Understanding the control strategies that underlie multijoint limb movements is important to researchers in motor control, robotics, and medicine. Due to dynamic interactions between limb segments, choosing appropriate muscle activations for initiating multijoint arm movements is a complex problem, and the rules by which the nervous system makes such choices are not yet understood. The aim of the dissertation studies was to evaluate some proposed initiation rules based on their ability to correctly predict which shoulder and elbow muscles initiated planar, two-joint arm movements in various directions. Kinematic and electromyographic data were collected from thirteen subjects during pointing movements involving shoulder and elbow rotations in the horizontal plane. One of the rules tested, which is based on statics, predicted that the initial muscle activity at each joint is chosen such that the hand exerts an initial force in the direction of the target, while another rule, based on dynamics, predicted initial muscle activity such that the initial acceleration of the hand is directed toward the target. For both rules, the data contradict the predicted initial shoulder muscle activity for certain movement directions. Moreover, the effects of added inertial loads predicted by the latter rule were not observed when a 1.8 kg mass was added to the limb. The results indicated, however, that empirically derived rules, based on ψ, the target direction relative to the distal segment, could predict which muscles would be chosen to initiate movement in a given direction. Furthermore, the relative timing and magnitude of initial muscle activity at the shoulder and elbow varied systematically with ψ. Thus, the target direction relative to the forearm may be an important variable in determining initial muscle activations for multijoint arm movements. These findings suggest a control scheme for movement initiation in which simple rules suffice to launch the hand in the approximate direction of the target by first specifying a basic motor output pattern, then modulating the relative timing and magnitude of that pattern.

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Thesis (MHSc--Health Science) -- Auckland University of Technology, 2004. / Also held in print (80 leaves, 30 cm.) in Akoranga Theses Collection (T 612.75 ROT)

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The increase in tissue extensibility and joint range of motion through stretching has often been demonstrated. However, the existence of an optimal stretch duration has not been proven and the identification of an ideal number of repetitions of a stretch has received little attention in the literature. The purpose of this study was to examine and clarify the relationship of duration and number of repetitions of a stretch and their effect on changes in hamstring extensibility (HE) resulting from a stretching program. The HE of 33 subjects who were randomly assigned to a control group or one of three treatment groups was assessed before and after a three-week stretching program. The subjects in the three treatment groups stretched once a day, five times a week, and either once for 15 seconds (1 X 15), twice for 15 seconds (2 X 15), or once for 30 seconds (1 X 30). A repeated measures ANOVA showed no statistically significant effect for treatment group (P=0.181) or for treatment by pre- and post-treatment measurements (P=0.140), but indicated a significant difference between pre- and post-treatment measurements (P<0.001). The HE of the control group (which did not stretch) changed only slightly (0.9��, SD 5.7), the 1 X 15 and 1 X 30 groups improved more clearly (4.2��, SD 3.1 and 3.8��, SD 6.7, respectively), and the 2 X 15 group's HE increased the most (6.8��, SD 3.9). The results suggest that varying durations and repetitions of a stretch may influence the magnitude of improvement of HE. However, with the relatively low power (0.46) in the present study, no statistically significant difference between stretching protocols of 1 X 15 seconds, 2 X 15 seconds, and 1 X 30 seconds could be found. / Graduation date: 1997

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Empirical and modelling studies are reported which explore ways in which the central nervous system might consider musculo-skeletal geometry when generating commands for single degree of freedom elbow flexion and extension movements. In a series of experiments it is shown that subjects do not perform rapid, goal-directed flexion and extension movements equally accurately in different parts of the elbow's workspace. In these experiments, movements of 10, 20 and 30 degrees in amplitude were tested using up to five different initial elbow angles. When performing flexions, subjects tended to overshoot targets when starting from extended positions, to undershoot targets when starting from more flexed positions, and to perform relatively accurate movements when starting from the centre of the workspace. Final position accuracy was more variable for extensions. When reliable differences existed for extensions, subjects tended to produce a pattern of results opposite to that of flexions: subjects overshot targets when starting from flexed positions and undershot targets when starting from more extended positions. A model of elbow movement based on the $ lambda$ version of the equilibrium-point hypothesis was used to assess the extent to which the pattern of errors obtained in the empirical studies could be reproduced by a control scheme that does not adjust commands in response to changing musculo-skeletal geometry, but rather uses one single invariant command throughout the workspace. The motivation for testing the invariant command notion was to explore the possibility that motion planning might be achieved without an explicit representation of musculo-skeletal geometry. Predicted patterns of final position errors across the workspace matched empirically obtained error patterns for flexions, but the model performed less well when predicting the pattern of errors observed for extension movements.

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