Department of Biological Sciences, California State University, Sacramento, CA 95819, USA.
Department of Zoology, University of British Columbia, Vancouver, BC, Canada, V6T 1Z4.
J Exp Biol. 2020 Nov 12;223(Pt 21):jeb225839. doi: 10.1242/jeb.225839.
Birds that use high flapping frequencies can modulate aerodynamic force by varying wing velocity, which is primarily a function of stroke amplitude and wingbeat frequency. Previous measurements from zebra finches () flying across a range of speeds in a wind tunnel demonstrate that although the birds modulated both wingbeat kinematic parameters, they exhibited greater changes in stroke amplitude. These two kinematic parameters contribute equally to aerodynamic force, so the preference for modulating amplitude over frequency may instead derive from limitations of muscle physiology at high frequency. We tested this hypothesis by developing a novel work loop approach to measure muscle force and power output from the whole pectoralis major of zebra finches. This method allowed for multiple measurements over several hours without significant degradation in muscle power. We explored the parameter space of stimulus, strain amplitude and cycle frequencies measured previously from zebra finches, which revealed overall high net power output of the muscle, despite substantial levels of counter-productive power during muscle lengthening. We directly compared how changes to muscle shortening velocity via strain amplitude and cycle frequency affected muscle power. Increases in strain amplitude led to increased power output during shortening with little to no change in power output during lengthening. In contrast, increases in cycle frequency did not lead to increased power during shortening but instead increased counter-productive power during lengthening. These results demonstrate why at high wingbeat frequency, increasing wing stroke amplitude could be a more effective mechanism to cope with increased aerodynamic demands.
使用高拍打频率的鸟类可以通过改变翅膀速度来调节空气动力，翅膀速度主要是翅膀振幅和翅膀拍动频率的函数。以前在风洞中对斑胸草雀（）在一系列速度下飞行的测量表明，尽管鸟类调节了翅膀运动学参数，但它们表现出更大的翅膀振幅变化。这两个运动学参数对空气动力的贡献相等，因此优先调节振幅而不是频率可能源于高频肌肉生理学的限制。我们通过开发一种新的工作循环方法来测量斑胸草雀的整个胸大肌的肌肉力量和功率输出，从而验证了这一假设。这种方法允许在几个小时内进行多次测量，而不会导致肌肉功率显著下降。我们探索了以前从斑胸草雀测量的刺激、应变幅度和循环频率的参数空间，结果显示肌肉的总净功率输出很高，尽管在肌肉伸长时存在大量的反生产功率。我们直接比较了通过应变幅度和循环频率改变肌肉缩短速度如何影响肌肉功率。应变幅度的增加导致缩短时的功率输出增加，而伸长时的功率输出几乎没有变化。相比之下，循环频率的增加并没有导致缩短时的功率增加，而是导致伸长时的反生产功率增加。这些结果表明，为什么在高翅膀拍动频率下，增加翅膀拍动幅度可能是一种更有效的机制，可以应对增加的空气动力需求。
