Fueling and performance in a high speed locomotor muscle

本研究探讨了运动肌肉的加油和机械性能随体型和肌肉活动频率的变化。蜂鸟独特的悬停飞行是所有脊椎动物中能量和机械成本最高的运动形式。胸肌和喙上肌分别负责为蜂鸟的翅拍提供向下和向上的动力，它们的活动频率范围与体重成反比，而小型蜂鸟的运动肌肉活动频率是脊椎动物中最快的。过去的研究已经揭示了蜂鸟胸肌的一些解剖学和生化特征，这些特征使其能够实现高速率的有氧代谢。然而，关于悬停过程中燃料氧化的来源、肌肉性能调节机制和高速运动肌肉收缩特性的量化，包括这些特征如何随体重或肌肉工作频率而变化，许多问题仍未得到解答。本提案的研究将解决三个主要问题：1)不同体型和悬停代谢率的蜂鸟在运动中使用新摄入的燃料和内源性燃料的能力是如何变化的？2)胸肌的神经肌肉控制模式如何随翅拍频率而变化？3)蜂鸟胸肌的最大力量和工作频率（收缩速度）之间是否存在权衡？为了回答这些问题，我们将结合对自由悬停蜂鸟的蜂鸟燃料使用、运动学和神经肌肉控制（EMG）模式的研究，以及对孤立的蜂鸟胸肌的收缩特性的研究，包括在不同体重范围内的不同翅拍频率的物种。结果将增加我们对肌肉性能和高频加油限制的理解，并确定适用于所有运动肌肉的肌肉生理学各个方面之间的权衡。

This research examines some of the ways that the fueling and mechanical performance of locomotor muscle varies in relation to body size and muscle operating frequency. The distinct hovering flight of hummingbirds is the most energetically and mechanically costly form of locomotion employed by any vertebrates. The pectoralis and supracoracoideus muscles, responsible for powering the downstroke and upstroke, respectively, of the hummingbird wingbeat, operate at a range of frequencies inversely related to body mass with small hummingbirds employing the fastest locomotor muscle operating frequencies of any vertebrate. Past research has revealed some of the anatomical and biochemical features that enable the hummingbird pectoralis to achieve high rates of aerobic metabolism. However, many questions regarding aspects of the source of fuels oxidized during hovering, mechanisms for modulation of muscle performance, and quantification of contractile properties of high speed locomotor muscle, including how these features vary in relation to body mass or muscle operating frequency, remain unanswered. The research in this proposal will address three main questions: 1) How do capacities for the use of newly ingested, versus endogenous fuels during exercise vary among hummingbirds of varying body size and hovering metabolic rate? 2) How do patterns of neuromuscular control of the pectoralis vary in relation to wingbeat frequency? 3) Is there a tradeoff between maximal force and operating frequency (contraction speed) in hummingbird pectoralis muscles? To answer these questions we will couple studies of hummingbird fuel use, kinematics, and neuromuscular control (EMG) patterns on freely hovering hummingbirds with studies of the contractile properties of isolated hummingbird pectoralis, including species across a range of body mass with varying wingbeat frequencies for study. The results will increase our understanding of the limits to muscle performance and fueling at high frequencies and identify tradeoffs among various aspects of muscle physiology that apply generally to all locomotor muscles.