It's been 25 years since my last ex phizz class, and honestly speaking even back then I was sleeping through a good part of my instructor's lessons, and when I wasn't he was. But that won't stop me playing physiologist on the web.
As my first felony act while impersonating a scientist, I'll mention that there are two terms that keep appearing in the (often equally inauthentic) physiology reading material in triathlon, and these two terms -- lactate threshold (LT) and anaerobic threshold (AT) -- seem to be used with no distinction between them.
As I understand these terms, the LT "line" is crossed a bit prior to the point where one reaches AT. Lactate threshold has been described as, "...the lowest work rate at which blood lactate appearance exceeds its rate of removal." If blood lactate does not rise -- if one has reached a state of lactate production and buffering equilibrium in his work -- then one can more or less go on indefinitely at that exercise pace. (Of course this isn't precisely so, rather something other than lactate acidosis is what will bring you down in the end.)
Anaerobic threshold is often described as that effort level one can sustain for, say, a 10k (running). In other words, to the endurance athlete it's a rather arbitrary designation signifying an effort level good for 40-ish minutes. This seems to me descriptive of a higher work rate than LT. My notion would appear to be backed up by the developer of the anaerobic threshold hypothesis, Dr. Karlman Wasserman, who has said of it: "The mean gas exchange AT was found to correspond to a small increment of lactate above the mathematically defined lactate threshold."
At the same time I recently read the following in an essay by a work physiology doctor: "The anaerobic threshold defines the point of exercise at which lactate production and lactate elimination are balanced." This description seems to me that which I'd bestow on lactate threshold, not anaerobic threshold. Go figure.
While AT may seem an arbitrary designation to me and perhaps to you, it is not arbitrary to those in the medical profession. Anaerobic threshold is not the sole or even primary property of endurance athletes. It is a diagnostic tool used by cardiologists, pulmonologists, occupational therapists, and others, and it is measurable (through gas analyzers and expensive stuff like that). Among many other things, using this tool a doctor can tell if a patient's got a heart condition, and with a doctor's help an insurance investigator can tell if a claimant is faking a condition.
About this AT versus LT designation, I'm not trying to be difficult. I'd just like to get the nomenclature right for the sake of additional ideas I'll pose below. Work performed at lactate threshold is work that you ought to be able to perform for a long time, all things equal. Limiting factors become muscular endurance, the durability of your connective tissue, your ability to store and replace fuel and electrolytes, and so forth. It's well established what the physiological changes occur upon commencement of training: increased capillarization, production of mitochondria, muscle hypertropy, and all that. That's not my interest here.
Rather, it is in the narrow mention of another set of physiological observations, some of which you've heard of, some you haven't: VO2 drift, the slow component of VO2, and heart rate drift. If you've heard of any of these it is the latter, and there is only a loose correlation between the increase in heart rate and in oxygen consumption or "uptake."
Oxygen uptake (VO2), remember, is the amount of oxygen your body consumes. Don't confuse this with inspiration -- the amount of air you intake into your lungs. "Consumption" denotes how much oxygen you burn. This is a measure of how hard you're working, and of how fit you are. It's analogous to your car. If you've got a big powerful engine it's going to consume a lot of oxygen. It's got big "muscles" and those muscles (pistons) require a lot of gas (hydrocarbons) to move them up and down. Oxygen is required for that process. A lot of oxygen. If you tune your engine, you'll burn gas more efficiently. In other words, less unburned gas (emissions) will go out the tailpipe, and the burning of that extra gas requires more oxygen. If you tune your body (through working out) you'll also burn more hydrocarbons (okay, we'll turn it around and say carbohydrates). In the same way as your car, if you're burning more carbohydrates, you're burning more oxygen.
Likewise as you work harder, your body consumes more oxygen, just like your car consumes more oxygen as you press on the gas pedal. When you go as hard as you possibly can, you reach the maximum level of oxygen consumption, (VO2 max). A max test will give you an idea how you line up against the rest of the world, and is measured in millileters of oxygen per kilogram of body weight per minute. A VO2 max of 65 is really good, and the highest reliable readings ever recorded are in the 80s.
