While on the trainer this evening, it's quite clear that while pedaling a steady 90rpm at a moderate power level, changing my position from hands-on-hoods to an aero position (aerobars) will raise my HR 5-6bpm.
What I'd like to know is/are the reason(s) for this. Is it because the less relaxed position demands more muscular energy (a bit more O2)? Is it because the aero position compromises breathing efficiency?