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Tuesday, March 31, 2009

"Influence of Gender, Power, and Hand Position on Pelvic Motion during Seated Cycling" Sauer et al 2007

Sauer, J.L., J.J. Potter, C.L. Weishaar, H.L. Ploeg, D.G. Thelen.  Influence of Gender, Power and Hand Position on Pelvic Motion during Seated Cycling.  Med. Sci. Sports Exerc., Vol. 39, No. 12, pp. 2204-2211, 2007.

This is the first installment in some interesting research I have been kicking around and using in my bike fitting practice.  I have decided to share a few bits here.

This first study took trained cyclists and they measured movement through the hips and pelvis at three different wattages (100 W, 150W, and 200 W), on three different saddles (Bontrager X-Lite 2006 mens, fizik Vitesse womens, and Bontrager Race Lite mens), and in two different hand positions (tops and drops).

It did not effectively determine much in the way of gender differences.  I think they set out to find out if riding on the drops versus the tops caused more pelvic motion for males or females.  Perhaps they were expecting more aberrant pelvic motion among females, I don't know.  Overall I think they tried to make their scope too broad -- they were trying to figure out too many things at once.  This, I think watered down their results a bit.

They fessed up to their short-comings in their Discussion, which is admirable but still doesn't help to improve the utility of the study.  The short-comings they listed had to do with the fact that the women were tested at the same wattage as the men and therefore at a higher percentage of their maximum - so asymmetries would be more pronounced in the women due to a greater relative workload.  
The women were also tested on the same handlebar (which had 145 mm of drop to it), and given that the women were smaller, they were forced to relatively lean further forward when they went in the drops.

One other thing I wish they had done, was to include more information and clear photos of the saddles they used -- it can be difficult to find saddles outside of their production year.  And saddles can be changed often from year to year, so finding a 2009 fizik Vitesse may not be very instructive.

Things I learned:

The women's ischial tuberosities (sit bones) were (on average) 134 mm apart center to center, while mens were 115 mm.  Nearly 2 cm difference in width of the sit bones -- that's significant.

What does this mean for bike fitting? Well, simply women's bike seats should be wider at the back of the saddle so their sit bones can rest on something properly, right?

Well, maybe.  Remember, these are averages -- some women have hips shaped like a 13 year old boy, so we need to think individualistically.  But also, this study found that the center to center distance between men's and women's hip sockets was NOT significantly different.  

This reinforces to me a long-held idea I look for in women's seats (as it applies to a woman who shares these "average" proportions -- remember, we need to take things on a case by case basis):  Yes, their seat should be wider toward the back to accommodate the wider ischial tuberosities, but it's my opinion that the saddle needs to narrow very quickly in the middle -- or as I call it, the transition --(essentially the part of the saddle below which the seatpost is clamped to the rails).  

To get a visual on the anatomy, check out this link for a view of the pelvis.  The bottom picture gives you a sense of where the femurs attach to the hip socket (acetabulum), so when you look at the male and the female structures above it, you can see there is a difference in how the femur relates to the ischial tuberosities.

Consider the fizik Arione saddles below.  The little hash marks along each side of the saddle are part of their "WingFlex" technology.  This is the transition area that I was referring to.  In the case of the Arione, this is very effective for some people -- mainly men over about 165 pounds seem to benefit.  Perhaps they are heavy enough to take advantage of the Wings and actually cause them to flex out of the way.  I have not found as many women that are comfortable on them.

I believe this is necessary because the gap between where their sit bones contact the seat and the path the femur takes during the pedal stroke is narrower, which can put more shear force on the soft tissue just distal and lateral to the ischial tuberosity.

I feel many women would do better -- and, again, this is a generalization -- on a saddle more like:

Well, not this exact saddle, but it's female equivalent.  Some of you may recognize it as a Selle SMP Stratos, and I think the fact that the saddle narrows down quickly (the angle of this picture does not do it justice) keeps the width where it is needed (in the back) and keeps material out of the way of those distal-lateral soft tissues by our sit  bones.  You can see the actual women's version of this saddle here.

Next post I'll talk about another reason that women's pelvic motions on the saddle differ from men's, since it can't be explained by hip joint or ischial tuberosity widths alone.

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