Whether I am working with a client as their coach, consultant, metabolic assessor or providing educational certifications and seminars, "THE" question is always asked, and "THE" debate is always thrashed out: "Is threshold power AeT, VT, LT, AT?" What follows is often the discussion of a variety of explanations of threshold, many as alien as E.T. Since "threshold power" is dynamic and one of the strongest trainable indicators of endurance and performance, a thorough understanding of what "threshold power" refers to is essential. That answer will be revealed as the article progresses. First, it's necessary to comprehend thresholds in general.
Although AeT, VT, LT, AT are used interchangeably, they are not the same thing and determined in a different way. The misconceptions and modified definitions can easily be spelled out based on knowledge of their scientific characterizations. To discover those meanings, it will be essential to go to the "lab", as this is where the practical relevance of these thresholds were established.
Stepping into my metabolic lab, you'll find metabolic gas analyzers, blood lactate analyzer and various exercise equipment. Speaking specifically to cycling, you'll find the CycleOps PowerBeam Pro trainer and CycleOps 400 Pro Indoor Cycle, which are both electronic trainers. This equipment allows me to perform accurate assessments of an individual's physiology (cycling specific in this case), determining true thresholds in a controlled manner. The data can accurately be used to establish training zones, for reassessment comparisons and to determine individuality. (outside of the lab - field testing - discussed at the end of this article).
Getting a better understanding of thresholds is as easy as breaking down the definition. First, let's start with the word "threshold" itself. The dictionary definition of relevance states threshold is, "any place or point on entering or beginning...the point at which a stimulus is of sufficient intensity to begin to produce an effect." Keep that in mind, as that definition is critical from this point forward.
AeT, LT1, VT1
Aerobic threshold (AeT) has become the common label to describe lactate threshold 1 (LT1) and ventilatory threshold 1 (VT1). In any case, these markers are used to establish an aerobic base. By definition of terminology, AeT would be the “edge” of aerobic metabolism; LT1 would be the first rise in lactate levels; VT1 would be the first rise in oxygen uptake. In other words, it is the intensity (stimulus) at which aerobic (with oxygen) energy pathways begin to produce an effect of aerobic conditioning. By determining an accurate AeT, LT1 or VT1, one can condition the specific physiological marker where the start of aerobic conditioning begins. Base training should be done at this marker, to ensure that an effect is occurring. To determine accurately, one would need to go through metabolic gas analysis or lactate measurement, to assess the point where the body begins to increase it's demand for oxygen to metabolize an increase in blood lactate. At this point, the effect of the effort results in one beginning to use more glycogen than fatty acids to fire muscle contractions. Due to the start of demanding an oxygen increase, ventilation will be noticed, and you can feel the first signs of a change in your breathing.
To determine LT1, blood lactate measurements would need to be taken. During lactate assessment, this marker is noted when the intensity begins to produce the first breaking point on the lactate curve, or where there is 1 mmol/L increase in blood lactate above the rest value. The graph shows that the stimulus of 175-200 watts begins to produce this effect.
During metabolic gas analysis (VO2), the type of fuel (Fat –vs- Carb) your system prefers to utilize is assessed at different efforts. Oxygen usage and carbon dioxide production provide a detailed evaluation (Respiratory Quotient) of how the body uses fat and glycogen. Since the metabolism of fat consumes a great deal more oxygen relative to the production of carbon dioxide than the metabolism of carbohydrates, the rise in demand for oxygen will result in a rise in glycogen for fuel (fat deceleration). VT1 is reflected as an increase in VE equivalent for O2 (VeVO2) without increase in VE equivalent for CO2 (VeCO2). The chart shows data from gas analysis, displaying the beginning rise of ventilation and glycogen fuel.
In summary, AeT, LT1 & VT1 are a similar marker. They are the place or point at which a stimulus (watts) is of sufficient intensity to begin to produce an aerobic effect.
LT2, VT2 By definition of terminology, LT2 would represent where the intensity begins to produce the second breaking point on the lactate curve. It is often referred to as the onset of blood lactate accumulation (OBLA) , which is a higher intensity than LT1, as the second increase more often than not occurs when the concentration of blood lactate reaches around 4mmol/L. In any case, there is a balance between lactate production and removal or a rate that glycogen utilization can be maintained for an extended time. This marker clearly is critical for performance, as it is associated with the highest work performed without a rapid rise in lactate concentration, due to the stability of production and removal. A rightward shift of this marker is truly a sign of positive training adaptations. The chart shows where the stimulus begins to produce the second effect.
