All references to thresholds here are in relation to VO2 testing. A search of the literature, or Google, will find references to other methods of determining some thresholds. But beware, there is much mis-information on the web!
For the Aerobic Threshold I found this explanation in Wikipedia(1):
"The aerobic threshold (AeT) is a term sometimes used by sports coaches and trainers to describe a level of exercise intensity somewhat below the anaerobic threshold. It, however, is not a defined physiological term.
The AeT is sometimes defined as the exercise intensity at which blood lactate reaches a concentration of 2 mmol/liter (at rest it is around 1 mmol/liter). This tends to be at a heart rate of approximately 20-40 bpm less than the anaerobic threshold."
In physiology, AeT refers to an increase in ventilation (breathing volume) that occurs rather early in an exercise session, sometimes also called the first ventilatory threshold (VT1).
The Lactate Threshold (LT): Blood lactic acid increases with increasing exercise workload. Most lactic acid is re-cycled in the muscles and liver, a very complex process. Every individual has a point (work rate) where the rate of lactate production exceeds the rate of re-utilization. At that point the lactic acid then increases disproportionately to workload. This point is called the lactate threshold. To determine this point it is necessary to puncture (lance) your skin (typically earlobe or fingertip) every 1 to 2 minutes and collect a drop of blood for analysis.
The Ventilatory Threshold (VT or VT2). At the LT, or very close to it, your ventilation also increases disproportionately to increases in workload. When plotted on a graph, this is often the most recognizable event demarcating the ventilatory threshold, see figure 1. When workload increases at a steady, linear rate, and ventilation initially follows with a like increase, then a sudden deviation is certainly an event that must have significance.
Figure 1 shows Ventilation and heart rate plotted against a linear
increase in workload. VT1 and VT2 are determined by changes in slope of ventilation.
The Anaerobic Threshold (AT). "Anaerobic" means "without air" and now we are stepping on thin ice. The term "AT" was originally suggested by Dr. Karlman Wasserman of UCLA, but the term and the scientific reasoning behind it have been seriously questioned in recent years. It's safe to say that the AT is going out of favor.
Ok, there is agreement that there is a threshold "T", a change in physiologic response to increasing workload, an unbalance. Scientists are still debating the cause. The issue is what to call it. Some have called it the Fitness Threshold, Thermal Threshold etc. The most important point here is that, for most people and under most conditions, the LT, VT and AT occur at the same point, although nit-picking scientists will quibble and split hairs that us Dummies either can't understand or don't care about.
There is also agreement that you can perform work for prolonged periods below the "T", but working above the T will quickly lead to exhaustion.
There is further growing consensus that the T measurement may be more useful than VO2max for most people. After all, unless you are a super athlete that wants to brag, or may otherwise benefit from knowing your VO2max, what's the point? The fact is, VO2max test results depend very much on motivation. Is a person that is trying to collect disability payments motivated to perform well?
The "T" clearly defines the limit of your day-to-day operational capability. For untrained individuals, the T may be approximately 50% of their VO2max, for trained athletes it will be more like 80 to 90% of their VO2max, therefore, the T is either expressed as a percentage of VO2max, or in liters of VO2 per minute, or as the heart rate at the T for training purposes. Nevertheless, this suggests valuable implications for athletic training, where the goal is to move the athlete's T as close to VO2max as possible.
Since the T is less dependent on motivation, and does not require exercising to exhaustion, it can be measured more often for a person engaged in a fitness program. Increasing your T is what you should aim for and even us Dummies know that frequent exercise of most any kind will get you there.
(1) Wikipedia is a registered trademark of the non-profit Wikimedia Foundation
Copyright (C) John Hoppe 2008