Most athletes have an intuitive idea of “threshold”. It refers to the idea that every athlete has a point at which working just a little bit harder means having to stop a lot sooner. Over the years, there have many explanations for this phenomenon. Like most processes in the human body, “threshold intensity” is likely multifaceted. It is complicated by the fact that most of the people writing or talking about it don’t have a good handle on what it really is.
For starters, we have a lot of terminology that people in sport use somewhat recklessly and inaccurately. Let me try to help clear things up a little bit by concisely defining the “scientific” words you have heard thrown around. In short, there are really two “thresholds”, if you like: Lactate Threshold (LT), and Critical Power (CP). Lactate Threshold is indicative of changes in your exercising body that may be difficult to detect on a conscious level, but which we can find using specialized tests. Critical Power is probably the threshold athletes actually feel.
Let’s first get handle on lactate. It is a substance that appears in the blood during exercise. The harder you go, the more of it there is. A lot of attention has been paid to it, because it is easily and cheaply measured. However, it is really only an indirect marker of exercise intensity. Basically, your body burns a mixture of fat and carbohydrate during exercise. For many reasons, exercising harder requires your body to burn a greater amount of carbohydrate, and one of the end products of this is lactate. It isn’t a waste product, and it does not in and of itself cause fatigue. Rather, it is an energy rich compound which is taken up and utilized by other cells / organs in the body.
So, how has lactate been used to understand the relationship between exercise intensity and stress on the athlete’s body? Essentially, scientists wanted to set up some ground rules for what was easy and what was hard. They looked at the behavior of lactate in the blood at different work rates, and looked for places where things seemed to change. This lead to the definitions of things called LT, OBLA, and MLSS. There are some other, lesser used terms, but these are the ones you want to know about if you want to communicate effectively.
What I would like you to do is imagine yourself jogging along easily, or going for a spin on your bike on a long, flat road. Now, I want you to imagine slowly speeding up, and hitting all of these different levels of exercise as you go.
At the low end of the spectrum, we have Lactate Threshold or LT. This is simply the point where the level of lactate in the blood rises by 1 mmol / L over exercise baseline. In other words, if you are zipping along at 1.5 mmol / L, and you speed up enough that you reach 2.5 mmol / L, you have crossed LT. This is actually a lot easier to do than most athletes realize. You could ride around at / about LT for a couple hours without much of a problem. Running, it is probably in the ballpark of your marathon pace, although elites can run a marathon a bit harder than this.
In the middle somewhere, we find OBLA, or Onset of Blood Lactate Accumulation. It is when you hit 4 mmol / L, irrespective of where your baseline was. This term is not used frequently anymore, and we should probably avoid it. It isn’t a great marker for many reasons, but a big problem is that it is an “absolute” level. For instance, let’s say your baseline lactate during easy exercise is 2.5 mmol / L. Hitting a level of 4 probably means something different to you than it does to someone who started at a level of 0.5 mmol / L.
At the high end, there is the MLSS, or Maximal Lactate Steady State. This is equivalent to something like the pace you could hold for a 20 to 40k TT, or a 10k to a Half Marathon, depending upon how well trained you are. In terms of lactate, this is simply the highest level of exercise we can manage while maintaining a steady concentration of lactate in the blood. If we try to go any harder, we see a progressive increase in lactate levels, even if we maintain a constant power or pace. MLSS is intimately related to the concept of Critical Power (CP), and is probably one of the biochemical manifestations of reaching Critical Power (if you are on a bike) / Critical Speed (if you are running or swimming or whatever). The physiological “stuff” going on at this point is probably what results in the “feeling” of “threshold”, and is what is partially responsible for what Andy Coggan has called “FTP”. FTP is not reflective of some unique, alternative threshold phenomenon. It is just convenient shorthand, and results from your body “understanding” that if it goes much harder, bad stuff is going to happen (like inexorable fatigue). In other words, FTP that you observe in the field by doing something like a 40K TT or running a half-marathon is probably pretty close to, but just slightly lower than CP in a well trained athlete.
This begs the question, what is CP and how do you measure it? If you did a bunch of all-out exercise tests…i.e. went as hard as you could for 3 minutes,
and then for 5 minutes, and then for 10 minutes, and you graphed them on a piece of paper, you’d get a curve. It would be high on the left, and then slope down to the right. The place where it seems to level off is CP. You can also make the same graph by using joules (this is easy… watts = joules per second), and you get a straight line, and the slope of that line (rise / run) is equal to CP. If you wanted to do it running, you would run a 3k, a 5k, a 10k, and then make the same graph: the distance on the vertical axis, the time it took you on the horizontal axis. Rise / Run = Critical Speed. (PLEASE NOTE: This is Critical Power, the real scientific one, not the bastardization that is incorrectly used by many people to refer to the hardest they can go for some period, like “CP30″ for “the hardest I can go for 30 minutes).
Basically, it goes like this. As you cross LT, you begin burning more carbohydrates. You also begin recruiting less efficient muscle fibers, which tend to use more glycogen / carbohydrate and less fat for fuel by their nature. The result of this is an increase in lactate in the blood we discussed above. This is not a problem. Like I said, the lactate is taken up by other cells / organs and is metabolized. (Again, lactate is an energy-rich compound, not a waste product. It doesn’t in and of itself cause fatigue. In fact, your brain, your liver, even your kidneys will actually use it for energy!) In triathlon, it is a problem only in the sense that your body glycogen / carbohydrate stores are limited, and that you can “bonk” if you aren’t getting enough carbs in while you race (if the race is long enough).
Something else interesting happens above LT. We see the emergence of what is called the “slow component” of oxygen use. In other words, let’s say your LT is 170W (not unreasonable for an age-grouper). Below 170W, you are using some constant amount of oxygen. If you made a graph, you’d see your oxygen use rise up as you started, and then become a straight line, which was perfectly level. Now, if you ride 180W, what you will see is that your oxygen use seems to kind of level off, and then (maybe a minute or two later) there is another “hump”. In other words, it appears as though your body suddenly realizes it needs more oxygen to do the job. However, this second “hump” also levels off, so you again end up with a steady state of oxygen use. It is just that you are using more oxygen than you might otherwise have expected.
The key is that so long as you stay below Critical Power, your body is able to maintain a physiological steady state. After several minutes, oxygen use levels off, and lactate concentration levels off. In the muscles, the concentration of creatine phosphate (PCr), ATP, inorganic phosphate and hydrogen ion (i.e. pH, or how acidic it is inside the muscle) stabilizes. Once you go hard enough to cross Critical Power, bad stuff happens. In other words, let’s say your CP is 240W. If you go to 250W and hold it, the result is a major metabolic upset inside the muscles . Even though the work rate is not changing, there is still a progressive increase in the amount of oxygen used, a progressive increase in lactate concentration in the blood, and a progressive decrease in ATP, PCr and pH inside the muscle. It is very cool, because you cat actually watch a lot of this stuff happen with a specialized MRI setup. Eventually, you reach some limiting level of these markers, fatigue, and must stop, or at the very least drop to some much easier intensity.
So, you may now be asking yourself, “Why should we even worry about lactate?” The answer is, we shouldn’t! I encourage my athletes and students to stop thinking in terms of lactate. Everyone does it, because it is so easy to measure. However, as you can see, it is really just a very indirect marker of some much more important / interesting stuff that is going on in the body, most of which is not easy to measure without expensive gear. From the perspective of the average (or even professional) athlete, it is simply important to realize that you don’t want to be crossing CP with any significant frequency or for any significant duration if you are expecting to do your best in a triathlon, particularly a long course triathlon.