) and a forceplate (AMTI OR6-7, Advanced Mechanical Technology Inc, Watertown, MA) were used to measure ground reaction forces and moments of 16 participants – 8 obese and 8 sex-, age- and height-matched healthy-weight – to analyze knee joint kinematics and kinetics at three walking speeds. Participants wore an accelerometer (ActiGraph GT3X, Fort Walton Beach, USA) for seven days to measure daily steps counts. Dependent t-tests were performed to determine group differences in ground reaction forces, knee angles and knee moments, as well as knee adduction moment impulse and cumulative knee adductor load (CKAL). The obese group walked at a significantly slower self-selected speed (p=0.013). While not statistically significant, the obese group did present with a more valgus mean dynamic knee alignment than the health-weight group. A significantly greater maximum abduction angle (p=0.009) and smaller minimum knee flexion angle at heel contact (p=0.001) was found in the obese group. A significant difference was found in the peak medial rotation moment in the transverse plane (p=0.003). A greater stance duration lead to a significantly greater knee adduction moment impulse (p=0.049) in the obese group. While significant group differences were not found in the steps per day, the obese group had a significantly greater CKAL (p=0.025). Obese young adults with healthy knees demonstrated a gait pattern of reduced medial knee joint compartment loading through greater knee abduction, medial knee rotation and a slower walking speed compared to matched controls. The ramifications of gait modifications on long-term musculoskeletal health remain unknown, but compensations may lead to increased risk of osteoarthritis of the knee.