Holt Natalie C, Azizi E
Department of Ecology and Evolutionary Biology, University of California, Irvine, CA 92697, USA
Department of Ecology and Evolutionary Biology, University of California, Irvine, CA 92697, USA.
Biol Lett. 2014 Sep;10(9). doi: 10.1098/rsbl.2014.0651.
Skeletal muscles are rarely recruited maximally during movement. However, much of our understanding of muscle properties is based on studies using maximal activation. The effect of activation level on skeletal muscle properties remains poorly understood. Muscle optimum length increases with decreased activation; however, the mechanism responsible is unclear. Here, we attempted to determine whether length-dependent calcium effects, or the effect of absolute force underpin this shift. Fixed-end contractions were performed in frog plantaris muscles at a range of lengths using maximal tetanic (high force, high calcium), submaximal tetanic (low force, high calcium) and twitch (low force, low calcium) stimulation conditions. Peak force and optimum length were determined in each condition. Optimum length increased with decreasing peak force, irrespective of stimulation condition. Assuming calcium concentration varied as predicted, this suggests that absolute force, rather than calcium concentration, underpins the effect of activation level on optimum length. We suggest that the effect of absolute force is due to the varying effect of the internal mechanics of the muscle at different activation levels. These findings have implications for our understanding of in vivo muscle function and suggest that mechanical interactions within muscle may be important determinants of force at lower levels of activation.
在运动过程中，骨骼肌很少会被最大程度地募集。然而，我们对肌肉特性的许多理解是基于使用最大激活的研究。激活水平对骨骼肌特性的影响仍知之甚少。肌肉的最佳长度会随着激活程度的降低而增加；然而，其背后的机制尚不清楚。在这里，我们试图确定长度依赖性钙效应或绝对力的影响是否是这种变化的基础。在一系列长度下，对青蛙的跖肌进行固定端收缩实验，采用最大强直刺激（高力、高钙）、次最大强直刺激（低力、高钙）和单收缩刺激（低力、低钙）条件。在每种条件下测定峰值力和最佳长度。无论刺激条件如何，最佳长度随着峰值力的降低而增加。假设钙浓度如预期那样变化，这表明是绝对力而非钙浓度是激活水平对最佳长度产生影响的基础。我们认为绝对力的影响是由于肌肉内部力学在不同激活水平下的不同作用。这些发现对我们理解体内肌肉功能具有启示意义，并表明在较低激活水平下，肌肉内部的机械相互作用可能是力的重要决定因素。
