I thought this was a good explanation; What Is Functional Threshold Power (FTP)? The term "threshold" has become synonymous with the word "confusion" in the minds of many athletes. There are many different words for essentially the same thing: anaerobic threshold (AT), lactate threshold (LT), maximal lactate steady state (MLSS), onset of blood lactate (OBLA), and just plain old "threshold." It seems that there are just as many possible definitions, with different versions of the concept based on heart rate, blood lactate, wattage, and so on. As a result, even in many scientific articles the authors have to present their "own" definition to clarify what they are talking about. For more than thirty years, exercise physiologists have known that the exercise intensity at which lactate begins to accumulate in a person's blood-that is, his or her functional lactate threshold (LT)-is a powerful predictor of that person's endurance performance ability. This is because although an individual's cardiovascular fitness-that is, his or her maximal oxygen uptake (VO2max)-sets the upper limit to his or her rate of aerobic energy production, it is the individual's metabolic fitness-that is, LT-that determines the percentage or fraction of this VO2max that he or she can utilize for any given period of time. The physiological factors determining functional lactate threshold are complex, but essentially, blood lactate levels serve as an indirect marker for biochemical events within exercising muscle. More specifically, a person's LT reflects the ability of his or her muscles to match energy supply to energy demand, which in turn determines the fuel "mix" (i.e., carbohydrate versus fat) used and the development of muscle fatigue. Consequently, functional lactate threshold-especially when expressed in terms of power output, which also takes into account cycling efficiency-is the single most important physiological determinant of performance in events ranging from something as short as a 3 km pursuit to a stage race lasting as long as three weeks. Just as important, because the effort that is experienced by an athlete when exercising at any given intensity is dependent upon his or her power output relative to power at LT, this parameter provides a physiologically sound basis around which to design any power meter-based training program. Functional threshold power is the highest power that a rider can maintain in a quasi-steady state without fatiguing for approximately one hour. When power exceeds FTP, fatigue will occur much sooner, whereas power just below FTP can be maintained considerably longer. So, how do you go about determining your functional threshold power? 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 FTP is to simply rely on data collected using your power meter in the field. There are a number of different ways to do this, each of which has its advantages and disadvantages, but all of which provide very similar estimates of threshold power. With all these different ways to test your FTP, you may wonder which one to start with. We believe that the best way to begin is to just go out and do a ride specifically designed to find your threshold, such as the one described in the next section. This is without a doubt the first big step in your new adventure in training with power.
