The Lactate Threshold - Reality or Fallacy?
For many years exercise science has perpetuated the concept of a lactate threshold - a point during exercise where a sudden, sharp increase is noted in the concentration of lactate in the blood. This phenomenon is supposedly noticed when blood samples are taken from subjects performing incremental to max exercise tests much the same as a VO2 max test. Traditionally, it has been noted that when concentration of lactate is plotted against running speed (or %VO2 max) on a graph, as the individual runs faster the quantity of lactate in the blood remains constant up to a certain speed, after which a sudden inflection in the gradient occurs. This inflection point has been dubbed the lactate threshold - the point during intense exercise where the muscles become increasingly anaerobic, generating vast quantities of lactate. Therefore, this phenomenon has also become known as the anaerobic or ventilation threshold.
As discussed in an earlier article on lactate featured on this website, early exercise scientists (and even some present day ones) attributed the increasing amounts of lactate in the blood during exercise to a lack of oxygen supplied to the muscles. This theory holds that the cardiorespiratory system must be inefficient at matching blood (oxygen) supply to the muscles and exercise intensity. Therefore, as the intensity of exercise increases, the muscles have to rely increasingly on "oxygen independent" (anaerobic) metabolism and its associated lactate generation. What these researchers failed to understand, or were unaware of was that as the intensity level (power output) of exercise progressively increases, there is a corresponding increase in the dependence of the glycolytic energy system. It is now universally accepted that energy systems are exercise intensity dependent. As explained in the previous article on lactate, this accumulation of lactate is a necessary consequence to maintain the increased flow of energy through the glycolytic pathway.
Tim Noakes at the University of Cape Town, South Africa states that it is highly unlikely that the muscles ever become truly anaerobic. He provides some powerful evidence to back up this statement and quite possibly the most convincing might be what he calls the "lactate paradox". In his studies of the results of several research papers on Everest climbing expedition experiments, Noakes noted that lactate accumulation in the blood actually decreased as attitude increased. This finding is the exact opposite of what one would traditionally expect since as altitude increases, the ambient barometric pressure decreases, as does the relative availability of oxygen in the inspired air. Therefore, one would suspect that near the peak of Everest, the exercising muscles must be truly anaerobic and generating large amounts of lactate but as mentioned previously, this does not occur. Professor Noakes explains that at such an extreme altitude, some internally regulated factor (possibly the brain) severely limits the intensity of exercise to protect the heart therefore also limiting the amount of lactate production. Additional research, has demonstrated that even at rest under more than adequate oxygen availability, muscles generate lactate. Furthermore, other researchers have failed to find conclusive evidence that muscles become anaerobic at exercise intensities approaching the lactate threshold or even during maximal exercise. Therefore, Noakes prefers to refer to anaerobic metabolism as "oxygen independent" metabolism since in his astute opinion there exists no such thing as anaerobic muscle.
Noakes dismisses the probability of a sudden increase in lactate concentration. He explains that if too few blood samples are taken - say for every three or four kilometers per hour increase, then when the rise in lactate is actually observed, it may indeed show a precipitous "jump" from the one observation to the next. If samples are taken more frequently however, say at every speed increment, then the increase is much more gradual, producing a smoother logarithmic or hyperbolic curve. Dr. Noakes indicates that lactate buildup during increasingly intense exercise is the result of its production rate exceeding its clearance. As mentioned in our article,Lactate is NOT the Culprit!, Brook's lactate shuttle is purported as being responsible for assisting with the transport, utilization and clearance of lactate during exercise. At lower exercise intensity levels, the rate of lactate clearance is able to match the rate of production. However, as exercise becomes increasingly more intense and more muscle mass is recruited, proportionately large quantities of lactate are produced, but it is unlikely that clearance via the shuttle will be able to maintain pace. Therefore, there exists a speed or intensity level at which the production of lactate surpasses its clearance and the blood concentration begins to steadily increase. This point is what Noakes prefers to call for lack of a better word - the "lactate turnpoint" and that it occurs at an intensity/speed where the concentration of lactate in the blood is approximately 3.0 mmol/L. If the individual were to run at this constant speed, this would be called running at the subjects "lactate steady state". In theory this is the fastest speed that can be maintained by the exerciser for extended periods of time such as marathon running.
Therefore, from discussions in this and previous articles, it is safe to conclude that lactate production in the body is the direct result of an increased reliance on the glycolytic energy systems, not from a lack of oxygen in the muscles. Paired with this increased use of oxygen independent metabolism is an increase in lactate production. The sudden jump in lactate production simply does not exist; it increases proportionally with the increased exercise intensity or power output. In addition, it should be obvious that the term anaerobic threshold is a misnomer and that perhaps the only appropriate term to refer to this phenomenon is the lactate turnpoint.
References and further reading: more information on the concept oxygen independent metabolism and the lactate turnpoint may be found in Lore of Running - a classic book in its fourth edition dedicated not only to running performance, but to cutting edge exercise physiology as well.
David Petersen is a Personal Trainer/Certified Strength and Conditioning Specialist and the owner and founder of B.O.S.S. Fitness Inc. based in Oldsmar, Florida. More articles and information can be found at http://www.bossfitness.com
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