Here’s another post in our series Ask Chad.  This question came in through e-mail.

I see you have both an 8min test and a 20min test to estimate FTP. Can you discuss the differences, how we should determine which one we should use and how often to re-test?

From my experience, even taking 95% of a 20min average is probably higher than what I can really hold for 60 min (at least on my KK Road Machine). I am guessing that using an 8 min test would even be further off, depending on what calculation you are using to determine FTP from the 8min Test.

Here’s Chad’s response.

The 2x8min format is more appropriate for riders who know how to really push themselves because it’s basically a measure of power at VO2max. And as the more experienced riders know, working at or very close to VO2max requires great focus and a willingness to endure a high level of suffering for 8 minutes, twice. The 20min format ratchets the intensity down a bit, but the challenge of properly pacing over a 20min effort tends to skew many riders’ final number.

Both formats are valid though, and the efforts – and consequently the data derived from the efforts – improve with time with respect to how accurately they assess maximal steady state power; practice make perfect.

I like my riders to re-test every 4 to 6 weeks and this depends on the time of training season. During the Base phases, there’s no need to reassess and adjust values any more frequently than every 6 weeks, but as the season progresses and the training intensity climbs, it behooves data-inclined riders to reassess more frequently, as often as every 4 weeks, in order to stay on top of their improvement and modify their target power figures as their capabilities increase.

And if you’ll indulge me, I’d also like to briefly address the misconception of estimated hour-power because this same notion plagued my understanding of assessment for quite some time. The concept is that we perform our assessment effort(s) and then derive a best-guess threshold value – HR, watts, or RPE – based on these sub-hour efforts. We then modify this number and voila, the theoretical output we should be able to sustain for a solid hour. But how many riders can actually ride at that power output for an hour, indoors or outdoors?

The answer is, the riders who have trained specifically and consistently in order to develop the muscular endurance and mental fortitude to endure a solid hour’s effort – translated, very few. This is not something that comes easily and many riders will never train with this intent, rather we assess this estimated sustainable output level to find a point where our power (and lactate levels) go from manageable to unmanageable, over the span of roughly one hour.

So while you might not be able to actually sustain this type of power for 60 minutes – give or take – as long as the format is consistent, both the 2x8min and 20min formats are still useful ways to quantify and track improvement, which is the real heart of the matter.


** Our default testing format to estimate an athlete’s FTP was changed to the TrainerRoad Ramp Test since this post was published