Most runners are familiar with threshold training. It's been the chief contribution to real-world training from the field of exercise physiology. In principle, it's simple: there is a "tipping point," physiologically speaking, when incremental increases in speed become progressively more difficult. Training right at this sweet spot should raise the threshold, moving that "tipping point" to a faster pace. But there is not a unanimous understanding of what exactly "threshold training" is, or about how to properly apply it to training. Threshold training is one of the topics du jour in any number of running magazines and websites, so I don't mean to unnecessarily repeat here what's been done to death elsewhere. No, today's post is about my own peculiar interpretation of threshold training, and the consequences thereof. In short, I think the real value of threshold training is that it involves practicing running fast and efficiently in a state of low metabolic stress. I'll cover exactly what all that means shortly, but the consequences of this are surprising: "threshold" does not always have to correspond to one specific pace. This opens up a lot of possibilities in training.
First, though, a crash course in exercise physiology and a thought experiment to illustrate: In any athletic endeavor, almost all of your energy is derived from aerobic and anaerobic respiration. You're probably familiar with these processes: both turn sugars into energy, either by burning them with oxygen (aerobic respiration) or without oxygen (anaerobic respiration). There are no problems with sustaining aerobic respiration for a long time, but anaerobic respiration is a different story. Metabolic byproducts like lactate and protons (H+) build up during anaerobic respiration, and eventually cause fatigue. "Lactic acid buildup" is commonly blamed for the fatigue associated with anaerobic respiration, but this is technically a misnomer. During an anaerobic effort, blood pH drops (indicating rising acidity) and blood lactate levels climb, but it turns out that these two are mostly independent processes, not the result of "lactic acid." It seems that the acidity is the chief cause of fatigue, not the lactate. Regardless, except for a narrow window of time where the body's buffering system dampens the buildup of acidity but lactate levels rise (this is called the isocapnic buffering zone), lactate levels and blood pH mirror each other very closely. In general, it's easier to measure blood lactate, so lactate levels are used as a proxy for how heavily an athlete is relying on anaerobic respiration. In any case, the body only increases its baseline rate of anaerobic respiration when the energetic demands of an athletic effort exceed the body's aerobic capacity.
Now, our thought experiment: let's say we take Sam, a hypothetical runner, and put him on a treadmill. Sam is a reasonably fit runner who has recently set a 3200m PR of 10:00 and a (track) 5k PR of 16:05. To measure his blood lactate levels, we insert a probe into a blood vessel. The first thing we'll notice is that before we even start the treadmill, our lactate probe will show that there is a small amount of lactate circulating in Sam's blood, even at rest: about 1.0 millimoles of lactate per liter of blood, or mM for short (mM is just a measure of concentration). This is because there's always some anaerobic respiration in your body, even at rest. But there's more than enough oxygen circulating to "mop up" the metabolic byproducts, so the lactate level is stable. So, we start up the treadmill. Very slowly at first, say at 10min per mile pace. Sam's lactate level will jump up a bit right away, perhaps to 1.5 mM, but it won't budge after that. Even if we speed up the treadmill to 9min/mi, 8min/mi, or 7min/mi, his blood lactate level will move by 0.1 or 0.2 mM. Furthermore, even if Sam runs on the treadmill for over an hour, his lactate level won't change. That's because he's running at a steady state effort. His body is getting all of the energy it needs from aerobic respiration.
So we keep cranking up the speed on the treadmill, to 6:30 pace, then to 6:00 pace. Sam's lactate level is probably 1.7 or 1.8 by now. Just under 6min/mi, we'll see Sam's lactate level start to move up a bit more, reaching 2.0 mM for the first time. If we stop changing the treadmill's speed, we won't see a change in lactate over time. Sam is still at a steady state effort. But if we increase the pace a little more, down to 5:50 or 5:45 pace, Sam will not be at a steady state anymore. His blood lactate level will rise over time, even if we don't change the speed of the treadmill. But at this pace, the increase in lactate concentration will be very slow. Sam can probably sustain 5:50 or 5:45 pace for 80-90 minutes before having to stop due to exhaustion. So we keep increasing the pace on the treadmill. Sam's blood lactate will change a bit after each increase in pace. Once we hit about 5:35, his blood lactate will be at about 4.0 mM. If we continue to increase the pace, Sam's blood lactate level will start to jump more quickly with each change in pace. Furthermore, the duration he could sustain a given pace before becoming exhausted will also drop sharply with each increase in pace. Sam could maintain 5:35 pace for 50-60 minutes with a race-level effort. But he could only maintain 5:25 pace for about half an hour. And he could only maintain 5:15 pace for 17 or 18 minutes. 5:05 pace; perhaps 11 minutes.
So clearly there is something going on between 6:00 and 5:35 pace. Whereas Sam could maintain 6:00 pace for well over two hours in a race situation, his ability to sustain paces for a long duration drops off significantly after 5:35 pace or so. As you might guess, in this range is where Sam's aerobic system becomes insufficient to meet his energetic requirements. As such, his anaerobic system needs to pitch in to help, resulting in higher blood lactate levels and an unstable metabolic situation. Numerous physiological events occur in Sam's body between 6:00 and 5:35 pace, all of which have garnered a name and all of which vie for being called the "real" threshold. At about 5:55 pace, Sam ceases to be at a metabolic steady state. Any speed above this point will result in the gradual accumulation of lactate. So some physiologists call this point the aerobic threshold. Others think the most important event is when Sam's blood lactate levels cease to rise linearly and instead rise exponentially. This happens at about 5:35 pace, and is often termed the anaerobic threshold. Some physiologists are uncomfortable with the term "anaerobic" (since it implies that less oxygen is getting to muscles), so they term it the lactate threshold. For others, even this isn't good enough: it has to be the onset of blood lactate accumulation (OBLA). Even others prefer to measure the increase in ventilation, or breathing, and term the sharp increase the ventilatory threshold. You can see how this has quickly gotten out of hand.