Hagen Daniel A, Valero-Cuevas Francisco J
Department of Biomedical Engineering, University of Southern California, Los Angeles, CA, USA.
Department of Biomedical Engineering, University of Southern California, Los Angeles, CA, USA; Division of Biokinesiology and Physical Therapy, University of Southern California, Los Angeles, CA, USA.
J Biomech. 2017 Jul 5;59:90-100. doi: 10.1016/j.jbiomech.2017.05.019. Epub 2017 May 31.
We investigated how kinematic redundancy interacts with the neurophysiological control mechanisms required for smooth and accurate, rapid limb movements. Biomechanically speaking, tendon excursions are over-determined because the rotation of few joints determines the lengths and velocities of many muscles. But how different are the muscle velocity profiles induced by various, equally valid hand trajectories? We used an 18-muscle sagittal-plane arm model to calculate 100,000 feasible shoulder, elbow, and wrist joint rotations that produced valid basketball free throws with different hand trajectories, but identical initial and final hand positions and velocities. We found large differences in the eccentric and concentric muscle velocity profiles across many trajectories; even among similar trajectories. These differences have important consequences to their neural control because each trajectory will require unique, time-sensitive reflex modulation strategies. As Sherrington mentioned a century ago, failure to appropriately silence the stretch reflex of any one eccentrically contracting muscle will disrupt movement. Thus, trajectories that produce faster or more variable eccentric contractions will require more precise timing of reflex modulation across motoneuron pools; resulting in higher sensitivity to time delays, muscle mechanics, excitation/contraction dynamics, noise, errors and perturbations. By combining fundamental concepts of biomechanics and neuroscience, we propose that kinematic and muscle redundancy are, in fact, severely limited by the need to regulate reflex mechanisms in a task-specific and time-critical way. This in turn has important consequences to the learning and execution of accurate, smooth and repeatable movements-and to the rehabilitation of everyday limb movements in developmental and neurological conditions, and stroke.
我们研究了运动冗余如何与平稳、准确、快速的肢体运动所需的神经生理控制机制相互作用。从生物力学角度讲，肌腱的移动是过度确定的，因为少数关节的转动决定了许多肌肉的长度和速度。但是，由各种同样有效的手部轨迹所诱导的肌肉速度分布有多大差异呢？我们使用一个包含18块肌肉的矢状面手臂模型，计算了10万种可行的肩部、肘部和腕关节转动，这些转动产生了具有不同手部轨迹但初始和最终手部位置及速度相同的有效篮球罚球动作。我们发现，在许多轨迹中，甚至在相似轨迹之间，离心和向心肌肉速度分布都存在很大差异。这些差异对其神经控制具有重要影响，因为每个轨迹都需要独特的、对时间敏感的反射调制策略。正如谢灵顿在一个世纪前所提到的，未能适当地抑制任何一块离心收缩肌肉的牵张反射都会干扰运动。因此，产生更快或更可变离心收缩的轨迹将需要在运动神经元池之间更精确的反射调制时间；从而导致对时间延迟、肌肉力学、兴奋/收缩动力学、噪声、误差和扰动具有更高的敏感性。通过结合生物力学和神经科学的基本概念，我们提出，实际上，运动和肌肉冗余受到以任务特定且对时间要求严格的方式调节反射机制的需求的严重限制。这反过来对准确、平稳和可重复运动的学习与执行，以及对发育和神经状况及中风患者日常肢体运动的康复都具有重要影响。
