Raise your hand if you’re a little confused about what the lactate threshold is. Don’t be embarrassed if your hand is in the air. Exercise scientists are also confused about the lactate threshold. Fortunately, it’s not a concept you have to completely understand to use effectively in your training.
The first thing you should know about the lactate threshold is that it is not a phenomenon, like language, but an artificial construct, like grammar. There are no fewer than four definitions of the lactate threshold, precisely because the lactate threshold is more like grammar than language. It’s easy to disagree about the right way to talk; much more difficult to disagree about whether talking exists.
What we know is that the blood concentration of lactate produced by active muscles during exercise tends to increase as the intensity of exercise does. What scientists can’t agree on is whether the lactate threshold should be defined as the highest exercise intensity at which the blood lactate concentration remains fairly stable, or the intensity at which the increase of the blood lactate concentration goes from linear to geometric, or the point where the blood lactate curve appears to spike upward on a graph (known as the eyeball test), or a fixed blood lactate concentration of 4 mmol per liter.
It is widely believed that it is important to “correctly” define the lactate threshold, because fatigue occurs much more quickly just above the lactate threshold than just below it, and because training precisely at the lactate threshold boosts fitness far more effectively than training either above or below it. However, there has never been any evidence of a performance threshold corresponding to the lactate threshold (consider that world record pace in the men’s marathon—which occurs a little below the lactate threshold—is a mere 6 percent slower than world record pace in the half marathon—which is at slightly above the lactate threshold—even though the distance is 100 percent greater). And recent research has shown that training a little above and a little below the lactate threshold boosts fitness just as well as training right at it.
So it really doesn’t matter that there is no consensus definition of lactate threshold. All competitive runners should, however, include some type of threshold work in their training. There’s abundant proof that doing a little running each week in that moderately high-intensity threshold zone yields better race results than training only at lower intensities or at lower and much higher intensities only.
However you define it, the only way to train right at lactate threshold intensity is to visit an exercise laboratory and have blood samples taken at various points during an incremental exercise test, not forgetting to determine the heart rate that corresponds to your lactate threshold, and then do all of your lactate threshold training at that heart rate. But since this sort of precision is unnecessary, you should feel free to find your lactate threshold by less technical means, and the means I recommend is perceived exertion.
Rating of perceived exertion is just that: a subjective assessment of how hard exercise feels at any given moment. The most commonly used tool for this type of measurement is the Borg scale, which goes from 6 to 20. On this scale, a perceived exertion rating (RPE) of 6 means “extremely easy” and an RPE of 20 means “extremely” hard. When subjects are asked to rate perceived exertion throughout a lactate threshold test, an RPE of 13 is usually found to correspond to the lactate threshold. Thirteen happens to fall exactly in the middle of that 6-20 scale, and is described as “somewhat hard” or, alternatively, as “comfortably hard.”
You know that feeling of pushing yourself to run hard, yet feeling very much in control of, or “on top” of, your effort? That’s your threshold intensity. At this intensity your breathing is deep and heavy but not strained. You’re forcing the pace but are still able to run relaxed, and you feel you could sustain the effort for some time.
When you perform threshold workouts in your training, you can simply feel your way to this perceived exertion level: 13, or somewhat hard. Better yet, perform your own lactate threshold test, wearing a heart rate monitor and/or a speed and distance device to learn the heart rates and paces that correspond to this effort level. Then you’ll have high-tech and low-tech tools with which to keep your training on track.