“ In walking, the will and muscles are so accustomed to work together and perform their task with so little expenditure of force, that the intellect is left comparatively free. ”
 Oliver Wendell Holmes Sr., The Autocrat of the Breakfast-Table (1858). copy citation

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“the second with the amount and kind of power, and the extent and kind of action. In all forms of active exercise there are three powers simultaneously in action,—the will, the muscles, and the intellect. Each of these predominates in different kinds of exercise. In walking, the will and muscles are so accustomed to work together and perform their task with so little expenditure of force, that the intellect is left comparatively free. The mental pleasure in walking, as such, is in the sense of power over all our moving machinery. But in riding, I have the additional pleasure of governing another will, and my muscles extend to the tips of the animal’s ears and to his four hoofs, instead of stopping at my hands and feet.” source