Running Functional Threshold Power

Running Functional Threshold Power - A Primer

By Steve Palladino

coach and consultant, Palladino Power Project

October 31, 2017

Functional threshold power (FTP) is a metric, developed by Andrew Coggan, PhD, that has been in use in the cycling world for more than 15 years. However, its use in the context of running is perhaps less than 2 years old. For runners that have not been following discussions on FTP over the years, or the nuances of estimating it, this article is intended to deepen the reader’s understanding of this important metric, and refine its use within the context of a run training program.

What is FTP?

Originally, FTP was defined as “the highest power that a runner can maintain in a quasi-steady state for approximately one hour without fatiguing”. The “approximately one hour” component of the definition has since been clarified to range between perhaps 30-75 minutes, depending on the individual. Thus, the correct statement might be amended to “the highest power that a runner can maintain in a quasi-steady state without fatiguing, where the duration may range from 30-70 minutes, depending on the individual” The maximal duration of time that one can hold their FTP is referred to as Time to Exhaustion (TTE).
FTP correlates with power at maximal lactate steady state (MLSS). This is the physiologic state that represents the maximal power that can be sustained while remaining in a steady-state blood lactate, aerobic condition. MLSS, lactate threshold (LT), and FTP all represent similar physiologic states. In part to obviate the need for formal lab testing of MLSS or LT, FTP was originated as a less-expensive, more conveniently scheduled, and self-performed field-test surrogate for those particular lab tests.
FTP, as surrogate for MLSS, is a measure of metabolic fitness.

Why monitor FTP?

FTP, as measure of metabolic fitness, if normalized to weight (in W/kg), correlates to race speed encountered in the range of 5k to marathon. It is one of the key determinants of performances in races of this range. The fact that, for most runners, races from 5K through the marathon are executed at an average power of FTP +/- 10% is rather significant.
Following FTP over time gives one the opportunity to track metabolic fitness. (Figure 1) During a period of racing on like courses (eg, the track), fitness might be monitored by times and performances. However, what of periods in which racing is not frequently performed, or when racing is done on non-comparable courses?
FTP enables the stratification of training zones. As coach Alex Simmons put it in his blog,

“it enables a <runner> to define and measure intensities of <running> (or power levels) relative to their own current level of fitness, expressed in a manner that relates to the primary physiological adaptation that occurs at each intensity (power) level. This is very useful for guiding training and making sure that the mix of intensity and duration during a workout or training cycle is appropriate for gaining the specific fitness required for a <runner’s> target events.” <runner/running substituted for rider/riding>

Although training in supra-FTP zones is enhanced by following an individual’s power-duration curve at those intensities, FTP nevertheless, retains importance in providing a descriptive framework for those higher intensities. Further, since most runners execute races from 5K to marathon at FTP +/- 10%, FTP guided training retains significance.

Figure 1. Example of tracking FTP.

Power and Chronic Training Load on y-axis, date on x-axis. White line is WKO4 modelled FTP. Blue line is WKO4 Chronic Training Load (CTL). The dotted vertical line on 6/10/2017 coincided with the runner’s 10,000m track PR (a 6.8% improvement in time versus prior to the season start). Also note that the FTP peak and PR coincided with a short taper of the runner’s CTL high.

Errors of assumption regarding FTP....

The primary error of assumption regarding FTP is that FTP is equivalent to average power over a maximal 60 minute effort. FTP was never defined this way, or certainly not intended to be defined this way. As mentioned above, the duration that one can hold FTP (also called Time to Exhaustion or TTE), can range from 30-75 minutes. In elite marathoners and half-marathoners, the 60 minute TTE at FTP might hold true. However, in most runners below the elite level, TTE at FTP is often between 35 and 50 minutes.
Another common error is the assumption that Critical Power (CP) = FTP. Although both metrics reflect similar physiologic states, it is not unusual that calculated CP does not equal estimated FTP. This is not to say that the two are not often close in value. Hill states that “exhaustion occurs after about 30 to 60 minutes of exercise at CP”. The sustainable duration for FTP, as mentioned before, ranges between perhaps 30-75 minutes, depending on the individual. Further, the CP reported by Stryd Powercenter is said to reflect a sustainable duration of about 50 minutes (perhaps 50-60 minutes) per the folks at Stryd. In the end, from the perspective of FTP, rather than considering FTP and CP as the same thing, it might be better to consider CP an approximate estimate of FTP based on the CP or Monod model.
A less common error of assumption is that an athlete’s running FTP is equal to their cycling FTP. Quite simply, for a given athlete, their estimated running FTP will not be the same as their cycling FTP estimate.

