I’ve received several questions asking about the difference between FTP and Lactate Threshold. Most people for sake of simplicity interchange the two. However, let us take a step back and objectively look at the differences between the two.
Functional Threshold Power
Functional Threshold Power (FTP) by definition, is the maximum power (watts) that an athlete can sustain for an hour. Since it’s mentally and physically taxing, most people resort to the 20min time trial to estimate FTP. I wish to emphasize the word estimate as it really is a basic derivation of FTP. Supposedly, 95% of the wattage for the 20 min time trial would yield the calculated FTP. Sadly, this does not take into account individual fatigue curves. Some can hold a higher percentage of 20min power than most, others tire out fairly quickly.

Going back to an article I posted roughly a year ago, I asked my athlete to do a standard 20min Time Trial (to the tune of Sufferfest’s Rubber Glove). Based on his test results, his calculated FTP is 229w (based on a 20min power output of 241w). A couple of days after, once he was fully rested, I asked him to hold that wattage for roughly an hour. Sadly, he couldn’t sustain that wattage for more than 30mins. He was bonking soon after despite being rested and well fueled.

Once he did his lactate test, the results shed light on the whole predicament. The working definition of Lactate Threshold is as follows: there is a rapid onset of fatigue at any point beyond this value. It is a more accurate representation of FTP since it takes into account how the body functions on a physiological level. Based on his curve, his OBLA (Onset of Blood Lactate) is at 217w and his Lactate Inflection Point (LIP) is at 197w. The LIP is the point which results in a sudden rise in lactate beyond it. This means that at any effort above this point, lactate is accumulating at a faster rate than the body can process. He can sustain 241w for 20mins but because of rapid lactate accumulation, he can only sustain somewhere close to 210w for an hour. Aside from lactate levels, residual fatigue greatly affects the amount of wattage one can sustain for this duration. One must be quite rested before he can perform the test. That being said, FTP actually changes on a daily basis. Objectivity becomes a problem because it’s hard to discern when someone is “fresh enough” to take the test. Blood lactate sampling on the other hand, is more impervious to fatigue and stress. Your body produces the same lactate concentration values for a particular wattage/pace whether fresh or tired. The only thing that changes is the effort needed to sustain those numbers. Since each segment is only 4 minutes long, this concern becomes irrelevant.

Going back to the definition of FTP, even if one were to calculate the correct value, this number is only valid for races lasting one hour. In triathlon, the distances raced often last well above . Even for Olympic distance races, one would have to average 40kph to appreciate one’s FTP. In short, time trial testing does not give an athlete his real zones. Some resort to calculating zones via percentages of FTP (76-89% Zone 3, 90-105% Zone 4 etc.) Again, this does not take into account that each person is unique. Individual fatigue curves are different as some athletes are stronger anaerobically while some are more aerobic. This means that certain individuals may not do so well in shorter distances, but excel in longer distances and vice versa. Time trial testing does not shed light on such insights. Aside from that, unlike LT testing which gives you the specific types of workouts you need to improve (based on the shape of the curve), FTP calculations do not yield a “plan of action” to help the athlete improve.

LT Testing on the other hand, graphs each individual’s lactate curve. Each zone is mapped out based on his/her lactate values. Each zone has specific lactate concentration ranges (e.g. Zone 4 has lactate values between 3.5mmol/L and 5mmol/L). This gives a more exact picture of how the athlete performs both on short distance events and longer events. In short, the curve helps determine whether the athlete is more anaerobic (“fast twitch”) or aerobic (“slow twitch”). More importantly, by knowing whether one is too aerobic or anaerobic, the necessary adjustments may be done to his program to help address these weaknesses.

To summarize, FTP time trials are attractive because it’s free, basic, and easy to calculate; however, realistically, it holds almost no value for the serious athlete. Brushing aside concerns regarding FTP accuracy due to individual response to fatigue, knowing one’s power for an hour becomes irrelevant for longer races. While it may be true that FTP is a good gauge of fitness, one’s lactate curve gives a richer, more precise, and more accurate representation of one’s conditioning.
Lactate Threshold Heart Rate
In the absence of a power meter, some athletes resort to a variation of the aforementioned time trial. This time, the time trial lasts 30 minutes. Instead of keeping track of pace and power, the average heart rate for the final 20 minutes of the 30 minute time trial is taken into account. Supposedly, this is a good estimate of the athlete’s Lactate Threshold Heart Rate (LTHR) which correlates with FTP. Based on most of the 120+ lactate tests I’ve done, the results aren’t quite exact. There are several problems that are encountered here. Since there’s only one data sample (the 20min segment), it is prone to external variables and errors. For example, some athletes have slow rising heart rates. This means that the target HR is not reached within the 20 or 30min time trial. Aside from that, going too hard at the start of the time trial will spike up HR unnecessarily and effectively skew results.

The beauty of LT curve analysis is that it’s more precise because of the number of data samples taken. Each test consists of 6-7 stages wherein the wattage and HR values are logged and graphed. Because of the larger sample size of data, there is redundancy in terms of calculating HR values and ranges. This results in a more accurate representation of one’s HR response.
Another drawback of calculating LTHR is that one cannot track progress with this number alone. For example, if one were to do the LTHR test today, this number would barely change 3 months from now even if he improved immensely. It only gives you the estimated heart rate at which lactate threshold occurs (which barely changes). It does not give you the power/pace coupled with LT. Even if one would keep track of the distance covered during the time trial, irregularities in terrain, temperature, and freshness would make the data subjective. Simply put, running 4min/km on a flat stretch of road at 27C is vastly different from running 4min/km on hilly terrain at 33C during race day. Objectivity and repeatability is thrown out the window with such tests.
Everything in Its Place
Before concluding, let me state that there is still value to doing FTP and LTHR tests. However, one should know how to find their place in designing training programs and race strategies. First of all, FTP is a good way of keeping track of form/progress between Lactate Threshold Tests. Most serious athletes get their curves updated every 3-4 months. More recreational athletes can resort to the 20min time trial to gauge where they are at in terms of fitness. Of course, this will not give one’s full range of zones but it’s a good way of seeing whether he/she improved somewhat. LTHR on the other hand, is an initial starting point for athletes who have no access to LT Testing. Despite being a mere estimate of one’s threshold, for recreational and/or beginner athletes it should be good enough (better than nothing).