My thinking on this has changed over the years. I used to believe triathletes should spend the majority of their training time in the aero position since that is how they race. I no longer hold that view. The answer is more nuanced, and getting the balance right can meaningfully impact both your training quality and your race-day performance.
Off-Season: Stay on the Road Bike
During the off-season and early base-building phase, the road bike should be your primary tool. The reasons are both practical and physiological.
On a road bike, you can push more power. The upright position allows greater hip opening, better breathing mechanics, and more effective force application through the pedals. This matters enormously when the goal is building raw fitness.
More importantly, you simply cannot achieve true VO2 max efforts in the time trial position. The mechanical limitations of the aero position - compressed diaphragm, restricted hip angle, compromised breathing - cap the intensity you can sustain. If your training plan calls for VO2 max intervals, doing them in aero is leaving adaptation on the table.
Even professional cyclists who race time trials spend 80 to 90 percent of their training time on the road bike. If the best time triallists in the world are mostly training upright, age-group triathletes should take notice.
Approaching Race Season: Get Serious About Aero Time
As you move into race-specific preparation, the balance shifts. You need meaningful time in the aero position to develop the muscular endurance, flexibility, and comfort required to hold that position for the duration of your race.
This does not mean every session moves to the TT bike. It means that your longer, race-pace sessions and your position-specific work should be done in aero, while your high-intensity sessions can remain on the road bike where you can access your full power range.
Build your engine on the road bike. Learn to use it in the aero position. The off-season is for power. The race season is for applying it.
The Critical Mistake: Testing in the Wrong Position
This is the point most athletes and coaches overlook entirely. If you race in the time trial position, you need to test your physiology in the time trial position.
Your heart rate, power output, and lactate numbers are almost certainly lower in the aero position than when riding upright. The compressed position restricts breathing and alters muscle recruitment patterns. An FTP test done on the road bike will give you numbers that do not accurately reflect what your body can sustain in aero.
If you set your race-day power targets based on an upright FTP test, you are likely setting yourself up to over-bike. Your actual threshold in the aero position could be 5 to 15 watts lower than what you tested upright.
The fix is simple: do your FTP, heart rate, and lactate testing in the position you will race in. Use those numbers to set your race-day targets. This single adjustment can prevent the pacing errors that ruin so many triathlon bike legs and the run splits that follow them.