Requisites for accurate FTP estimation

For FTP estimation to be a reasonable representation of reality, it requires two things:

A useful model of FTP estimation

Within the model, the number of parameters used may be a factor

Maximal or very near maximal efforts for each parameter used in the model

The accuracy of the estimate falls if; a) the model is inferior, b) the effort(s) used is/are not maximal or very near maximal, or c) both a and b.

Methods of estimating FTP from running powermeter field data

How many parameters does your estimation model use?

>3 parameters. The WKO4 power-duration curve model uses multiple parameters (>3, but probably less than 10), each representing a point along the individual’s meanmaximal power-duration curve. The model can be quite accurate at estimating FTP (along with TTE @ FTP) - if the individual’s meanmaximal power-duration curve contains sufficient true maximal or near-maximal data points across a range of durations. There are other models that likely use >3 parameters, such as those used by TrainingPeaks, Xert, and Golden Cheetah. I am most familiar with and use WKO4.
2-3 parameters. The Monod critical power model often uses two parameters. The critical power calculator on Stryd’s Powercenter uses two parameters - either a) the powers from both, 3 lap (1200m) and 6 lap (2400m) tests, or b) the powers from both, 3 minute and 9 minute tests. As stated above, the CP reported by Stryd Powercenter is said to reflect a sustainable duration of about 50 minutes (perhaps 50-60 minutes) per the folks at Stryd.

Two parameter models work best with a shorter duration maximal effort, and a longer duration maximal effort. The short duration effort should be at least a minute long. The larger the difference in duration between the short duration test and long duration test (with the longer duration extending to as much as 20 minutes), the more reliable the estimate (as long as both tests are maximal). To independently estimate CP outside of Powercenter, there is at least one online calculator that allows the user to use either two or three parameters.

1 parameter. The concept of a single parameter model is to use a single race effort in the range of 5k to 21.1k, and convert it by formula to an estimate of CP/FTP. The modified Riegel formula (presented in Van Dijk and Van Megen’s book, The Secret of Running, p98, 2017) and further discussed by Ron George, in his 2017 articles New Fatigue Factors for Running : Men's Road and Track Racing, by Ron George, 2017. and New Fatigue Factors for Running : Women's Road and Track Racing, by Ron George, 2017.) can be a useful single parameter model.

Protocols - the WKO4 power-duration curve model

When using the WKO4 power-duration curve (PDC) model for FTP determination (referred to as mFTP), your maximal efforts are taken into consideration along the breadth of your own mean maximal power-duration curve. (Figure 2) The model samples at least 3, but probably not more than 10 locations on your mean maximal curve - likely between the far left of the curve (Pmax) through at least the point where the flatness of your mean maximal curve begins to drop (approximately Time to Exhaustion).

Figure 2. Meanmaximal power curve (MMP curve) and resultant power-duration curve (PDC) modelling, including associated PDC metrics.

Meanmaximal power curve (runner’s actual maximal efforts for last 90 days) = yellow line

WKO4 PDC model (based on the runner’s MMP curve) = red line

From the PDC model, mFTP (among other PDC metrics) is estimated. (Figure 3) The accuracy of the mFTP estimate is most dependent on maximal or near-maximal efforts represented across the parameter sampling area of the MMP curve. Note that in Figure 3, the MMP curve (yellow line) sags below the PDC model prediction at perhaps 35s through 1m30s, which may be an area of the MMP curve that is underrepresented with true maximal or near-maximal efforts, and consequently, tethering the model down in this case. In addition, note the flatness of the MMP curve (yellow line) from 1 second through 30 seconds. This likely represents the lack of a true maximal effort in the realm of these durations, which may also impact the model. (Note: there is a short time lag in Stryd recording from a standing start - it is best to test the far left of the athlete’s power-duration curve with flying start efforts.) To help assess the accuracy of the FTP estimate, the standard error of the estimate and coefficient of variation can be calculated and reported. (Figure 3)

Figure 3. Meanmaximal power curve (MMP curve) and resultant power-duration curve (PDC) modelling, including associated PDC metrics and S.E.E and CV of mFTP

Meanmaximal power curve (runner’s actual maximal efforts for last 90 days) = yellow line

WKO4 PDC model (based on the runner’s MMP curve) = red line

In this example, mFTP is estimated to be 233W with a S.E.E. of 2.8W and CV of 1.2%.

