TT positions these days are more uniform than they have been in the past, partly due to UCI restrictions (e.g., saddle noses cannot extend beyond the bottom bracket, arms must be horizontal on aerobars) and partly due to what's taken as common aerodynamic principles. These include the principle of minimizing frontal area, which has, generally, four elements:
(1) getting low;
(2) getting narrow;
(3) getting long;
(4) lowering the head;
Then there's the other idea, which is simply to be in a position which allows you to ...
(5) maximize power output.
Elements 1-4 are often at odds with 5, but the best time trialers manage to do all 5. The aim is speed, and most riders don't achieve their top speeds at their most aero position; they achieve it by balancing aero benefit with power sacrifice.
We're not talking Uncle Pappy of the arthritic back and sanctimonious taint here; we're talking pros. They can legally arrive at a position which places their navel on the nose of their saddle, but even they can't put out any power when so constrained.
This past weekend I received a fit on my TT bike from Scot Epsley, a physical therapist with Georgetown and a fitter for a number of pro riders in the past. We did it in his garage. I'd put myself in an extremely forward position, mimicking the position of Norman Stadtler. Here's Stadtler's bike, to give you an idea:
Take a look at the seat. Not UCI legal (although legal for you or me).
In this tri/forward position, common to triathletes, you look like this:
Most achieve a forward position with three elements:
(1) Seat extremely forward;
(2) Nose directly above front hub;
(3) Upper arm angled at 90 degrees or thereabouts;
(4) Closed space between elbows and knees: compactness.
This position places load on their quadriceps rather than their hamstrings and glutes--muscles especially taxed in the following run portion of their uncouth sport. Thankfully, you and I need not worry about saving ourselves for a run leg. We can whore our entire musculature out to the purpose of going fast.
In more civilized tones, this is what Scott explained to me as he...
(1) moved my seat backward;
(2) flipped my stem, moving it up slightly;
(3) shortened my aerobars.
I'd been having tenderness in the taint, and I'd moved myself forward thinking to put more weight on my upper body, so I was suspicious about Scott's approach.
Aside from solving my taint issues, the element of the fit that most surprised me was compactness. My lower and upper body are now more slightly folded together, and it feels surprisingly powerful and comfortable.
Compactness seems to be a common element of time trial and triathlete positions. It's apparent in the above video of triathletes. Check out Cancellara's position:
Though Cancellara and Millar have slightly different body types and positions, they both have compactness: knees close to elbows.
Cancellara and Millar have vertical upper arms, as do Zabriskie and Evans--who achieves a kind of hyper-compactness where his knees actually come inside of his elbows:
They all also tend to ride on the nose of their saddles, almost as if they'd assume triathlon positions if the UCI allowed their seats to extend forward.
The position of Tony Martin seems to me to be totally different. His elbow angle is greater than 90 degrees. Even though his elbows are pushed out, though, he still manages to achieve compactness. This may be something he's changed over the past year, at least when you look at positions from 2010 and this year.
2010 Tony Martin Position
2011 Tony Martin Position
To be clear, Martin's bike setup could be unchanged: saddle neutral or slightly forward, elbows narrow and bent at more than 90 degrees, head down well below his back. But clearly his body is more compact--in 2011 the space between his elbow and knee is gone. In fact, the two seem to be touching.
Other time trialist who manage to achieve compactness despite great elbow bend are Contador and Janez Brajkovic:
Both have slight torsos, which allow them to put out power in extremely folded positions. This may be a weakness in the position or physiology of notoriously slight-torsoed Andy Schleck, who, at least in this photo, fails to achieve compactness despite planting his ass on the very rivet of his saddle. Despite having a 90 degree elbow bend, he still doesn't manage compactness:
In the end, I'm not sure why compactness matters--whether it's benefits are aero or powerful. Still, if all the best riders do it, it's probably a good idea to at least try to emulate it.