what exactly does saddle setback do?

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so im looking into my fit, as per the problems ive described here before. but i need to know a bit about saddle setback, ive read as much as ive come accross. but. what exactly does it do, muscle wise? i have 9.6cm of setback and about an 80cm bb to seat height. with long femurs, i get fit by those serotta guys like this.

yet, how can someone like wiggins (since i saw his bike on cyclingnews), who undoubtedly has long legs, have close to no set back (his seat is jammed forward on an integrated seat post), i guess he is known for his pursuiting abilities.

i think i may have too much setback, as my glutes and then hamstrings start to hurt when im on the hoods in that jens voigt-esque power position for my intervals.

so what exactly does saddle setback do.

I would guess your fitter was trying to get your knees over your pedal spindles with your feet at 3 o'clock - if you have long femurs then he'd have to push your saddle back further and/or give you a setback seatpost. It sounds like with your saddle back that far you may be having to reach farther for your handlebars than the flexibilty of your hamstings and glutes can tolerate right now. How is your arm position - are your elbows bent? or are they straight? If they are straight out you might need a bike with a shorter top tube or at least a shorter stem. If your arms seem OK, maybe you need to put your handlebars up higher for a while, let your body get used to the position, then move them down gradually. Stretching exercises for your legs and back might help some too.

Take a look at the fit/positioning articles posted here:
http://www.cyclefitcentre.com/further%20reading.htm

Dave