Practical guidelines for using the WKO4 model for FTP estimate (mFTP)

For greatest accuracy, one’s MMP curve should be composed of maximal or near-maximal efforts across a wide range of durations. The PDC metrics are based on MMP actuals. If the data are not sufficiently robust across durations, the PDC will be less accurate
It is not critical whether the maximal or near-maximal efforts are generated from testing, racing, or exceptional workouts wherein a maximal or near-maximal effort was produced.
If one were to implement testing, the testing durations may be standardized, and should be distributed across a variety of durations. Alternatively, one may elect to selectively test the apparent areas where the MMP curve and PDC model are tethered down. Regardless, test efforts do not have to be completed on the same day.
A 90 day representation of one’s MMP curve is best for determining mFTP, except perhaps in a period of rapid decline in metabolic fitness (eg - after injury or illness).
Caution: the MMP curve can be corrupted by data spikes. The data spikes, may, in turn, corrupt the PDC model estimation of FTP. These spikes generally will be found to impact the far left side of the MMP curve.
Caution: as data from runs expire outside of the 90 day window, they may carry meanmaximal performances that were supporting the PDC model. Consequently, the estimated mFTP may change due to data expiration from the sampling window.
Caution: changes in PDC metrics other than mFTP can and will have an impact on the mFTP estimation. Changes in maximal power in one duration area of the one’s MMP curve will have a ripple effect on other PDC model duration areas. A classic example is that if the FRC* value increases, mFTP estimation typically drops.

*(FRC = PDC metric Function Reserve Capacity - similar to anaerobic capacity - or the extent to which, in kJ, one can exert above mFTP before fatiguing)

Caution: if the athlete is continually emphasizing one limited area of the PDC (eg 8 minute intervals), other areas of the MMP curve may relatively lag in improvement, and consequently, the PDC will be 'tethered down' by the relative underdeveloped areas, masking the improvements made in the focused area and their effect on mFTP.
The standard error of the estimation and coefficient of variation of the mFTP estimate can be assessed. Think of FTP lying in the range of mFTP +/- S.E.E.

Protocols - the Critical Power model

Stryd Powercenter critical power prototcols

The Stryd Powercenter critical power model uses two parameters. Testing involves performing two maximal effort tests in one test session. Resultant values from each of the two tests performed in the session are then input into the respective Stryd Powercenter critical power calculator to arrive at a critical power estimation.

Option 1: 3 lap and 6 lap tests performed in one test session on a 400m track

1. Warm up for 5 minutes. Do two to three 100-meter strides at approximately 80% maximum effort during warm up to enhance the blood circulation and ready your muscle for intense use.

2. 800 meters Easy-pace run. Two laps on a 400-meter track, please use the innermost lane. Run at an easy pace, such that you can comfortably maintain conversation.

3. Warm up for another 5 minutes.

4. 1200 meters Maximum-effort run. Run at a consistent pace throughout the test, but so that you are nearly exhausted at the end of the test.

5. Recovery for 30 minutes. Throughout the 30-minute recovery period, the runner should walk or jog slowly.

6. 2400 meters Maximum-effort run. As was the case for the three-lap run, it is important to maintain a consistent pace during this run instead of dramatically changing pace (and effort) during the run.

7. Cool down.

Option 2: 3 minute and 9 minute tests performed in one test session

1. Warm up for 10 minutes. Do five 100-meter strides at approximately 80% maximum effort during warm up to enhance the blood circulation and ready your muscle for intense use.

2. Maximum distance run for 3 minutes. Run at a consistent pace throughout the three-minute test, but to be nearly exhausted at the end of the test.

3. Recovery for 30 minutes. Throughout the 30-minute recovery period, the runner should walk or jog slowly.

4. Maximum distance run for 9 minutes. Again, maintain a consistent pace during this run instead of dramatically changing pace (and effort) during the run.