You burn more oxygen as you recruit more muscles, and therefore cyclists and nordic skiers have especially high VO2 max results because they've developed and refined more and bigger muscle groups than, say, arm wrestlers.
You might suspect that as a person ups his effort level his blood lactate and VO2 would increase in a linear fashion. Exercise physiologists believed this to be the case until perhaps a decade ago. Then they began to realize that one's oxygen uptake increased at a higher rate than expected, and this "extra" increase is the "slow component" of oxygen uptake. Even if you're working at just slightly over LT, your VO2 will rise faster than your blood lactate.
Why does it do that? Some have surmised that it is because of the recruitment of ancillary, secondary, and perhaps less efficient, muscle groups. When I first read about this I surmised that this was descriptive of what happens when one's form "falls apart." I've been told, however, that it is more likely due to a recruitment of fast twitch muscle fibers to do the work that the slow twitch fibers are no longer, after a period of time, able to perform. At any rate, the line of VO2 increase -- the rate of the slow component -- is subject to reduction through training. In other words, you can delay the point of "red lining."
One way I measure my adaptation to heavy work while cycling is via a specific Computrainer workout I do. I like to do a ride in "ergometer" mode at, say, 240 or 260 watts, while keeping my cadence up to 95 - 100 beats per minute. (In the ergometer mode the Computrainer makes sure that whatever gear I'm in and at whatever rate I'm pedalling I'll always be putting out a set amount of power). I'll do this for 45 minutes, or until my body cries uncle, whichever comes first. This accomplishes a variety of aims on which I won't elucidate now, but one thing I measure -- a sort of workout inside a workout -- is what happens to my perceived level of exertion after I hit 170 beats per minute.
I'll explain. This is a hard workout for me, and is designed to take me to a place of pain by the time I'm done. It's 15 easy minutes, 15 reasonably hard minutes, and 15 downright uncomfortable minutes. Depending on how fit I am and how many watts I'm pushing, I'll hit 170 beats per minute inside of the first 15 minutes. The question now becomes, How long can I ride at 170+? If I'm not in shape I'd have a hard time riding for 10 minutes after I first hit 170 bpm. If I'm in good shape I can ride for a half-hour at the same effort rate after I first hit 170. This, to me, indicates that my slow component line is flattening out.
What you might expect is that it'll take longer for me to hit 170bpm in the first place after I've gotten myself fit. Funny thing, this is less likely to be my physiological response to training. What is more likely to happen is that my pulse goes up to a certain point relatively quickly, but I'm able to sustain that heart rate for a longer period of time. Knowing this about my body makes it easier for me to understand how my body responds to training, and how to measure an increase in fitness.
Let's go back to AT for a moment. In light of the above -- and if you define AT as a work rate in which one's lactate production slightly outstrips one's capacity to buffer it -- it is axiomatic that there is no such thing as a steady heart rate one can maintain during an exertion which will lead to exhaustion in 40 minutes. Well, okay, yes, one could certainly run at a specific heart rate for a period of time, but -- assuming your running above LT -- your pace is going to slow. Record attempts in swimming, running and cycling over a period of decades indicate that the most efficient way to go fast is to do so at a steady work rate. Therefore, if the work rate remains steady, ipso facto your pulse rate is going to go up throughout the duration of the effort.
Therefore, the idea of a specific AT heart rate becomes less helpful, at least to me. Rather, my interest is in my body's ability to sustain an exertion for a longer period of time after a target heart rate is first noticed.
Shrewd readers will notice that in the last two paragraphs I've jumped from oxygen consumption drift to heart rate drift. There is not necessarily a linkage between the two, but there is often a general correspondence. Perhaps if we're one day wearing oxygen consumption monitors on our wrists - - and I suspect that might be the case -- we could look at VO2 as an in vivo diagnostic device measuring training adaptations. Until then heart rate is all we've got.