Yogācārabhūmi-śāstra: Volume 34 (Part 16)

How to contemplate the impermanent nature of internal activities characterized by toil and transformation?

By observing the weariness and exhaustion of the body—whether one’s own or another’s—resulting from running about, leaping, jumping up and down, riding horses, or performing various swift bodily actions. Then, at another time, seeing that same person free from such weariness and exhaustion, upon observing this, there arises the notion: "All conditioned things are indeed of an impermanent nature." The rest is as stated previously.

Explanation: How does one observe the impermanent nature of transformations arising from bodily toil within the internal sphere? By observing the weariness and exhaustion of the body—whether one’s own or another’s—caused by running swiftly, leaping, repeated jumping, horseback riding, or performing various rapid physical actions. Later, when such extreme fatigue has ceased and vanished, upon observing these phenomena, one forms this understanding: "All conditioned things are truly of an impermanent nature."