Andrew R. Coggan, Ph.D.
For more than 30 years, exercise physiologists have known that the exercise intensity at which lactate begins to accumulate in a person's blood - that is, their lactate threshold (LT) - is a powerful predictor of their endurance performance ability. This is because although an individual's cardiovascular fitness, i.e., their maximal oxygen uptake (VO2max) sets the upper limit to their rate of aerobic energy production, it is their metabolic fitness, i.e., their LT, that determines the percentage or fraction of their VO2max that can they can utilize for any given period of time. The physiological factors determining LT are complex, but in this context blood lactate levels essentially serve as an indirect marker for biochemical events within exercising muscle. More specifically, a person's LT reflects the ability of their muscles to match energy supply to energy demand, which in turn determines the fuel "mix" (i.e., carbohydrate vs. fat) used and the development of muscle fatigue. Consequently, LT - especially when expressed as a power output, which also takes into account cycling efficiency - is the single most important physiological determinant of performance in events ranging from as short as a 3 km pursuit to as long as a 3 week stage race. Just as importantly, because the metabolic strain experienced when exercising at a given intensity is dependent upon the power output relative to power at LT, this parameter provides a physiologically sound basis around which to design any power meter-based training program.
TrainingPeaks WKO+ software explicitly recognizes the crucial importance of power at LT by allowing you to enter a value for your current "threshold power" (and threshold heart rate) into your "Athlete Settings" file. This value is then used to calculate the intensity factor and training stress score for every file you analyze [see "What are normalized power, intensity factor (IF), and training stress score (TSS)?"]. In addition, if you wish TrainingPeaks WKO+ will use your threshold power to automatically calculate seven suggested training ranges, or levels. Alternatively, TrainingPeaks WKO+ also allows to custom define your own power-based training levels.
So, how do you go about determining your threshold power? Obviously, one way is via laboratory testing with invasive blood sampling, but few people have access to such testing on a regular basis. In addition, power at LT as determined in this manner is often significantly below what athletes and coaches tend to think of as a "threshold". A more convenient and possibly more accurate way of determining your functional threshold power is therefore to simply rely on data collected using your power meter in the field. There are a number of different ways of doing so, each of which has its advantages and disadvantages, but all of which provide very similar estimates of threshold power. In order of increasing complexity, these are:
Since one goal of any training program is to increase power at threshold, the value you have entered into TrainingPeaks WKO+ should be periodically reassessed to be certain it is still accurate. (In particular, an intensity factor of more than 1.05 - meaning that normalized power is more than 5% greater than threshold power - during a ~1 hour mass start race is often evidence that threshold power is greater than the value entered into the program.) How often threshold power will change significantly will depend in part on an individual's training history and habits - for example, someone who is just beginning in and/or returning to cycling may see large and rapid changes in their threshold power, whereas an experienced rider who has been training for many years and/or an athlete who maintains a high level of conditioning year round will probably experience much less variation. In general, however, assessing threshold power a few times per year (e.g., near the start of training as a baseline, partway through the pre-competition period to track improvement, and during the season to determine peak fitness achieved) is probably sufficient.