My dentist balanced on one leg for ten seconds during a cleaning last month. Okay — not literally. But she did make me do it, right there in the chair, after I mentioned I’d been feeling a little wobbly stepping off curbs during my morning runs. Timer on, foot off the ground, eyes forward. I lasted the full ten seconds, but I felt something I hadn’t felt since my heptathlon days: my ankle, knee, and hip having a frantic three-way conversation about who was in charge. That tiny wobble stuck with me. So my team and I spent six weeks…