5. Cool down.

Independent critical power protocols

One can create their own critical power protocol, independent of the Stryd critical power model. It may be, for example, that an ultrarunner is not able to execute solid maximal efforts at 3 and 6 laps or 3 and 9 minutes. Perhaps the runner would like to use maximal effort tests of 2 minutes and 20 minutes. Also, the athlete may want to test on similar terrain to which they train or race (for example, trail runners). In these cases, the runner could design their own protocol and input the values into an online critical power calculator. The CP Calculator - Monod Critical Power Calculator from Cycling Power Lab . com will take 2, or even 3, test results, ranging from 1 to 60 minutes. Alternatively, the CP Calculator - Cycling Zone Calculator from BeginnerTriathlete.com will take 2 test results, with the shorter test being 3m00s-5m59s, and the longer test being 12m00s-20m59s. Keep in mind, when designing your own protocol, the larger the difference in duration between the short duration test and the longer duration test, the more reliable the estimate (as long as both tests are maximal).

Practical guidelines for using the Critical Power model for FTP estimate

For more valid comparison, use the same estimation protocol and model over time.
Maximal efforts are necessary for each test duration.
The rest (taper) leading up to the test date should be similar each time.
The rest between each test effort should allow for complete recovery and be very similar each time
Cherry picking best efforts from separate dates is not advised, since the practice is likely to produce an invalid estimate.
The venue or course should, ideally, be the same over time/test sessions.
It is best to test and use values generated under very similar environmental conditions (wind, temperature, humidity) to which you will be training.
Make sure the the footpod is on the same foot, and relatively the same location each time. A CP estimate in raw Watts is only specific to the weight setting that was present on the pod at the time of the test session.
If using a CP protocol other than the Stryd protocols, make sure that the tests are sufficiently different in terms of duration. The short duration effort should be at least a minute long. The longer duration can be extended to 20 minutes. The larger the difference in duration between the short duration test and long duration test, the more reliable the estimate (as long as both tests are maximal). When the longer test is < 12 minutes, the risk of overestimation is higher.
Realize that the CP estimation of FTP is applicable for a limited period of time (perhaps 6 weeks +/- 3 weeks, depending on which training phase the athlete is in). As fitness changes, re-testing and re-estimation is warranted.

Protocols - estimating FTP using a single parameter model (using modified Riegel formula)

It is possible to arrive at a reasonable FTP estimate from a single parameter input model. However, as stated previously the potential for error may be higher than with properly used multiple parameter input models.

The Riegel formula serves as a useful model for estimation of FTP. Modification of the formula yields the following equation:

estimated FTP = (known power) * (predicted duration/known duration)^exponent

The issues with this formula for FTP estimation are:

The “predicted duration” function in the equation.
The actual “exponent” to be used for the given athlete.

For the “predicted duration” function, I somewhat arbitrarily recommend using 50 minutes. Fifty minutes aligns fairly well with what the Stryd folks say is the approximate TTE (time to exhaustion) for their CP value. Fifty minutes also lies within the ranges cited for TTE for both FTP and CP. Lastly, it likely lies much closer to most runners’ TTE for FTP than does 60 minutes. That said, in practice, I typically will bracket my estimates for FTP using this method by using “predicted duration” values of 45 and 50 minutes. Admittedly, using a standard “predicted duration” of 50 minutes as TTE is akin to a broken clock being right twice a day. However, it is likely that this “predicted duration” produces a reasonable FTP estimation under the circumstances.

The exponent for the equation is tied to what Riegel referred to as the fatigue factor. The exponent used in the equation may vary, depending on; a) aerobic fitness level and stamina, b) gender, c) age, and d) perhaps even anaerobic capacity. The actual “exponent” to be used is likely unique to the athlete for any given point in their training cycle.

Most attribute an exponent of -0.06 to Riegel’s formula. This is the exponent used in many online prediction calculators.
Van Dijk and Van Megen, in their book The Secret of Running, recommend using an exponent of -0.07 for “most runners”. They recommend -0.06 for a runner with very good fatigue resistance. They demonstrate in their book, The Secret of Running, that the data points of both men’s and women’s world records from 5k to marathon fit the regression line with an exponent of -0.07.
Lower aerobic fitness level and stamina, female gender, age >50, and perhaps high anaerobic capacity may prompt the use of a higher exponent, such as -0.08. Some have even suggested a higher exponent when predicting marathon from shorter efforts.
I will typically use an exponent of -0.06 in higher caliber runners, but have found -0.05 to best fit the races of an emerging elite female runner.
Ron George has found an even lower exponent for elite female and male runners.
It is possible to calculate your own personal best-fit exponent with races over various distances, as Andrew Burke has done (his calculator is forthcoming), and as Ron George did for elite athletes using established world records.

