on sensory feedback, ignoring the effect of descending and rhythm generating neurons, which have been shown to be crucial in coordinating proper walking. Coupled oscillator models Dynamical system theory shows that any network with cyclical dynamics may be modeled as a set of weakly coupled phase oscillators, so another line of research has been exploring this view of walking. Each oscillator may model a muscle, joint angle, or even a whole leg, and is coupled to some set of other oscillators. Often, these oscillators are thought to represent the central pattern generators underlying walking. These models have rich theory behind them, allow for some extensions based on sensory feedback, and can be fit to kinematics. However, they need to be heavily constrained to fit to data and by themselves make no claims on which gaits allow the animal to move faster, more robustly, or more efficiently. Control based models Control-based models start with a simulation based some description of the animal's anatomy and optimize control parameters to generate some behavior. These may be based on a musculoskeletal model, skeletal model, or even simply a ball and stick model. As these models generate locomotion by optimizing some metric, they can be used to explore the space of optimal locomotion behaviors under some assumptions. However, they typically do not generate plausible hypotheses on the neural coding underlying the behaviors and are typically sensitive to modeling assumptions. Statistical models Phenomenological models model the kinematics of walking directly by fitting a dynamical system, without postulating an underlying mechanism for how the kinematics are generated neurally. Such models can produce the most realistic kinematic trajectories and thus have been explored for simulating walking for computer-based animation. However, the lack of underlying mechanism makes it hard to apply these models to study the biomechanical or neural properties of walking. Animals Horses The walk is a four-beat gait that averages about . When walking, a horse's legs follow this sequence: left hind leg, left front leg, right hind leg, right front leg, in a regular 1-2-3-4 beat. At the walk, the horse will always have one foot raised and the other three feet on the ground, save for a brief moment when weight is being transferred from one foot to another. A horse moves its head and neck in a slight up and down motion that helps maintain balance. Ideally, the advancing rear hoof oversteps the spot