Publication

Understanding motor control and impairment: An upper-limb model for muscular assessment

Abstract

The human upper limb is a complex musculoskeletal system that can still perform various tasks with impressive efficacy thanks to the ability of the central nervous system to control and modulate the activation of more than 40 muscles.Stroke is a leading cause of long-term disability, and individuals who have experienced a stroke often show unusual muscle activation patterns in the limb that was affected. A thorough understanding of these changes in muscle activation is crucial for the development of efficient rehabilitation plans.So far, the study of the recorded electromyographic (EMG) signals has been the primary option for investigating muscle activation during movement. Unfortunately, given the number of muscles acting on the shoulder and arm and their positions with respect to each other, a complete upper-limb's muscles EMG recording is not feasible in practice. Numerical musculoskeletal model could represent a very useful alternative approach to gather this kind of information.This thesis aims at extending an existing upper-limb musculoskeletal model, to capture the overall muscle activations of a subject from its scaled recorded kinematics and a limited number of recorded muscle EMG. The original model was force-based and its aim was to obtain the muscle forces from inverse dynamics (ID). However, these could not always be physiologically feasible, indeed, after modeling the musculotendon dynamics with a Hill-type model we further reduced their boundaries leading to unfeasible activations solutions. The ID was then reformulated to be activation based ensuring feasible activation and force given the muscle state. Moreover studying patients performing reaching tasks with an exoskeleton can lead to non-smooth kinematics, further restricting the ID possible solutions. A flexible formulation allowing for tolerance of torques at the joints was added.The purpose of this thesis is to present a tool that can be used to study variations in muscle activations from healthy and stroke patients during their rehabilitation. This means that a large dataset with multiple repetitions of the different tasks and subjects has to be simulated which would not be practical with the initial model. However, with a rigid tendon muscle assumption as well as the precomputation of multiple functions of the model, the computation time could be reduced by almost 2 orders of magnitude when studying multiple movements.In the last part of the thesis I could show how the model could help with a quantified evaluation and functional diagnosis of stroke patients which is in contrast to current assessment methods. It also establishes a study of the muscle co-contractions and synergies given the full set of muscles acting on the shoulder and arm. I also developed a forward dynamics methods of the model which aims was at first to be solely driven by muscle activations, but without the closed-loop formulation, it is too unstable in its current state because of the accumulation of numerical errors over a simulation. It is however setting the ground for possible further development. A graphical user interface was also developed for simpler and adaptable use of the model, in the forward and inverse modes, and for the study of multiple subjects and repetitions.The achievements presented in the thesis can provide a tool to better understand the mechanisms underlying upper limb control as well as its impairments. Moreover, it sets a

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Ontological neighbourhood
Related concepts (36)
Muscle contraction
Muscle contraction is the activation of tension-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such as when holding something heavy in the same position. The termination of muscle contraction is followed by muscle relaxation, which is a return of the muscle fibers to their low tension-generating state.
Skeletal muscle
Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscle tissue, and are often known as muscle fibers. The muscle tissue of a skeletal muscle is striated – having a striped appearance due to the arrangement of the sarcomeres. Skeletal muscles are voluntary muscles under the control of the somatic nervous system.
Muscle cell
A muscle cell is also known as a myocyte when referring to either a cardiac muscle cell (cardiomyocyte) or a smooth muscle cell, as these are both small cells. A skeletal muscle cell is long and threadlike with many nuclei and is called a muscle fiber. Muscle cells (including myocytes and muscle fibers) develop from embryonic precursor cells called myoblasts. Myoblasts fuse from multinucleated skeletal muscle cells known as syncytia in a process known as myogenesis.
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