Examples

The female athlete, whose WKO4 mFTP development chart is depicted in Figure 1, ran 34:51 for 10,000m at an average power of 242W. The modified Riegel formula, using an exponent of -0.05, produces a 50 minute FTP estimate of 238W, and a 45 minute FTP estimate of 239W. Her WKO4 mFTP estimate after the race was 241W.
The same athlete ran 17:07 for 5,000m at an average power of 243W. The modified Riegel formula, using an exponent of -0.05, produces a 50 minute FTP estimate of 230W, and a 45 minute FTP estimate of 232W. Her WKO4 mFTP estimate after the race was 229W.
The same athlete ran 58:55 for 10 miles at an average power of 232W. The modified Riegel formula, using an exponent of -0.05, produces a 50 minute FTP estimate of 234W, and a 45 minute FTP estimate of 235W. Her WKO4 mFTP estimate after the race was 236W.

Practical guidelines for using the modified Riegel model for FTP estimate

The model requires a maximal effort - ideally a race performance.
The model works best when using known duration and power in races from 5k to half marathon. The model loses accuracy when using races with a higher anaerobic contribution, for example, <5k. If using the marathon, the exponent may have to be adjusted upward, depending on the stamina/ endurance of the athlete.
It is best to apply this model to races or maximal efforts generated under very similar environmental conditions found in training (altitude, temperature, humidity).
The “predicted duration” function of the model is likely best when arbitrarily set at 45-50 minutes, and perhaps bracketing your estimate is warranted.
The selection of an applicable exponent is unique to the athlete for any given point in their training cycle. Sometimes, bracketing by exponent may also be helpful.
This method is likely superior to assuming that 10K time = FTP. 10k power may happen to be correct for some athletes, and close for others, but it could be unacceptably off in others.
Note: this method does not need a distance input, only a duration input. For example, it matters not that maximal power over 42:15 duration was achieved in a 12K race or 10K race or 8K race. It’s all about power-duration.
This method is likely superior to using an assumption that 95% of 20 minute power = FTP. Or for that matter, assuming that a population-based fixed percentage of any duration = FTP. While the percentage may be correct for some athletes, and close for others, it could be unacceptably off in others.
Note: this method may be used effectively with an all out time trial of a set duration (for example, a 20 minute test).
If using a race and the single parameter model, the issue of choosing a race vs formal two parameter CP test lies more in how the race or test fits into a training program, and how much residual fatigue one is willing to introduce at a particular time. For this reason, I typically advise 5k and 10k races in this model as surrogates for formal CP testing. That said, the model does work quite well between 5K and half marathon distances (perhaps even to marathon distance).
The possible margin or error in using a one parameter model may be offset relative to formal CP testing in that racing is more likely to achieve a truer maximum effort.
Realize that this estimation of FTP is applicable for a limited period of time (perhaps 6 weeks +/- 3 weeks, depending on which training phase the athlete is in). As fitness changes, re-testing and re-estimation is warranted.
Make sure that your running powermeter is as accurate as possible. For Stryd, this means making sure that your weight is correctly set. Of lesser importance, but still worth attending to, is making sure the the footpod is on the same foot, and relatively the same location each time.

Other FTP Estimation Protocols

There are other FTP estimation protocols that have been proposed for runners. For completeness, I’ll cover them here. For various reasons, I do not use any of them.

Run with Power 30 minute time trial method

(Jim Vance, Run with Power, p60-61, 2016)

Warm up 15 minutes, preparing for a hard effort afterward.
Start a 30-minute time trial (best effort) on a flat road or track, collecting power data (collect pace and HR data as well, if possible).
Cool down 10 to 15 minutes.
Take the average power for the last 20 minutes of the time trial; this is your rFTPw

Note: This methodology, on the surface, seems that it could overestimate FTP - particularly in faster runners.

Run with Power 3+9 minute test method

(Jim Vance, Run with Power, p57-60, 2016)

Warm up 15 minutes, preparing for a hard effort at the end of it.
Conduct a 3-minute interval at maximal effort.
Recover with a 5-minute walk, 10-minute very easy jog, 5-minute walk, 5-minute easy jog, and 5-minute walk again (30 minutes total).
Conduct a 9-minute interval at maximal effort.
Cool down 10 to 15 minutes.
Add the 3-minute average power value to the 9-minute average power value.
Divide that total by 2.
Take 90 percent of the quotient; that is your estimated rFTPw value.