VT2 is the second rise in VE equivalent for O2 (VeVO2) and this time will also elicit a rise in VE equivalent for CO2 (VeCO2). One of the reasons for this rise in CO2 is due to its use as a buffering agent. The chart below shows VT1 (blue marker) and VT2 (green marker).
In summary, LT2 and VT2 is a similar marker. This marker has been closely associated with an estimate of work corresponding to a rate glycogen utilization can be maintained for 1 hour. Since this marker reflects a balance between lactate production and removal, LT2 and VT2 would then reflect the maximum steady state pace that could be sustained. A rightward shift of this marker would be a sign of a positive adaptation and a leftward shift would be a sign of digression. With that in mind, LT2 and VT2 would be where the stimulus begins to produce the "highest sustainable effort" effect. Training below this marker will develop aerobic conditioning. Training above it will develop anaerobic conditioning (we'll talk AT next). Training around it develops THRESHOLD, or the stimulus (watts) that elicits the max steady state pace. The label Threshold or Threshold Power is established. Without drawing blood or wearing a gas mask, this marker can be determined during field testing or by taking the Power Test, by determining the point where one can no longer "sustain" the effort. This would be the place or point at which a stimulus is of sufficient intensity to begin to produce an effect. Go to http://www.saris.com/t-CPTC.aspx?skinid=2 to learn more about the Power Test.
AT
By definition, anaerobic (without air) threshold would be the point of entering anaerobic metabolism or the point at which a stimulus is of sufficient intensity to produce an anaerobic conditioning effect. Here the cardiopulmonary system can no longer supply adequate oxygen to dynamic muscles, even when it is available. Because the demand for oxygen surpasses the ability to expire CO2, ventilation increases at a much higher rate to blow off the excess. The oxygen deficit discourages fat utilization and rapidly depletes glycogen stores. Due to the fact that anaerobic metabolism produces lactic acid, blood lactate levels increase suddenly and additional CO2 is produced to assist in maintaining the acid/base balance (buffered by bicarbonate). This lactic acid accumulation makes the effort very hard to sustain. With a rightward swing of AT, you can work at higher efforts while postponing fatigue and increased levels of acidity in the muscles which cause one to slow down or stop.
The gas analysis graph shows the anaerobic threshold, marked at the point where fat decelerates abruptly, while CO2 increases abruptly.
Summarizing anaerobic threshold, note that it is not the same as lactate threshold by definition. AT is the level beyond lactic acid accumulation and the entrance to anaerobic metabolism and inadequate oxygen to the muscles. This prevents sustaining work. While lactate levels are increasing at LT2, there is a balance of accumulation and removal. This allows a maximum steady state effort. I like to make the comparison to that of a funnel. If you were to pour water into the funnel quickly, there comes a point where the funnel can no longer keep up with the water demand, and the water begins to spill over top. LT2 would correlate to the balance of keeping the funnel full and draining the funnel. AT would be the point where the water begins to spill overtop.
The similarity in AT and LT2 is that they can be determined without drawing blood or gas analysis, as they mark the point of highest sustained work, which can easily be determined by feel. Although lab assessments provide accurate data in a controlled environment, real performance results should occur during real performance. Also, AT and LT2 are both dynamic and trainable. A rightward shift of this marker would be a sign of a positive adaptation and a leftward shift would be a sign of digression. As with LT2, training above AT will develop anaerobic conditioning. Training around it develops THRESHOLD, or the stimulus (watts) that brings forth anaerobic metabolism. Training below it will develop aerobic conditioning. The label Threshold or Threshold Power is established.
Since cycling involves constant repetitive pedaling and watts are an instantaneous measurement, training with power focuses on staying within a zone range instead of a specific watt. Since LT2 and AT are within a very close range, basing your Threshold Power on either point will keep you honest. The walk away from this is that your Threshold Power is a range of work correlating to the point at which the work is of sufficient intensity that produces an effect of your maximum sustained effort (or produces an effect where you can no longer sustain effort), which is one of the strongest trainable indicators of endurance and performance.
Provided by Angie Sturtevant CycleOps Power Fitness Education Director & Master Training Specialist Owner of Specialists in Sports Performance
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