There is a big difference between the Triathlon and Time-Trial bike fit.
The most obvious difference is that at the end of a Time Trial the rider should be completely spent; at the end of the Triathlon bike-split, the rider needs to run as fast as possible to the finish, sometimes to the tune of 26.2 miles.
No triathlete has ever won an event by his/her bike time, so it makes sense to fit a person for the best transition into running as possible. This usually entails having the seat forward, increasing hip-angle for transition to running and better aerodynamics via. lower front-end. Elbows should rest under shoulders (approximately), with enough distance between arm-pads to ensure optimal diaphragm function and comfort. Most top-fitters will suggest a more relaxed position over aerodynamics for Triathletes.
Time-trialists are UCI- restricted to position the saddle nose 5cm behind the bottom bracket (though most proficient racers ride the nose, anyway). This creates a problem at the hip-angle (significantly reduced) that impacts the rider’s ability to increase saddle to bar differential, which many consider essential to optimize aerodynamics. The fit is speed-specific, tuned to maximize aerodynamics and power for a specific distance/terrain/condition. Elbows are closer and anterior to shoulders, back is more kyphotic, and diaphragm position is altered to breath from the thoracic/lumbar junction.
Most TT and Tri customers I meet are positioned with femoral/inferior iliac crest contact at TDC, and “head-up” posture. Easier to fix for the Triathlete than the TT person, obviously. The problem is that compression between pelvis and thigh is hardly perceptible, like pinching the skin at your elbow — neurologically, just not very proprioceptive. Symptoms can occur at the acetabulum, unilateral or bilateral back, medial or lateral knee . Want to check if this applies to your position? Have someone place a finger at the crease of your hip, and see if it gets crushed by the femur at TDC. Also check knee position at TDC. Splayed knees may add confidence to your findings if positive for the finger-crush-test (rule-out psoas). The leg has to go somewhere at TDC, and lateral is the only option if restricted by hip angle.
Additionally, the diaphragm shares attachments of the quadratus lumborum (low back) muscles. It makes sense, anatomically, that tight QL’s = lesser exhalation = shallow breathing = poor O2 intake = bad performance.
Head-Up vs. Head-Down?
Tom Coleman has a picture of a fish on his computer screen, to remind him about aerodynamics. Fish are aerodynamic — humans, not so much. Our face is like a wall, creating drrrrraaaaaagggg; conversely, Head-Down with a traditional cycling helmet (with big holes) is also creating drrrrrraaaaaaggggg — like a plane, landing. Wind tunnel testing indicates Head-Down with an aero helmet is the best option.
Bend your knees, feet hip-distance, and place hands, approximate to knees, on your thighs. Shift your gaze and face to horizontal, and take a really deep breath. Feel good? Now, do the same exercise, but with head “neutral”, and gaze at the horizon of your eyelids. Get it?
In closing, remember that it is a fine line between aerodynamics and performance. A quality fit-technician should know how to optimize both, specific to the DNA of each cyclist.