Resonance tuning in a neuro-musculo-skeletal model of the forearm |
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Authors: | B W Verdaasdonk H F J M Koopman F C T Van der Helm |
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Institution: | (1) Faculty of Engineering Technology, Department of Bio-mechanical Engineering, University of Twente, P.O.Box 217, 7500 AE Enschede, The Netherlands |
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Abstract: | In rhythmic movements, humans activate their muscles in a robust and energy efficient way. These activation patterns are oscillatory
and seem to originate from neural networks in the spinal cord, called central pattern generators (CPGs). Evidence for the
existence of CPGs was found for instance in lampreys, cats and rats. There are indications that CPGs exist in humans as well,
but this is not proven yet. Energy efficiency is achieved by resonance tuning: the central nervous system is able to tune into the resonance frequency of the limb, which is determined by the local reflex gains. The goal of this study is to investigate if the existence of a CPG in the human spine can explain the resonance tuning behavior,
observed in human rhythmic limb movement. A neuro-musculo-skeletal model of the forearm is proposed, in which a CPG is organized
in parallel to the local reflexloop. The afferent and efferent connections to the CPG are based on clues about the organization
of the CPG, found in literature. The model is kept as simple as possible (i.e., lumped muscle models, groups of neurons are
lumped into half-centers, simple reflex model), but incorporates enough of the essential dynamics to explain behavior—such
as resonance tuning—in a qualitative way. Resonance tuning is achieved above, at and below the endogenous frequency of the
CPG in a highly non-linear neuro- musculo-skeletal model. Afferent feedback of muscle lengthening to the CPG is necessary to accomplish resonance tuning above the endogenous frequency of the CPG, while feedback of muscle velocity is necessary to compensate for the phase lag, caused by the time delay in the loop coupling the limb to the CPG. This afferent
feedback of muscle lengthening and velocity represents the Ia and II fibers, which—according to literature—is the input to
the CPG. An internal process of the CPG, which integrates the delayed muscle lengthening and feeds it to the half-center model,
provides resonance tuning below the endogenous frequency. Increased co-contraction makes higher movement frequencies possible.
This agrees with studies of rhythmic forearm movements, which have shown that co-contraction increases with movement frequency.
Robustness against force perturbations originates mainly from the CPG and the local reflex loop. The CPG delivers an increasing
part of the necessary muscle activation for increasing perturbation size. As far as we know, the proposed neuro-musculo-skeletal
model is the first that explains the observed resonance tuning in human rhythmic limb movement. |
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