Why Power Based Bike Fitting Doesn’t Work

Are you considering working with a bike fitter who guarantees that they can quantifiably show you an increase in your power from their fitting?  It is not uncommon that I hear people say their fitter promised, and then showed them, a quantifiable 20 to 50 watts of power increase.  The reality is that bike fitters can’t honestly quantify increased power in a normal fitting session.  While a proper bike fitting can certainly improve power output for many riders, quantifying results in real-time is challenging at best.

Here’s why:

Why it doesn’t work when working with a well intentioned, but misinformed, fitter: 

Let’s assume your bike fitter is honest and not just using “smoke and mirrors” behind the scenes to “create” a power increase; they are truly trying to assess and measure power improvements through position changes.  We’ll also assume that as you move through the fitting, things start to feel better and you are actually riding with 20 more watts of power than where you started.  If that 20 watts did not come from positioning changes, where did it come from?

  • You might just be pushing harder on the pedals.  It sounds simple, but if your legs and body warming up or the adrenaline of ‘wanting’ the new position to produce more power have not been accounted for, you’ll likely see a nice power increase from where you started regardless. We’ve done power based assessments with riders in the same position in five minute intervals over a sixty minute period and seen over 20 watts of variability between intervals at the same heart rate.  Heart rate simply is not that consistent.
  • Heart rate (and even VO2) is a “noisy” variable.  On paper, if the rider can produce more power at the same heart rate, or find his/her heart rate dropping at the same power output, then we can make a case that we’re improving the power output.  Measuring heart rate response during a bike fitting is tricky at best though.  If you own a power meter and heart rate monitor, try riding a set of intervals on the trainer at the same power output and analyze your heart rate response from the power file.  Most riders will find that their max and average heart rates are different on each interval – not very different, but different enough to confuse things. These slight differences and uncertainties in measured data are “noise”.  Heart rate is generally a “noisy” response as it is dependent on factors ranging from adrenaline, cadence, how many intervals you’ve ridden, to what you are thinking as you pass the time during the workout.  I know it is not uncommon to find myself riding on the trainer, watching a cycling DVD and trying to maintain a certain power, only to find my power unintentionally 30 watts higher during an exciting sprint finish.  Measurements get even noisier when you consider stopping, making changes to the bike, the rider continually having to warm-up again, and a different starting heart rate for each cycling bout.
  • Existing muscle dominance patterns.  How can you account for allowing your body/muscles to adapt and develop to the new position over time?  We often build positions that we know are biomechanically neutral, balanced and safe, but may initially result in the rider losing a little power.  Why?  The power will come back (and then some…) in the future once the muscles develop and balance out.  If a higher heart rate based power number steers you away from a position change that your body would have adapted over time, you are simply chasing existing muscle dominance.
  • There is a difference between sustainable/Functional Threshold power and short-term power gains.  For example, when you stand on your bike, you usually increase your power.  However, if you try to maintain that standing position for an entire ride, your power is not sustainable.   This is similar to what happens in a car; when you drive faster mph, your mpg drops simultaneously.

In a bike fitting, you can favor muscle groups that could increase short-term power, but that can actually compromise long-term/sustainable power in the process.  The goal of our fittings is functional sustainable power coming from a body set-up in a biomechanically neutral and balanced position.  You do not want to do something that may increase power now, but will put the rider in jeopardy for increased joint and muscle based issues years down the road.  How much power are you going to produce if your back is always giving you problems?

Even when used with the best of intentions, power measurement in a bike fitting just isn’t that useful a piece of information.  For the reasons above, power based fitting can even be misleading.

How power can be used as “smoke and mirrors”:

So, what happens if your bike fitter is trying to pull the wool over your eyes.  Sadly, it does happen.  Perhaps they feel they need to justify their results or possibly give you a watts improvement that you can itemize in a customer testimonial or online review.   How do they do it?  That is a good question.

Most fitting bikes are attached to constant load power meters.  Regardless of the position changes made, if the rider is spinning at 90 rpm, the power output will remain constant.  In order to increase power, the cadence will have to increase or somebody will have to add resistance to the system at a given cadence.  Adding resistance can typically be done through the computer, shifting gears on the bike, or manually adjusting trainer load/rolling resistance calibration in the machine.   If your cadence remains constant, and you haven’t shifted a gear, and you magically start to pedal at 20 watts higher, then you should start asking questions.  I’ve heard stories of changes being made in a fitting and suddenly riders see that 20 watt increase, followed by an adjustment back to their “old” position and seeing the 20 watts disappear.  If you ever experience this keep a close eye on the hands of the person fitting you and make sure they aren’t adjusting the displayed power through the computer as there are a number of ways to do that. This represents the worst of the worst things you could experience in a bike fitting, but it’s unbelievable how many stories I’ve heard of this happening and how it doesn’t translate outside…

As I said before, it’s common to see a power improvement as a result of a bike fitting. That’s one of the reasons many cyclists and triathletes come seek out a bike fitting in the first place – to make sure they’re in position to produce power as efficiently as possible.  While some riders will experience an immediate power increase after a fitting, accurately quantifying increases in sustainable power is inconsistent at best; when it comes to power, the human body is much too complicated for generalities.  This is why we focus on maximizing sustainable power through encouraging optimized muscle firing patterns in our fittings and not absolute power.  Be highly skeptical of any fitting methodology or promises involving power based assessment.  The goal of a quality bike fitting should be maximizing functional sustainable power, not quantifying potentially temporary power gains.

About Dean Phillips:   Through self-implemented power based aerodynamic field testing, Dean gained 3 mph in speed without increasing his power output over a three year period.  Dean is a co-owner and the lead fitter of Fit Werx in Peabody, MA.  He is also a former pro triathlete and holds multiple TT records, many of which had been held by some of the fastest pro riders in the world.  Dean knows how power assessment works and is happy to answer questions on fitting and power from a scientific and research based approach. 

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