What I've Learned as a Bike Fitter - #2 Tools are Just Tools
I was recently glancing through the website of another fitter and smiled a little at the subtle jab at those fitters who rely on “computerized angles” or something to that effect. He was no doubt referring to Retul, which is the dominant fitting specific motion capture system on the market (there are others, and even Shimano rolled out an impressive system a couple of years ago). I’ve come across this before from other fitters who, naturally, don’t use Retul, and have been the recipient of outright hostility for my use of a “$10,000 goniometer” or a system based on “smoke and mirrors.”
No doubt the equipment is expensive, and presents a pretty large barrier of entry to the small, independent fit specialist trying to make a go at an obscure niche industry, and I recognize the pushback as rooted partially in insecurity or “mocap envy”. Some of the criticism, however, is not without validity. For example, simply fitting a client “to the numbers” or looking at one angle measurement of one individual joint is a misuse of what is a pretty powerful tool, and motion capture does have its own set of limitations.
The root of the problem here is the use of the Retul normative ranges for the host of angles and distances that the system records. What I tell my clients when explaining what all of that gobbledygook of numbers means on the screen is that we look at these angles within the context of normative ranges - the angle range under which most cyclists tend to fall. For example, Knee Angle Extension, or the angle of the knee when a rider is at the bottommost portion of the pedal stroke has a normative value range of 35-40 degrees. This means that most people, when measured with Retul, should fall more or less within this range, and I’m usually quick to explain to a client that this merely represents a bell curve - that it represents the value for most riders, not all. It’s not prescriptive, but descriptive; it merely shows what a rider is doing and puts that number within a context. It is merely information. Information is good. Data is good, but beware of overly simplistic interpretations of data or trying to put it in a box. For one, this angle is influenced not only by the saddle position, but also the natural amount of plantarflexion of the rider. If, after a deliberate process of making positional changes and getting feedback, that number winds up being 42 degrees, then I’m perfectly happy with number. This is something I learned and adopted early on.
I’ve also developed through practice and a continually growing sample size my own set of normative ranges. For example, Retul’s normative range for back angle on a road bike position is 40-50 degrees. This is so broad and generalized as to be almost meaningless. In practice, I find that most cyclists with adequate core strength and an absence of any limiting back dysfunction are best somewhere between 40-45 degrees, and for those fortunate cyclists with a natural or developed ability to lower their torsos further with more elbow flexion often are lower than 40 degrees.
Knee Forward of Foot is another measure whose normative range I typically disregard. This is the measure of the knee marker in relation to the foot marker placed on the 5th metatarsal. It is roughly analogous the old plumb bob over the knee axle measurement, and is influenced primarily by the fore/aft adjustment of the seat or seat tube angle. The range Retul considers normative is 0 to -10 mm, with the knee falling behind the foot in the power phase of the pedals stroke. In practice, very seldomly does a client of mine fall within this range. 15mm to 20mm behind the 5tht metatarsal is pretty typical for me, and I’m also double checking with a laser to determine where the knee aligns with the pedal spindle. Saddle setback for me is determined by weight balance between the two contact points - saddle and handlebars - and the ability to produce power up through the top of the pedal stroke when in the drops. Moving the saddle back, usually, tends to shift the weight back and take weight off the hands, arms and shoulders when in the hoods. Conversely, moving the saddle forward opens up the hips when in the drops and helps with power development with a lower back angle. I try to balance both, and this is wholly dependent on the type of rider I’m fitting and his level of flexibility, weight, body proportions, riding history, core strength, the brand and style of saddle and probably some other factor that doesn’t immediately come to mind. In other words, it’s complicated, and is dependent on variables independent of the normative range for a single measure in a vacuum.
Motion capture is a powerful tool, and I still consider the tool to be indispensable to me when performing a precision fit. The analogy I like to tell clients is that Retul is to bike fitting like an MRI is to medicine. An MRI is not a treatment, but does allow a doctor to look more deeply and accurately at what is going on with his patient. Retul allows me to a look a little more deeply and globally at what the rider is actually doing on the bike, and gives me the objective data to better understand the interrelationships between, for example, knee angle and the amount of individual foot movement such as plantarflexion, or the differences in function and kinematics between the left and right sides. But I’ve learned to filter out the noise and interpret the data in my own way based on my own objectives and my own criteria.