The first scientist to draw the connection between exercise and lactic acid was Jöns Jacob Berzelius, Alex Hutchinson explains, the Swedish chemist who devised the modern system of chemical notation (H2O, etc.):
Sometime around 1807, he noticed that the chopped-up muscles of dead deer contained lactic acid, a substance that had only recently been discovered in soured milk. Crucially, the muscles of stags that had been hunted to death contained higher levels of lactic acid, while deer from a slaughterhouse who had their limbs immobilized in a splint before their death had lower levels, suggesting that the acid was generated by physical exertion.
A century later, physiologists at the University of Cambridge used electric stimulation to make frogs’ legs twitch until they reached exhaustion, and observed high lactic acid levels. The levels were even higher if they performed the experiment in a chamber without oxygen, and lower if they provided extra oxygen. That finding helped establish the prevailing twentieth-century view: your muscles need oxygen to generate energy aerobically; if they can’t get enough oxygen, they switch to generating energy anaerobically, which produces lactic acid as a toxic byproduct that eventually shuts your muscles down.
Athletes going lactic feel the burn and typically back off a bit:
In interviews with athletes who’ve begun using baking soda, a common theme is that they’re able to push harder for longer before feeling that burn in their legs, which in turn enables them to race faster.
One theory about the feeling of going lactic is that you’re literally starving your brain of oxygen. If you push hard enough, it’s not just your muscles that go more acidic; your whole bloodstream follows. Thanks to a phenomenon called the Bohr effect, rising acidity reduces the ability of your red blood cells to ferry oxygen from your lungs to the rest of your body, including your brain. In one study, all-out rowing caused oxygen saturation to drop from 97.5 to 89.0 percent, which is a big drop—big enough, perhaps, to slow you down and contribute to the out-of-body feeling at the end of hard races.
We also have nerve sensors that keep the brain informed about the metabolic status of the muscles. These group III/IV afferents, as they’re known, keep tabs on the real-time levels of molecules like lactate and hydrogen ions. If you block these nerves with spinal injections of fentanyl, exercise feels great—too great, in fact, because you’ll lose all sense of pacing, go out too hard, then hit the wall.
The most telling finding about the lactic burn, in my view, was a 2013 study where they injected various molecules into the thumbs of volunteers in an attempt to reproduce that familiar feeling. Injecting lactate didn’t do it. Neither did injecting hydrogen ions, or ATP, a fuel molecule whose levels are also elevated during hard exercise. Injecting them in pairs didn’t do it either. But injecting all three at the levels you’d experience during moderate exercise produced a sensation of fatigue in their thumbs, even though they weren’t moving them. And injecting higher levels turned fatigue into pain.
That’s a distinction I try to keep in mind in the late stages of hard workouts, and at the crux of races. That burning feeling is real, and it’s associated with lactate and acidity and muscular fuel levels. But it’s just a feeling.