  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 \emph{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, $COE_-$, as the
generalization of the well-known oxygen cost of transport, $COT$, in the frame
of our compact locally linear non-equilibrium 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.
