Trouble pinning down your FTP?….this could be why
Over on the intervals.icu forum (https://forum.intervals.icu/t/inaccurate-power-meter-or-anaerobic-contribution/1062) Benjamin_Turner asks:
“I was l aid off at the start of the pandemic so I’ve had plenty of time to follow my trainerroad training plan. This has been great FTP is moving up in leaps [but my FTP is variable in various tests and hard to pin down]….was it a power meter issue or this anaerobic contribution I keep hearing about or something else entirely?
Normal Daily Fluctuation
Think of your FTP akin to your body weight. You are measuring multiple times over one month on multiple scales and getting a variety of results clustering around 279. Your FTP just like your body weight actually varies every day, every week, every month according to multiple internal and external factors. It is not unusual for body weight to fluctuate as much as 2–3% per day!
In fact, even a variable that we consider fixed: your height can vary up to 1% morning vs night (https://pubmed.ncbi.nlm.nih.gov/11293727/)! Owing to gravity, our body shrinks by about 1″ throughout the day and gets back to normal every night.
The reason behind it is the elasticity and malleability of our spine discs. Over the course of the day, when we stand or sit, pressure and gravity compress the discs between our vertebrae by about 1%. Conversely, at night, our body expands and gets back to its normal height once we are stretched out between the sheets with the pressure off the spine. Cellular re-hydration occurs overnight without the need to fight gravity. One study study shows the height loss between 7 am and 7 pm of minimum 0.3″ (0.7 cm) and maximum 1.1″ (2.7 cm).
According to this study, changing posture and body position causes fluctuations in measurements up to 2.8″ (7 cm).
These include the following body measurements:
Waist girth varies by 4.04%
Abdominal girth varies by 6.96%
Hip girth varies by 3.86%
Thigh girth varies by 4.05%
Leg length varies by 0.86%
Daily Fluctuation in Physiology
A related factor is that your body is subject to a large number of physiological changes which occur typically morning and night (aka diurnal). Examples include body temperature, cortisol (and other stress hormones) and melatonin. This is part of a system that has evolved through the animal kingdom to deal with the effect of the sun on our bodies (circadian rhythms).
Daily Fluctuation in FTP
Considering the above circadian and gravity dependent daily variations, I would say it is not unreasonable to expect your FTP to fluctuate daily by the same % as weight (and of course even more following a hard ride the previous day). It will definitely fluctuate more than your height because FTP is a performance variable whereas height is an anthropomorphic variable.
Here is a real example back on FTP
Take a look at this chart published on ICU (http://fft.tips/icu):
Notice that body weight is fluctuating every month by several kg. It is actually fluctuating every day but this regular ICU user (https://twitter.com/nettietinker) (ooops is was not nettietinker my bad!) is measuring weight maybe every few days, or every week. In addition, her FTP (actually eFTP but it’s the same principle) is varying dynamically too. ICU is smoothing the variation but the point is, FTP is not static its completely continuous.
Therefore combining weight and FTP means your w/kg is subject to a double fluctuation!
When is your FTP high and when is it low?
One of the main effects here is your distant and recent riding history. If you are in a strong build phase your daily FTP is likely to be suppressed (meaning fatigue is high…in a good way…..aka your TSB is negative) but within an upward trend. If you are in a taper your FTP is likely to be peaking, providing the taper is short (excessive tapering is a common mistake in train for race day). If you are in a post-peak or injured your FTP is probably trending down (hopefully not as rapidly as mine!).
Power Meter Precision
Do you notice how every manufacturer is fighting over their power meter accuracy claims. It was 5%, then 3%, now they are claiming 2%. Remember these are generally unsubstantiated claims! But I did find one big study (Maie et al) that examined 54 power meters ( 23 different models) from nine manufacturers, including SRM, PowerTap, Quarq, Stages, Verve, power2max, Garmin, Polar and Rotor. The mean deviations of the power meters were –0.9±3.2% with 6 power meters deviating by more than±5%.
There is no one single FTP
So thinking about these factors then there is really no precise fixed FTP figure just what I would call the “best current estimate”.
If you change test conditions (eg ramp vs 8×2 vs 20min vs 60mn vs zwift race estimate) they would all give a slightly different figure even if you could test on the same day and remain fresh. The main purpose of having your FTP is to plan appropriate training loads. I think within 2% is reasonable for finding the correct zones. This is because your power meter varies by at least 2%, your daily fluctuation is >2% and the *zones themselves are a human construct* of a categorical system imposed on a completely continuous physiological variable which could also be a 2% error. Coggan and Allen admit it themselves here: https://www.velonews.com/training/fast-talk-ep-72-do-we-need-training-zones-with-dr-andy-coggan-dr-stephen-mcgregor-and-hunter-allen/.
Ultimately, I said to Benjamin_Turner. you are progressing, which is great, and you have a very good idea of your best estimate of FTP which is also great! Keep it up. Happy training!
Accuracy of cycling power meters against a mathematical model of treadmill cycling by Thomas Maier, Lucas Schmid, Beat Müller, Thomas Steiner and Jon Peter Wehrlin, published in the International Journal of Sports Medicine.