THE IMPACT OF THE JOINT POSTURE ON FINGER NEUROMECHANICS
This dissertation presents a series of studies investigating the neuromechanical properties of the index finger with special emphasis on how these properties varied with finger posture. A cadaver study was performed to measure the mapping relationship between tendon force and the resulting fingertip force. A strong linear relationship between magnitude of the tendon force and magnitude of the produced fingertip force was confirmed, up to the force levels examined. The finger posture, however, strongly influenced both fingertip force direction and magnitude. Two fundamental hypotheses of finger biomechanical modeling were then tested in further cadaveric studies. The preliminary results supported the assumption of linear superposition of the contributions of individual tendon forces to the overall fingertip force, at least for two muscles up to the force levels tested. It is also confirmed that the index finger force was almost exclusively produced by the muscles of the index finger, with relatively minor contributions from the other compartments of flexor digitorum superficialis (FDS), flexor digitorum profundus (FDP), or extensor digitorum communis (EDC). An in-vivo human subject study was performed to examine muscle activation patterns during isometric fingertip force production across task directions and finger postures. It was shown that significant changes in muscle excitation patterns occurred, especially with variations in interphalangeal (IP) posture. These changes involved fundamental variation in the underlying pattern rather than just scaling of a given activation pattern. An anatomically correct force-based computer model of the index finger was built to study the roles of different structures and characteristics in finger task performance. The model was validated with the cadaver study result to display its reliability in predicting fingertip xiii force and dynamic movement for given tendon forces. The model was then employed to estimate some hard-to-measure variables including joint contact force and joint passive torque components. The simulation of tendon injuries with the model, including Boutonniere deformation and mallet finger, was able to predict the abnormal finger joint movements under these patho-mechanical conditions. The model simulation also suggested that fingertip force capacity was largely maintained after selected tendon pulley excision surgeries.