Here is something twenty years of competitive training never taught me: my legs have been lying to me. Not on purpose — they simply report effort the way a gossip reports news, with a delay and a bias. Heart rate, that gold-standard metric I have worn on my chest since my heptathlon days, has the same problem. It lags. It drifts with heat, caffeine, sleep debt, and plain old anxiety. So this summer, when a researcher I work with kept mentioning muscle oxygen monitors — the little infrared sensors that sports scientists have been strapping to elite cyclists for a…