Quote Originally Posted by OakLeaf View Post
I'm not sure why the pros round their backs so extremely - I don't think it's for "shock absorption" as Sheldon suggests, so maybe the racers here will chime in. But I'm guessing it's so they can still pedal when they're in a deep aero tuck. For me, if I'm in even a shallow tuck, with very my shallow drops and high stem, if I'm in neutral spine, my knees hit my chest.

If you're more concerned about the long-term health of your spine, than about your momentary performance, then never force your spine out of neutral for extended periods of time. Some say never intentionally bring your spine out of neutral ever. (I don't necessarily agree, but there are prominent fitness experts who hold that opinion.)

Do use your core muscles to support your back, just as you would sitting or standing. I think the Sheldon Brown piece is talking about people who completely relax their bellies and sink into their shoulders in an extreme hyperlordosis, not people who maintain neutral. The forces of gravity are obviously acting on your spine differently in a tuck than they are standing, but neutral is still neutral, and compressing a disc unevenly is still compressing a disc unevenly.
A rounded back is not ideal, even for a pro. A rounded back means that you have less room for your lungs to expand fully. It may be efficient in terms of power output, but it will tax your aerobic capacity. You can be a freak like Lance and have a big hump in the back and manage to breathe more or less through the gut (his diaphram breathing is kind of extreme--looks like an alien is about to pop out of there). Pros tend to have above average saddle to bar drop with compact frames and large stems, so they need to have a long reach when tucked in the drops. Some are more effective at bending from the hip and keeping the back relatively flat than others. Of course, if you are too flat in the back when bent over like this, you will need a periscope to be able to see the road. Some hunching of the upper back allows the rider to look forward. At any rate, an extreme tuck is difficult to maintain for long periods and should only be used when aerodynamics are essential. Otherwise, a more neutral back will allow better breathing and improve overall efficiency, but it will be at the cost of aerodynamics. Much of the time, that's ok.