Postscript
A friend of mine and I, many years ago, were playing squash. Rather, he was teaching me how to play squash. He was a former tennis player and I was not, so I was more than a little overmatched. A squash court of course is very different from may other athletic spaces in that one runs at rapid speed towards walls and then must stop or crash into them gracefully before bouncing off. At one point I ran at rapid speed into one of those very unforgiving walls, making a spectacular wreck of my glasses (note: you should definitely wear proper sports googles) and leaving myself momentarily dazed.
“that's how you know you're alive”
That was his response — besides the laughing — and it stuck with me. Metaphor for life and all.