Neuromuscular dynamics in the context of motor redundancy

Motor redundancy in neuromuscular systems exists on multiple levels. The term “motor redundancy” represents the availability of infinitely many different solutions to perform a motor task. This dissertation is concerned with three particular of those levels: muscle redundancy, wrench redundancy and posture redundancy, which are successively more general forms of redundancy, each of which will be explained in detail.;The first level corresponds to the phenomenon that for a given constant vector of submaximal limb endpoint force in isometric tasks, an infinite multitude of muscle coordination patterns exists. The motor control research community refers to this kind of redundancy as muscle redundancy, and traditionally, the selection of a particular muscle coordination pattern has been considered a computational problem for the nervous system. In the present work, I am studying a potential benefit of muscle redundancy, namely the mitigation of muscle fatigue through the dynamic switching between muscle activation patterns. Based on my results, I am proposing to abandon the view of muscle redundancy as a computational problem for the nervous system, since in the presence of muscle fatigue even the alleged simplification of this problem, that is, dimension reduction through muscle synergies requires awareness of the full dimensionality of the motor task. Instead, future research should focus on how the nervous system responds flexibly to the challenge of time-variance due to fatiguing and actually leverages muscle redundancy.;The second level of motor redundancy is concerned with the phenomenon that in addition to the redundancy of muscles, infinitely many different combinations of endpoint forces and moments all achieve successful task performance. I will show how mathematically independent wrench space dimensions are actually controlled in quite different ways, reflecting their specific roles in achieving dexterous manipulation. This work shows that a purely spatial analysis of endpoint force variability is not sufficient and that temporal correlations can reveal important aspects of motor control. In particular, the dynamics of forces indicate a hierarchy of task dimensions in terms of task-relevance and contradict the view held by some that task variables can be separated into task-relevant and -irrelevant (i.e. the Uncontrolled Manifold Hypothesis). According to this view, large variability in a mechanically task-irrelevant dimension reflects the lack of control of this dimension by the nervous system. Based on these results, I am proposing to abandon the view of wrench redundancy as a purely spatial problem and to espouse the use of time series analysis to determine neural control strategies. In the second study of wrench dynamics, I will show how in a non-redundant dexterous manipulation task, where all wrench dimensions are task-relevant due to simultaneous force and motion requirements, the control of different task dimensions is likely coupled through neurophysiological pathways, whose separation during evolution has been incomplete. Specifically, I will show how different wrench space dimensions of the motor task, though mathematically independent, are nevertheless coupled in the performance of the task, thus limiting the ability to match the perfect mechanical solution of the task. We see here an important interaction between the wrench and the muscle level: when the wrench level becomes non-redundant, the muscle level also seems to hit a boundary and reveals limitations in the independence of control of muscles across fingers.;Finally, the third level is concerned with postural redundancy, meaning that during the performance of a motor task the task goal can be achieved with different limb configurations, described in terms of joint angles. Once again, this level of redundancy is a generalization of the previous level and potentially extends the potential benefits of the former: a selected posture that enables motor task performance will constrain the admissible endpoint force space, which in turn will constrain the muscle coordination space. One common task taking advantage of postural redundancy is quiet stance. During quiet bipedal stance, two-legged animals are usually swaying or shifting from one posture to another, the former of which can be attributed to motor noise and the latter of which is likely a fatigue mitigation strategy. In this dissertation, I will present results of an analysis of postural control in one-day old domestic chicks (Gallus gallus) that reveal differences in prenatal motor development, which were induced by different amounts of light exposure during incubation. (Abstract shortened by UMI.).