I might specifically mean "Rupa-jivitindriya", the physical form of jivitindriya.
I was initially skeptical of the notion of qi, but through mindfulness of my mind and body, and some clarification (distinguishing qi from western misunderstandings and Asian superstitions), I do feel as though there is a generalized principle and substance of energy, vitality, or life. It's identified with the "breath" in Chinese, because it seems particularly present there, where you can observe the interconnectedness between the physical breath and the emotional breath (of the mind).
While running vigorously, I always feel a tightness where my lower dantian is supposed to be. I'm not overweight (I have a BMI of 23), but I have a decent sized gut and, as I run, I am burning that fat, and so it feels a bit like a tiny black hole, which the fat gets sucked into, and turned into energy (and so, after running, my stomach feels empty and I tend to feel hungry). Earlier today, while running, I was reflecting, as I sometimes do, on developing mental concentration and energy, to overcome the pain of exercise. I was in the middle of my 20-minute run (a little bit past 10 minutes), so it hurt a bit to breathe, but I was concentrating, reflecting on notself, on voidness, on Indra, on effort, and suddenly I felt a burst of energy beginning in my head, then moving down through the rest of my body, and every breath felt nurturing and I felt so strong and alive. When I meditate deeply, too, I feel a sense of light-headedness, as if someone were taking a bicycle pump and inflating my head with air.
In a recent dharma talk by Gil Fronsdal, a Theravadin Buddhist, I was interested to hear him describe how some people experience a burst of energy at the bottom of their torso, rising upwards (and what he seemed to be describing was the kundalini).
Are the above experiences simply imaginative delusions? What is meant by rupa-jivitindriya? And is there some similar notion to "Qi" or "Prana" in Theravada?
The best things in life aren't things.