PDF(Pubmed)
Abstract：
Neuromuscular control loops feature substantial communication delays, but mammals run robustly even in the most adverse conditions. In vivo experiments and computer simulation results suggest that muscles\' preflex-an immediate mechanical response to a perturbation-could be the critical contributor. Muscle preflexes act within a few milliseconds, an order of magnitude faster than neural reflexes. Their short-lasting action makes mechanical preflexes hard to quantify in vivo. Muscle models, on the other hand, require further improvement of their prediction accuracy during the non-standard conditions of perturbed locomotion. Our study aims to quantify the mechanical work done by muscles during the preflex phase (preflex work) and test their mechanical force modulation. We performed in vitro experiments with biological muscle fibers under physiological boundary conditions, which we determined in computer simulations of perturbed hopping. Our findings show that muscles initially resist impacts with a stereotypical stiffness response-identified as short-range stiffness-regardless of the exact perturbation condition. We then observe a velocity adaptation to the force related to the amount of perturbation similar to a damping response. The main contributor to the preflex work modulation is not the change in force due to a change in fiber stretch velocity (fiber damping characteristics) but the change in magnitude of the stretch due to the leg dynamics in the perturbed conditions. Our results confirm previous findings that muscle stiffness is activity-dependent and show that also damping characteristics are activity-dependent. These results indicate that neural control could tune the preflex properties of muscles in expectation of ground conditions leading to previously inexplicable neuromuscular adaptation speeds.
摘要：
神经肌肉控制回路具有大量的通信延迟，但是哺乳动物即使在最不利的条件下也能健壮地运行。体内实验和计算机模拟结果表明，肌肉-对扰动的直接机械反应-可能是关键因素。肌肉预屈在几毫秒内起作用，比神经反射快一个数量级.它们的短期作用使机械前曲难以在体内量化。肌肉模型,另一方面,在扰动运动的非标准条件下，需要进一步提高其预测精度。我们的研究旨在量化肌肉在预弯曲阶段（预弯曲工作）所做的机械工作，并测试其机械力调制。我们在生理边界条件下用生物肌纤维进行了体外实验，这是我们在计算机模拟扰动跳跃中确定的。我们的发现表明，无论精确的扰动条件如何，肌肉最初都会以刻板的刚度响应抵抗冲击，该刚度响应被识别为短程刚度。然后，我们观察到与扰动量相关的力的速度适应性，类似于阻尼响应。预弯曲功调制的主要原因不是由于纤维拉伸速度（纤维阻尼特性）的变化而引起的力的变化，而是由于在扰动条件下的腿部动力学而引起的拉伸幅度的变化。我们的结果证实了先前的发现，即肌肉僵硬是活动依赖性的，并且表明阻尼特性也是活动依赖性的。这些结果表明，神经控制可以在预期地面条件下调整肌肉的前屈特性，从而导致先前无法解释的神经肌肉适应速度。
