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Sunday, July 29, 2012

Bike Fit 101: Proper Knee Extension on a bike

I was asked a  really good question this 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.

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