Department of Biomedical Physiology and Kinesiology, Simon Fraser University, Burnaby, BC, V5A 1S6, Canada
Department of Biological Sciences, University of Massachusetts Lowell, Lowell, MA 01854, USA.
J Exp Biol. 2020 Oct 7;223(Pt 19):jeb224410. doi: 10.1242/jeb.224410.
Most of what we know about whole muscle behaviour comes from experiments on single fibres or small muscles that are scaled up in size without considering the effects of the additional muscle mass. Previous modelling studies have shown that tissue inertia acts to slow the rate of force development and maximum velocity of muscle during shortening contractions and decreases the work and power per cycle during cyclic contractions; however, these results have not yet been confirmed by experiments on living tissue. Therefore, in this study we conducted work-loop experiments on rat plantaris muscle to determine the effects of increasing the mass of muscle on mechanical work during cyclic contractions. We additionally simulated these experimental contractions using a mass-enhanced Hill-type model to validate our previous modelling work. We found that greater added mass resulted in lower mechanical work per cycle relative to the unloaded trials in which no mass was added to the muscle (=0.041 for both 85 and 123% increases in muscle mass). We additionally found that greater strain resulted in lower work per cycle relative to unloaded trials at the same strain to control for length change and velocity effects on the work output, possibly due to greater accelerations of the muscle mass at higher strains. These results confirm that tissue mass reduces muscle mechanical work at larger muscle sizes, and that this effect is likely amplified for lower activations.
我们对整块肌肉行为的了解大多来自于对单个纤维或小肌肉的实验，这些实验是通过增加肌肉的大小来进行的，而没有考虑到额外肌肉质量的影响。之前的建模研究表明，组织惯性会减慢肌肉在缩短收缩过程中的力发展速度和最大速度，并降低循环收缩过程中的每周期功和功率；然而，这些结果尚未通过对活体组织的实验得到证实。因此，在这项研究中，我们对大鼠跖肌进行了工作环实验，以确定增加肌肉质量对循环收缩过程中机械功的影响。我们还使用增强质量的 Hill 型模型模拟了这些实验收缩，以验证我们之前的建模工作。我们发现，相对于未加载试验（即肌肉未增加任何质量的情况下），增加附加质量会导致每周期的机械功降低（对于肌肉质量增加 85%和 123%的情况，均为 0.041）。我们还发现，相对于未加载试验，在相同应变下，更大的应变会导致每周期的功降低，以控制长度变化和速度对功输出的影响，这可能是由于更高应变下肌肉质量的加速度更大。这些结果证实，组织质量会降低较大肌肉尺寸下的肌肉机械功，并且这种效应可能会因较低的激活而放大。
