The first source is hatha yoga, or physical yoga. The main goal of yoga (which means meditation) is not to get into shape or to become more flexible (although this does happen if one is practicing effectively) but to use the process of trying to get into shape as a vehicle for spiritual growth. This most obviously takes on the form of increasing one's discipline and power over their own body.
The second source is Taoist texts. In Taoist spiritual alchemy (which includes physical exercises), there is an attempt to use the physical body as a template for creating what I might call a "spiritual body", or as translated by Arthur Avalon, "the immortal fetus in the state of complete serenity".
A third source is my own intuition Also zen, the meditations and concepts associated with it are not very mentally taxing. I don't think that exercise by itself can be sufficient, it needs to be mentally categorized and contextualized somehow.
One early goal I have is to cross reference Hindu and Taoist concepts of the chakra points in the body with publicly available information we have about health. The idea is to create a workout regimen that is not just effective physically (since that is what people want) but in practice is more than just "working out" and is really a spiritual exercise.
As I wrote in my much maligned "deep thinks" thread, what was often an excess of mental energy in past eras is being increasingly replaced with an excess of physical energy, as highly intellectually demanding jobs become more common, we don't have the luxury of pursuing transcendence with our mental energy if we also want to satisfy or fulfill other kinds of obligations.