Note: For a number of reasons, this methodology is likely more prone to a larger margin of error than any of the primary three methods previously discussed.

The Secret of Running 10 minute time trial method

(Hans Van Dijk and Ron Van Megen, The Secret of Running, p118, 2017)

Perform all out time trial test for 10 minutes.
FTP is equal to 88% of 10 minute power.

Note: This methodology assumes a) that FTP is equal to 60 minute power, and b) a Riegel exponent of -0.07. These assumptions may lead to an FTP estimate that errs low.

The Secret of Running 20 minute time trial method

(Hans Van Dijk and Ron Van Megen, The Secret of Running, p118, 2017)

Perform all out time trial test for 20 minutes.
FTP is equal to 93% of 20 minute power.

Note: This methodology assumes a) that FTP is equal to 60 minute power, and b) a Riegel exponent of -0.07. These assumptions may lead to an FTP estimate that errs low.

Stryd Power Center Race Time conversion to FTP

Input a time for a recent 5K or 10K race, and the Powercenter calculator computes your CP

Note: Speed = Power * Running Effectiveness. The calculator only calls for entry of a race distance and time (thus speed is the basis of computations). Since this method does not call for the runner to input their Running Effectiveness, the calculator, no doubt, assumes a standard Running Effectiveness (RE). The calculator then assumes that all runners with the same race speed (or distance/time) must produce the same power (in W/kg) to produce that speed. A runner with a 5k time of 20:00 and RE of 1.04 is likely to be computed to have the same race power, and consequently, the same FTP as a runner with a 5k time of 20:00 and RE of 0.99. This assumption means that the calculator result for CP errs high for more “effective” (higher RE) runners and errs low for less “effective” runners (low RE). The percentage of error can be as much as 5% in some runners at the extremes of RE. Further, it is unknown whether the calculator converts to FTP as 60 minute power, or some other duration for FTP.

The Secret of Running Race Time conversion to FTP

Utilize a table in the book (Hans Van Dijk and Ron Van Megen, The Secret of Running, p132-133, 2017)

Look up your time in a recent race and determine relative FTP

Alternatively, use The Secret of Running online calculator

Enter your time in a recent race and determine relative FTP

Note: The Secret of Running uses the metric Energy Cost of Running (ECOR) rather than Running Effectiveness (RE). ECOR is the inverse of RE. Thus Speed = Power / ECOR. The Secret or Running uses a default ECOR of 0.98 in their calculations (the same as a RE of 1.02). The calculator only calls for entry of a race distance and time (thus speed is the basis of computations). Since this method does not call for the runner to input their own ECOR, the calculator, no doubt, assumes a standard ECOR of 0.98. Further, based on early conversions reported on page 118 in the book, it is likely that the conversions are to an assumed 60 minute FTP. The percentage of error can be as much as 5% in some runners at the extremes of ECOR.

The Secret of Running alternatives do allow for the input the time result of a greater variety of races distances than does the Stryd Powercenter calculator, which allows only 5K and 10K inputs at this time.

Matt Fitzgerald 30 minute time trial

(from Intensity Guidelines for 80/20 Running, by Matt Fitzgerald and David Warden)

“Warmup for 15 minutes with brief sprints of 10 seconds at estimated threshold power.”
“Then, perform a 30-minute time trial.”
“Your average power for that 30 minutes is your running threshold power, or rFTP.”
FTP = power from all out 30 minute time trial

Note: This method will likely overestimate your FTP. It would be better to apply the Riegel single parameter model for less potential error.

Matt Fitzgerald 20 minute time trial

(from Intensity Guidelines for 80/20 Running, by Matt Fitzgerald and David Warden)

“Perform a 20-minute test”
“Take 95 percent of that value to determine rFTP”
FTP = 95% of power from all out 20 minute time trial

Note: This is one of the most common mis-applied assumptions in estimating FTP. It will be spot on for a few runners, close for more runners, but associated with too much margin of error for many. Again, it would be better to apply the Riegel single parameter model for less potential error.

