The thumb is responsible for approximately 40% of hand function. It is critical for interacting with the environment, specifically, for applying forces produced at the thumb-tip to hold and manipulate objects. In order for the thumb to generate a well-directed force at the thumb-tip, a combination of nine uni- and multi-articular muscles are dynamically coordinated to simultaneously generate thumb-tip force and maintain thumb posture across three joints. Current analysis of this complex system relies on static methods, resulting in limited insights.

Therefore the first aim of this dissertation was the development of a novel dynamic biomechanical model of the thumb to simulate thumb-tip force production through EMG-driven forward simulations. Simulation inputs were adapted from a previous study that quantified thumb-tip force production and the corresponding muscle activity measured in human subjects. We concluded that a representation of how the muscles are activated relative to each other (e.g., a coordination strategy) was a critical factor that contributed to successful simulation of the thumb-tip forces produced by human subjects.

Based on what we learned by analyzing thousands of simulations, we designed a humansubjects experiment to investigate how thumb-tip force and muscle activation patterns vary by individual. To expand upon the status quo, we also measured joint kinematics. One unique contribution of our data is that it documents the variability in the joint postures adopted by subjects during the production of functional thumb-tip forces. Our data provide evidence that thumb joint posture changes during force generation, despite the fact that lateral pinch and opposition pinch are commonly evaluated using static biomechanical analyses.

In the final aim, we demonstrated the utility of integrating the dynamic model developed and experimental data collected in this dissertation into a simulation framework to investigate the mechanism that causes carpometacarpal osteoarthritis, a common and debilitation orthopedic condition. This study illustrates the value of utilizing dynamic simulations to further address questions related to pathological conditions in the hand.