Verdaasdonk B W, Koopman H F J M, Van der Helm F C T
Faculty of Engineering Technology, Department of Bio-mechanical Engineering, University of Twente, 7500 AE Enschede, The Netherlands.
Biol Cybern. 2007 Feb;96(2):165-80. doi: 10.1007/s00422-006-0112-6. Epub 2006 Nov 30.
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.
在有节奏的运动中，人类以一种强健且节能的方式激活肌肉。这些激活模式是振荡性的，似乎源自脊髓中的神经网络，即中枢模式发生器（CPG）。例如，在七鳃鳗、猫和大鼠中发现了CPG存在的证据。有迹象表明人类也存在CPG，但尚未得到证实。通过共振调谐可实现能量效率：中枢神经系统能够调谐到肢体的共振频率，该频率由局部反射增益决定。本研究的目的是调查人类脊柱中CPG的存在是否可以解释在人类有节奏的肢体运动中观察到的共振调谐行为。提出了一种前臂的神经 - 肌肉 - 骨骼模型，其中CPG与局部反射回路并行组织。与CPG的传入和传出连接基于文献中关于CPG组织的线索。该模型尽可能保持简单（即集中肌肉模型，神经元组被集中到半中枢，简单反射模型），但包含了足够的基本动力学，以便定性地解释诸如共振调谐等行为。在一个高度非线性的神经 - 肌肉 - 骨骼模型中，在CPG的内源性频率之上、之时和之下都能实现共振调谐。肌肉拉长对CPG的传入反馈对于在CPG的内源性频率之上实现共振调谐是必要的，而肌肉速度的反馈对于补偿由将肢体与CPG耦合的回路中的时间延迟引起的相位滞后是必要的。这种肌肉拉长和速度的传入反馈代表Ia和II纤维，根据文献，它们是CPG的输入。CPG的一个内部过程，它整合延迟的肌肉拉长并将其馈送到半中枢模型，在低于内源性频率时提供共振调谐。增加的共同收缩使更高的运动频率成为可能。这与对有节奏的前臂运动的研究一致，该研究表明共同收缩随着运动频率增加。对力扰动的鲁棒性主要源于CPG和局部反射回路。对于不断增加的扰动大小，CPG提供了越来越多必要的肌肉激活。据我们所知，所提出的神经 - 肌肉 - 骨骼模型是第一个解释在人类有节奏的肢体运动中观察到的共振调谐的模型。
