Aero body--back shape

In fiddling with my (self-evaluated) time trial bike fit, it seems that I have a choice: rolled forward hips and flat back at the cost of comparatively square (and broad) shoulders, or roll the shoulders forward and down into a narrower position but which also results in a slightly more rounded back. Short of wind tunnel data, any guess as to which is more aero?

The forward shoulders approach also makes me want more reach–in that scenario would it be better to extend armrests and drop the bars (presently 12 cm below the saddle) or to move my saddle back? As it is I’m in a semi-slammed back position on a road frame–about 3.5 cm behind the bb (kind of short femurs, this one).

Sorry if this is too vague and general to be of use. Thanks for any opinions.

Hmmm, hats off to you for doing what amounts to some real work on your position. My “default” recommendation would be to stick with the position that gives you the best power output. I am a believer in subordinating aerodynamics to power output.

Cobb says theres no problems with a rounded back, cause it fall in line behind your head—as long as your chest is parallel to the floor. Also, your elbows only need to be wide enough to block your legs. So look at narrow shoulders and if the wind can see your legs/knees —than those shoulders/arms are too close together. also, cobb says that the closer the elbows are back to the rider (if the elbows bend is 90 degrees) thats more powerful (and less aero) than the arms way out in front of you.