Bike Fit 101: Proper Knee Extension on a bike

I was asked a  really good question this week.....one I look past a bit and take for granted since I do so many bike fittings.  It has to do with the amount of knee extension one should have at the bottom of their pedal stroke.

When I pointed out to this client that his knee extension on the right compared to the left side was nice and even at 37 degrees from full extension on both sides he asked, "I thought that should be between 25 and 35degrees?"

He's not wrong, especially if you look at much of what has been written about bike fitting in the last 20 years.  Most of the texts label the acceptable range at 25-35 degrees.

So where did I go wrong, when I assumed that 37 degrees was "good"?  Actually, I didn't.

The discrepancy lies in the fact that most of the books on bike fitting were written using static measuring methods -- that is stopping the cyclist mid pedal stroke and lining up a goniometer (joint angle measuring device) at their knee.  Actually, most bike fits are still done this way and that's a shame.  

When you use dynamic measuring methods, as I do with the infrared motion capture system in my Studio, you get much better data about the rider's actual pedaling posture.

What the static measuring fails to take into account is ankle and hip motion.  Because the rider has to stop their pedal stroke, it's rarely possible to replicate the correct ankle placement when measuring at the knee.  Some fitters have tried to use video to see what position the ankle sits at at the bottom of the stroke.  But really this is little more than a guess -- you're eyeballing whether it looks close to what appears on the video, and whether the client can actually hold it there is another thing altogether.  An incorrectly mocked up angle at the ankle can have a drastic effect on the knee measurement as well.

Why not measure right off the video with Dartfish software or some other program?  The short-coming here is that you end up setting a pixel on a screen at the joints, and even on a very large screen the displayed size of the rider is very small, and getting that joint marker off by even a small amount contaminates the data.  Generally the margin of error with video runs about +/-10% while with infrared motion capture it runs about +/-0.5%.

So what angle does the knee work at across most populations?  Well, roughly between 30-40 degrees...but there are numerous situations where a rider is best fitted by allowing them outside those vague lines.  That is another post entirely.

Next post we'll look at how pelvic posture and mobility affects saddle placement -- maybe even dip into how some sacro-iliac (SI) joint dysfunctions play into the fitting.

Talkin' bike seats

I mentioned in my last post that, for the average woman, and ideal seat design would be wide in the back to support the wider ischial tuberosities, but then needs to quickly narrow to avoid compressing the tissues distal and lateral to the sit bones. 

This narrower space between the femur and sit bones that we tend to rest (which ends up being the proximal hamstring -- medially the semi-membranosus and laterally the biceps femoris) is not the only reason for this saddle shape.  The woman's sit bones are oriented more in the frontal plane (more side to side) than a man's.  The male sit bones are set more in the sagittal plane (front to back).

When you factor in the natural translation of the hips and pelvis downward at the bottom of the pedal stroke, you can visualize that the male sit bones can more readily follow this path of movement -- sort of like a knife blade slicing through the dirt.  The female sit bones can't move as easily in this path -- imagine running the same knife through the dirt now turned to it's side a few degrees, like a plow.  The amount of shear force (or at least the potential for shearing) is much greater.

Essentially, all the angles of the pubic and ischial rami (the structures that form the "loops" on the bottom of the pelvis, and that we sit on) are steeper and sharper and because of this, less contact with saddles is probable.  I think this is the reason women often struggle with saddles -- more contact and shear forces -- and not just the fact that they have wider sit bones.

Saddle position

Of course, the right saddle is nothing without it being fit in the right position.  Many cyclists are on saddles they are unhappy with, but the reason is that they are not sitting on the part of the saddle that is meant to be sat on.  Most are scooted too far forward, even to the point where the sit bones don't rest on the saddle, but rather the saddle is squeezed in between them and the rider is resting more on their soft tissue -- this is a problem, obviously.  A huge mistake I see all too often is having the saddle tilted down --- yes, even a little is generally not a good thing.  

A bike seat needs to be in the right place fore and aft so that the sit bones can contact the wider, more cushioned portion of the saddle, and then it needs to be level so that the sit bones can rest on it.  If you aren't perched on your bike seat, then you aren't effectively stabilized to make full use of your pedal stroke. 

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Think about this: 

If you have a seat slid all the way back on the rails, so that the seatpost clamp is at the front of the seat, and it is level.  What happens when you sit on the saddle?  What if the rails are made of Steel?  Titanium?  What I'm getting at, is that a saddle has a static (or unweighted) position and a dynamic (weighted) position.  The dynamic position is the only one that really matters.  It has been my experience that especially with titanium railed seats if the seatpost clamp is to the back of the rails the seat will flex downward, if towards the front of the rails the seat will flex backward.  Therefore I have allowed some seats to leave my Studio tilted up or down at times to accommodate.

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This leveling of the seat brings me to my last point about a good seat -- for a man or a woman.  The seat should have at least some portion of it's surface should be flat and not fully sloping. 

This FSA saddle is a good example of when some seat designs can cause trouble for people.

The centerline of the seat is the high point and the cover slopes downward to either side.  I am sure there are people who find this saddle comfortable, but I haven't met them yet.

I am intrigued by the new fizik Antares -- the entire saddle looks flat.  I will have to try it out and get back to you on that one.

Next up :  Some top secret stuff going on in the lab.  Well, not really secret, but it should be pretty cool.  We are combining the use of the Retul dynamic "mo-cap" with a very sensitive biofeedback system so we can see what exactly some muscles are doing when we pedal, and using all the information (and there is tons!) to try to determine what the leg muscles are doing when....say, a knee tracks laterally more then the other side. 

From the preliminary findings, I think I can say that many will be surprised at what we are finding.

--J