Muscles are biological actuators extensively studied in the frame of Hill’s classic empirical model as isolated biomechanical entities, which hardly applies to a living organism subjected to physiological and environmental constraints. Here we elucidate the overarching principle of a living muscle action for locomotion, considering it from the thermodynamic viewpoint as an assembly of actuators (muscle units) connected in parallel, operating via chemical-to-mechanical energy conversion under mixed (potential and flux) boundary conditions. Introducing the energy cost of effort as the generalization of the well-known oxygen cost of transport in the frame of our compact locally linear nonequilibrium thermodynamics model, we analyze oxygen consumption measurement data from a documented experiment on energy cost management and optimization by horses moving at three different gaits. Horses adapt to a particular gait by mobilizing a nearly constant number of muscle units minimizing waste production per unit distance covered; this number significantly changes during transition between gaits. The mechanical function of the animal is therefore determined both by its own thermodynamic characteristics and by the metabolic operating point of the locomotor system.

中文翻译：

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动物运动的热力学

肌肉是在希尔（Hill）经典经验模型的框架中作为孤立的生物力学实体而被广泛研究的生物促动器，它几乎不适用于受到生理和环境限制的活生物体。在这里，我们阐明生活的总体原则从热力学的角度来看，用于运动的肌肉动作是并联连接的执行器（肌肉单元）的组合，在混合（势能和通量）边界条件下通过化学-机械能转换进行操作。在我们紧凑的局部线性非平衡热力学模型的框架内，将精力的能量成本引入众所周知的氧气运输成本的一般化模型中，我们分析了有记录的能源成本管理实验中的耗氧量数据，该实验由马匹移动时进行三种不同的步态。马通过调动几乎恒定数量的肌肉单位来适应特定的步态，从而最大程度地减少单位覆盖距离内的废物产生；在步态之间的过渡期间，此数字会发生显着变化。