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Monday, August 6, 2012

Bike Fit 102: Elvis the Pelvis

The pelvis is not typically addressed directly by the vast majority of bike fitters, and that's not much of a surprise.  The pelvis and all its parts (yes, each side of the pelvis is made up of at least three distinct bones, and that's not including the sacrum or tailbone) is a difficult-to-visualize structure that nevertheless provides our main base of support on the bike.

Some important features of the pelvis are the acetabulae or hip sockets and the location of these is usually poorly understood.  When you mention "hip" to most people they typically point to the bone on the outside of the thigh/buttock area, but this is actually the greater trochanter -- merely an exterior bony landmark.  The actual hip joint exist a few inches further in toward the center line of the body from this landmark (and above it as well, so when palpating the trochanter of the hip think "in and up" and you'll have an idea where  the hip joint actually lies).

The perineum is also extremely important in bike fitting because this is usually the sensitive tissue that gets very irritated with a poorly fit bike.  Whatever you want to call it -- taint, under-carriage, naughty bits -- it's important to understand how and why it comes into contact with the saddle in different fit postures.  In my experience, the strength of a rider's pelvic floor muscles (muscles that form a sling or hammock on the underside of the pelvis) combined with their pelvic posture and mobility will go a long way toward determining what positions on the bike are feasible and which are not.

The sacrum is the tail (bone) end of the spine and it rests pinched between the ilium and so the joint on each side of the tailbone where they articulate is called the sacroiliac (SI) joint.  The SI joint is a common source of back pain, and as I've mentioned in previous posts (here and here) the small amount of movement (or lack there of) at the SI joint can cause some deviations in the pedal stroke from one side to the other.

Another important note is that the hamstrings attach to the ischial tuberosities (the part of the pelvis you actually sit on -- which is why they're also called sit bones),so the relative strength and mobility of the hammies can affect how the pelvis moves as well.

All of the features above (and many more) all contribute to what I call the "effective pelvic position" (EPP).  The EPP is the tilt that an individual's pelvis "wants" to perch on the saddle.  It can sit at a different angle than this but then aberrant forces will be put through the pelvis creating a disconnect between the upper chain (the spine) and the lower chain (the hip joint and femur).

A couple very common examples:

I may see an older male rider that has tight hamstrings and decreasing lumbar and SI flexibility and in order to effectively fit them I'll have to get their pelvis tilted forward only about 10-15 degrees from vertical -- fairly upright.

Conversely, a very fit young rider, even one with  moderately restricted hamstrings but excellent lumbar mobility (and very strong pelvic floor and transverse abdominal muscles) may be most efficient with an EPP closer to 25-30 degrees from vertical.

The ultimate goal, as with all of bike fitting is to get the rider balanced on the bike as best as possible -- and this is usually evidenced by a smooth pedal stroke, a decrease in lateral hip and knee deviations, and better engagement of the abdominal muscles, along with more comfort, power, and fewer aches and pains.

On rare occasions we will see someone with a fully vertical EPP or even tilted posteriorly (backwards).

Why is any of this important?

In my experience, a rider's pelvic posture is one of the biggest determinants of their handlebar position.  This is an enormous generalization, but the more anteriorly (forward) tilted pelvic postures match with longer and lower bar positions.

When the pelvis is tilted more forward then the lumbar spine is directed towards the handlebars more than with a vertical EPP which angles the spine more toward the sky.  When the spine is directed more vertically, then in order to reach a longer and/or lower handlebar position the rest of the spine needs to be exceptionally flexible to flex forward and complete this posture.  If it's not, then the motion will inevitably come from the shoulders and shoulder blades in the all too familiar super-stretched position:

Obviously an exaggeration but it goes to my point - clearly the pelvis can't roll forward any more (even though it's only tilted about 35 degrees or so) and so the extra distance is compensated by an enormous extension through the shoulders.

There are certainly times when a fit, strong rider will have a fairly vertical EPP but still will tolerate an aggressive long and low bar position.  As I mentioned before, they'd need to have a lot of abdominal stability as well as flexibility in their spine, but it certainly is possible and sustainable for a few riders.

Once we understand the EPP idea then we can address probably the most under-rated aspect of bike fit -- saddle tilt.  Saddle tilt plays, I think, the biggest roll in assuring the rider is balanced on the bike.  If we start with an understanding of the right saddle height (for more read here), we get the tilt and the fore-aft of the saddle right then we can be half way to a stellar bike fit.

My simplest explanation of how to adjust this tilt (and fore-aft) -- basically you need to know where and how the pelvis wants or needs to rest and then move the saddle under it to that position.  I know it sounds a little abstract, but once you have a good understanding of the pelvis you can make this happen.

Happy Pedaling.

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