If you want to annoy a physiologist, say something along the lines of "Boy, I can really feel the lactic acid in my legs from yesterday's workout." This is one of those exercise myths that refuses to die, despite decades of research showing that lactic acid (or, more correctly, lactate) is more of a help than a hindrance during exercise. Over at Letsrun, physiologist Matthew Goodwin has an amusing and detailed rant about lactate misconceptions in the popular press. It's a bit technical in places, but worth a read if you're interested in getting a fuller picture of what lactate does and doesn't do.

Two of his points that I wanted to highlight.

First, lactate doesn't cause next-day muscle soreness. There's no known connection between the lactate molecule and the processes that cause soreness -- and moreover, the kinds of exercise that cause the most soreness (eccentric muscle contractions, like from walking or running down a hill or lowering a heavy weight) don't actually produce much lactate.

That means that warming down to "flush out lactate from the legs" doesn't do anything. Goodwin shows some nice data (his Figure 6) of the typical pattern for lactate levels after hard exercise: a dramatic spike that returns to normal within about an hour after stopping. If you add a warm-down, you can slightly speed up that clearance rate, but either way all traces of lactate will be long gone with a few hours, let alone a day or two later when the soreness peaks.

The other point he makes that I thought was interesting is more about semantics. What is the lactate threshold? The usual quick explanation is that, when you're working so hard that you can't get enough oxygen to your muscles to fuel them aerobically, you start relying on anaerobic processes (which produce lots of lactate) instead. So above a certain effort level -- your threshold -- lactate levels in your blood rise exponentially.

But blaming the lactate threshold on insufficient oxygen is confusing correlation with causation, Goodwin points out. The difference between aerobic and anaerobic energy sources isn't just the presence or absence of oxygen -- it's also about how quickly the energy can be made available. Aerobic energy is slow, so when you're working really hard you have to rely on anaerobic (and lactate-producing) energy. That would be true even if your muscle cells had an infinite supply of oxygen! So the lactate threshold does signal the transition to anaerobic energy sources, but not necessarily because of a shortage of oxygen.

(There are two other factors that contribute to the rapid rise of lactate at higher intensities. One is that "hard" exercise recruits more fast-twitch muscle fibers, which produce more lactate. The other is that you produce more adrenaline at higher effort levels, which constricts the tissues leading to the liver, where lactate is removed from the bloodstream.)

Bottom line: the actual picture of what's going on inside your body during intense exercise is quite a bit more complicated than the way most of us talk about it. Does that actually matter? In most cases, probably not. Athletes spend a lot of time training at lactate threshold because it works, regardless of what the theory behind it is. Still, like Goodwin, I hope that the language we use to talk about training will gradually shift to reflect reality.

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