Many triathletes and time trialists (including previously this writer) will tell you there's no doubt that their power drops when they move from upright (non-aero) to an aero position. As a result, when the going gets tough, it's a natural reaction to bail out of aero to grasp for those last few matches available in sitting. However, a study looked specifically at this issue and found - not surprisingly - that power in upright or aero is DIRECTLY CORRELATED TO TRAINING. For those who don't train regularly in aero position, OF COURSE the aero position provides less power. However, in those who train regularly in the aero position, peak power was actually HIGHER in aero than in the non-aero position and there was no difference between the two in terms of mean power. Combine this with what the research tells us about the speed enhancement in aero position at otherwise equal power output, and the solution is straight-forward. Aero is the answer.
The abstract is below. Here's the direct link if you'd like to take a closer look: http://www.asep.org/files/PevelerV2.pdfEFFECTS OF TRAINING IN AN AERO POSITION ON ANAEROBIC POWER OUTPUT. Peveler W, PBishop, J Smith, M Richardson. JEPonline 2004; 7(5):52-56. Anaerobic power is a good measure of cycling performance. While previous studies have found that there is an increased metabolic cost when riding in an aero position, the difference in anaerobic power between upright and aero positions have not been examined. This study examined cyclists who never train in aero bars (CT) and triathletes who always train in aero bars (AT). Participants performed two 30 s Wingate tests on two separate days (one trial upright and one aero). For CT, peak power (PP) was significantly (p=0.000) higher in the upright position (768 ± 74.7 Watts) than in the aeroposition (706 ± 69.4 Watts). In AT PP was found to be significantly (p = 0.003) higher (749.6 ± 111.53 Watts) in the aero position than in the upright position (697 ± 75.9 Watts). CT had significantly (p = 0.022) higher mean power (MP) in the upright position (598.0 ± 45.95 Watts) when compared to the aero position (562 ± 47.9 Watts). There was no significant (p = 0.250) difference found in MP between aero (564.7 ± 63.4 W) and upright (560 ± 59.1 Watts) in AT. From these results it is recommended that athletes train in the position in which they race.