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Monday, February 16, 2009

Bike Fit: change the bike or the body?

I am writing this to elucidate further my thoughts on bike fitting and how it can successfully be accomplished.  I always learn anytime I write.

Undoubtedly, this whole post (along with my entire thought process with regard to fitting bikes) stems from my education and clinical work as a physical therapist.  

When I browse the internet or talk with bike fitters (from all over the world), most of the time I am a bit surprised that most bike fittings are approached as static events.  A rider comes in, position is assessed as "off", position is altered to make them "neutral", said rider goes off to ride their bike happily ever after.  I have just found this to not be true to how the human body acts or how cyclists progress (or digress).

First of all, we all have asymmetries.  Some greater than others.  Bicycles, if constructed well, are nearly perfectly symmetrical.  So if you try to take this asymmetric person and plop them on a symmetric bike, the ball of twine begins to unravel.  Obviously the bike doesn't move much -- a seat may become depressed on one side from increased pressure of a sit bone, for example -- but our body drapes itself on the bike any way it can to exert maximum force through the pedals for the longest period possible.  

A common demonstration of this is when a cyclist's right sit bone sits too far right of center on the saddle and it also sits further forward on the saddle than the left.  This leads to the right knee being further forward relative to our pedal, meaning this right leg "acts" longer while the left is now functionally shortened.  The body wants to keep the head facing directly forward so a counter-rotation occurs through the spine, and since the hands are forced to be in the same place the left shoulder blade has to elevate and the left shoulder is flexed to a greater degree.

The above is a common and very clean demonstration of how an asymmetric sitting position can translate up and down the body.  Unfortunately things aren't always this "clean."  At times a body part may not be oriented how you'd expect it (i.e. if both knees sat in opposite positions to those described above despite hard evidence of a right anterior and lateral sit bone position), but this is another post for another time.

So now you have assessed all these joints and how they are positioned, it's important to try to figure out where these alignment issues are coming from.  If you don't know what the root cause is for them, you can't correct them.  Many people do have structural or "fixed" deformities -- a longer right tibia, an anteverted hip, a scoliosis in the spine -- but I have found that most are quite small -- less than a quarter of all leg length discrepancies are greater than 1 cm.  

(Incidentally, the ONLY way to get a true measurement of a leg length difference is to have full length leg X-rays done and have the radiologist take measurements off that.  Anyone who looks at you in standing or takes measurements with a tape measure and then tells you with certainty you have a 2.2 cm leg length difference is blowing smoke - it can't be done; not well at least.)

More often than not, most of the deficits and asymmetries I come across are functional in nature.  This means they aren't related to underlying bony deformities.  For example, most of the leg length differences I find are related to tight hip musculature, or a tight sacro-iliac (SI) joint.  Differences of up to 2 inches can be perceived with the standard assessments, but it is clear from further investigation that the femur or tibia are not off by this large margin.  If we correct the underlying problem -- by mobilizing their SI, for instance -- often their "leg length difference" disappears.  This same phenomenon can be seen in just about any joint in the body:

- one rotator cuff can be significantly more restricted than another causing a discrepancy in how far that arm can reach in a given direction, 
-one hip flexor may be inhibited (neuromuscularly) causing a difference in step length during walking. 

The possibilities are endless.

So you might think, "what does it matter if a difference is functional or structural.  It's still a difference and should be adjusted for."  I can appreciate this line of thinking.  It does make some sense....until, you realize that a functional discrepancy can be CHANGED.  It can get better.  If you are willing to work at it a little bit then it can cease to be a problem .  

You have a functional leg length difference due to a tight psoas muscle, and this causes you to sit unequally on your bike.  You lengthen the psoas through a stretching regimen and over time you can begin to sit squarely on the seat again (and get rid of that saddle sore in the process).

The other problem with adjusting or compensating for a functional difference is that, left alone, they will get worse.  When we have a deficit, say a tight right hip flexor, it affects how we do other tasks, like while running it can increase the amount we rotate our trunk to the left to balance ourselves as we stride.  This becomes a functional deficit in trunk right rotation.  These patterns develop synchronously and they feed into one another.  Meaning, one leads to another, one worsens and leads the other to worsen.

Long story short: if you take a functional limitation and leave it alone (you don't stretch or strengthen the area appropriately) it will get worse.

This may be a good time to explain the difference between Correction and Compensation.

A correction is taking the deficit or underlying problem and working to fix or remedy it to return the body to a previous standard of function. 

A compensation is accepting a deficit and working on other avenues to work around the deficit and return the body to as close to the previous standard of function as possible. 

For example, when a right-handed person has a stroke leaving the right arm paralyzed, the rehabilitation therapist has to make a decision:  should they work on returning function to the right arm (correction) or work on teaching the left hand to do all the things the right hand did (like eat, write, drive etc.)?

So to get back to the realm of bike fitting....

Again most of the deficits that present themselves during bike fits are functional, not structural.  They can be corrected to at least some degree.

Most bike fittings done are adjusting and tweaking the bike to correct for these functional deficits, which is fine except almost no attention s payed to how the rider can affect their body to help it fit their bike better.

So most bike fits are compensating for (rather than correcting) functional deficits.  The glaring problem here is that these functional deficits are likely to get worse and the changes made to tghe bike will no longer be adequate.

Without working on our problem areas, we fit our bikes worse every year.

So, what's the answer?

I think it's clear that we need to adjust the bike to make it more comfortable during a fitting, but equal time needs to be paid to instructing the client on  how they can ensure that their bike will fit them well for years to come. 

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