I'm not seriously injured -- some scraped bits of skin, some large bumps, one particularly picturesque bruise, some places that feel like bruises but don't yet look like bruises -- because falling down the stairs is something I do routinely every now and then. I hadn't fallen down the stairs for a couple of years and I was probably overdue for another go, and when I'm falling down the stairs I don't usually pick up serious injuries. I'm actually quite good at falling down the stairs. I didn't hit my head, or damage my spine, or tear any muscles, and so the only injuries I have are those arising from impact.
I think there's a fanzine article waiting to be written about just how to fall down the stairs properly, achieving the maximum noise and comic effect coupled with the minimum of injury and lasting consequences.
Unless someone's already written it, and I haven't found it yet.