Wednesday, July 23, 2014

Lactate threshold and VO2-max testing (or: On Being a Lab Rat)

My kinesiology buddy Nate needed some pilot data for his thesis project, which involves lab testing on runners to find two commonly-tested physiological parameters: lactate threshold and V02-max.  I was happy to volunteer (plus I owe him)...and it was a good opportunity to revisit some physiology that I really should be keeping up on.  Here's the phys lesson I got from Nate, as well as some refresher reading. (Two good articles here and here.)

The Phys:"Lactate threshold" (LT), "anaerobic threshold" (AT) and "maximal lactate steady state" (MLSS) are concepts that refer to slightly different physiological checkpoints that occur during exercise.  Confusingly, folks have been using all of these terms interchangeably.  Really they refer to slightly different things that are of interest, but physiologists (and especially coaches--dammit coaches!) can't agree on which terms mean what, or don't care to distinguish between them. But these values are useful because they are powerful predictors of endurance performance, and follow-up measurements can help an elite athlete track their physiological progress in training. VO2-max means the maximum volume of oxygen consumed, per unit of body mass, per minute.  The units are a confusing (and awesome) ml/kg/min.  This is a blunt way to measure how well an athlete's cardio-respiratory system takes in and transports oxygen to the muscles, and how well the muscles can use this oxygen in the (well, oxidative) reactions that produce cellular energy. There are hundreds of links in the physiological chain that determine endurance performance, and VO2-max is a (sortof good) measure of all of them taken together. Note, however, that V02 max is far from a perfect predictor of endurance sport performance.  How you put that power to the ground is what matters (not to mention your mental toughness, etc etc).

When exercise begins (we're talking endurance exercise--running, etc.), muscle cells begin producing measurable amounts of lactate and hydrogen ions (together, "lactic acid", but this stuff dissociates in the blood anyway I think, so henceforth "lactate").  For years we thought this stuff was all bad, the product of muscle cells working anaerobically (without oxygen), and we assumed it limited the action of muscle cells.  Not quite right, it seems.  Rather, lactate is always produced, as anaerobic metabolism is always used to some degree in conjunction with the aerobic pathways.  But regardless, lactate production increases with exercise, as muscle cells want for oxygen.  The point at which lactate accumulates in the blood and is measurable (usually agreed to be 2 mmol per liter of blood) is referred to as the "aerobic threshold".  This is just about the pace that a well-trained elite runner can race a marathon on the roads.  As intensity increases, other "thresholds" are reached.  "Anaerobic threshold" (AT) or "lactate threshold" (LT)--terms usually used interchangeably-- is usually defined as the pace/effort at which blood lactate hits 4 mmol/L.  This is really a proxy for (in Nate's words) the gold standard, the real threshold of interest: maximal lactate steady state (MLSS), defined as the hardest effort at which lactate remains steady, before a steep increase.  In a well-trained athlete, this usually corresponds to an effort level that can be sustained for 45-60 minutes-- 10 mile race pace or so, on the roads, for a runner. Finding MLSS requires frequent consecutive blood lactate measurements with increasing pace, thus producing a neat curve on a graph.  Because this is hard to do, many researchers simply say that 4 mmol/L is a close proxy for MLSS.  Trouble is, some evidence indicates that blood lactate at MLSS will actually vary from runner to runner, perhaps wildly so.

VO2-max is only reached at a very high effort, usually corresponding to over 90% of maximum heart rate.  It's a pace that a trained runner can sustain for about 15 minutes. Blood lactate levels rise dramatically at this pace, upwards of 10 mmol/L.  Diminishing ATP production in the muscles due to the mismatch between oxygen need and oxygen availability, and other factors such as decreased muscle contraction function due to acidic pH, will eventually cause exhaustion.

The test
Mercifully, Nate isn't measuring MLSS for his study.  Instead he's using the agreed-upon 4 mmol/L benchmark to define LT.  I got strapped up with a gas collection mask (for the VO2-max test), which was very hard to breathe in.  The mouthpiece cut up my gums a bit, but more awkward was the sensation of breathing through a tube, not to mention that my nose was held shut with a clip.  Very weird to get used to, especially when running hard (I need my nose, thanks!). But the gas must be measured; so be it.  The LT test consisted of four 3-minute runs at increasing intensity, starting somewhere around 5:40 pace, and ending up around 5:15 pace*.  Each segment had a 30 second (more like 60 second) break so Nate could prick my finger and test my (capillary) blood lactate levels. (Implicit here is the assumption that capillary blood reflects the current, or at least recent, state of things in the working muscles.) When a measurement yields 4 mmol, another segment is run at a faster pace to get past 4 mmol as a final data point. The VO2 max portion came next--basically, super hard running, getting faster every 2 minutes.  I only got 3 minutes in before I panicked a bit in that freakin' mask and bailed.  The point is to go as hard as possible to get the biggest VO reading you can.

The results: LT*
5:33/mile = 3.4 mmol/L (got the blood right in; a good data point)
5:24/mile= 5.9 mmol/L (errant data point? blood may have been out too long.)
5:15/mile= 5.4 mmol/L (another good data point)
--> so, 4 mmol was probably reached around 5:24 pace (halfway between the 2 good data points.)
*Dec 2014 edit: Nate claims the treadmill was reading 4.5% slower than observed.  This changes the above speeds to 5:18/mile, 5:10/mile for LT, and 5:01/mile, respectively. 

Final pace reached before I bailed, which I only ran for a minute or two: 4:51/mile (Dec 2014 edit-4:37/mile).  VO2: 63.8 ml/kg/min.  Not a bad figure; I think I could have gone a bit higher.  To further sandbag, I think the mask didn't form a perfect seal, so there was some leakage.  (My real VO2 max is 93, I say! World record stuff!)

So what?
Any runner loves to nerd out, and it was pretty rad to see some numbers that represent the otherwise abstract/unseen fruits of my labors.  And Nate got to test out his methods without having to worry about the data.  For me, I'm mostly happy to revisit this physiology and use this super cool (super, SUPER expensive) equipment, because I see running physiology as a potential methodology for future research.  My thesis project is a bone biology study, hopefully with applications to the interpretation of fossils, but I'm also really interested in how our particular brand of endurance physiology evolved.  How does it compare with other animals, especially mammals? What can this type of comparative physiology tell us? And can I have a $200,000 treadmill in my basement? All good questions for future study.

1 comment:

  1. So you changed the track work out for this......