Other tidbits from Intensity Guidelines for 80/20 Running, by Matt Fitzgerald and David Warden

They also link to Jim Vance’s 3+9 minute protocol (presented above)
They also present the Stryd CP 3 lap and 6 lap CP test protocol, but go on to write that “Your rFTP is calculated as follows: (6 lap power x 6 lap time) – (3 lap power x 3 lap time) / (6 lap time – 3 lap time)”

Note: I am unsure whether this is the same formula that the Stryd folks have built into their Powercenter CP calculator.

l actate threshold testing with concurrent powermeter use

The determination of lactate threshold, and associated power at LT, may be somewhat protocol-dependent.
Maintaining a consistent protocol is necessary for test to test comparisons.
If the ramp protocol calls for change of grade on a treadmill, then for Stryd, the grade change must be concurrently be input into the Stryd app during the test protocol.
The financial cost of repeating this testing as frequently as may be needed in some cases may be prohibitive for some.
The convenience of repeating this testing as frequently as may be needed in some cases may be prohibitive for some.

How do I know my FTP is reasonably estimated?

The RPE for various intensities is perhaps the biggest tell.

While running easy, seeing power consistently exceed the upper level of Zone 1c (easy aerobic running) could mean that FTP is underestimated (or, on the other hand, the runner is simply running too hard when they are supposed to be going easy).
If long intervals near 100% FTP are consistently felt to be too hard to complete, then perhaps FTP is overestimated (or alternatively, perhaps the runner is in an overtrained state).
If long intervals near 100% FTP are consistently felt to be too easy, then perhaps FTP is underestimated, or has improved due to training.

If the resultant % of FTP for various race distances is off.

A % of FTP for a race that is below expected may indicate that FTP is set too high, and above expected may indicate that FTP is set too low.
If a runner that typically races 5k races at 105-107% of FTP runs a 5k race at 109% of FTP, their FTP is likely underestimated at that point.
If a runner that typically races 10k races at 102-104% of FTP runs a 10k race at 100% of FTP, their FTP is either overestimated (or alternatively, they did not run the race at full effort).

Alex Simmons mentions this, and it holds true for running as well: if a high CTL ramp rate is easily maintained in a power-based Chronic Training Load (CTL) model, then it is possible that FTP is underestimated.
Comparison of FTP estimates arrived at from differing methodologies may not be valid - especially when following FTP over time. Be suspicious of incongruous FTP estimations from different methodologies. On the other hand, reasonably congruous FTP estimations from different methodologies might be interpreted as useful confirmatory cross-referencing data points.
Be suspicious of FTP estimates from effort(s) that are not maximal.

Using the Riegel single parameter model, a less than maximal effort will yield an underestimated FTP.
Using a two parameter CP model, a less than maximal effort on the shorter test duration will result in an overestimated CP, and a less than maximal effort on the longer test duration will result in an underestimated CP.
Using the WKO4 power-duration model, if there is insufficient representation of maximal or near-maximal efforts across a variety of durations, the model will carry a higher margin of error.

Be careful with FTP estimations across periods of weight change. It is best to compare W/kg results during these periods.

For example, a runner estimates FTP of 300W while at 70kg, but then 2 months later, estimates FTP at 290W while at 67kg. The first estimate yields a FTP of 4.29 W/kg, while the second estimate yields 4.33 W/kg. If using just raw Watts during this period of weight change, the conclusion might have been that FTP dropped (and in raw terms and applications, it did). However, when W/kg are used, the interpretation is that the runner’s relative FTP (and metabolic fitness) may have actually improved slightly.

Summary

FTP is one of the key metrics in running with power. It has the significant advantage of being something that every runner with a running powermeter can estimate from field data. The estimate represents the highest power that a runner can maintain in a quasi-steady state without fatiguing, where the duration may range from 30-70 minutes, depending on the individual. The FTP estimate correlates with power at MLSS.

Most runners execute races from 5K to marathon at FTP +/- 10%. FTP is one of the most foundational metrics in running performance. Monitoring FTP over time can be as important as an indicator of metabolic fitness as race results themselves.

Besides enabling the monitoring of metabolic fitness, FTP can be used for training guidance and establishment of personalized training zones. Further, FTP can be used in race planning and “pacing”, intensity scaling, and training load modeling.

In using FTP, it is important to understand and correctly apply the various models and protocols for estimating FTP. These have been presented here for reader reference. Avoiding the pitfalls and misconceptions of FTP estimation is essential to arriving at and maintaining good FTP estimations over time.