For those who train on both TT and road bike, just wondering… how does your HR compare on them at the same power?
(This is assuming in full TT position on aerobars vs riding in hoods on road bike)
I just did the first session on the TT bike in over a year, and was surprised my HR stayed so low. On the one hand I am leaning on the arm rests, but on the other I am in a pretty low position with restricted breathing. Here’s the data from today’s session with a Z2 road bike session in blue:
Better late than never…lol. In general mine is lower while riding the TT bike at the same power as well. While I don’t know for sure I think it’s due to less upper body and core muscle utilization while riding the TT bike. Less utilization due to being supported by the arm rests.
Mostly my HR on a TT bike will rise to about the same as on a road bike I could be wrong but I think my HR raises faster and less sustainably sometimes on a road bike and its probably affected more by cardiac drift too. Ive never actually looked at what they (TT HR & Road HR) are doing at the same power though but I’d guess on the lower sustainability of the road bike means that the HR would be higher if I was achieving the same power as on a TT bike.
I briefly searched the topic and found mostly rider reports. Many said HR was actually higher on the TT bike yet, many were reporting lower. However, most did not hold power or speed constant so most of the statements were basically useless. Only found one study (so far) that was remotely related but, didn’t really address the question at hand. Interestingly, I found more information comparing running HR to cycling HR which suggests the difference in achieving some max sustainable HR is usually much higher running compared to cycling. Presumably this is due to higher total muscle utilization.
I was listening to a podcast recently (Maybe the Cycling in Alignment Podcast about the importance of core???) and it was mentioned that heart rate is lower on the TT vs. the Road bike because in that body position the blood does not have to travel as far to the heart.
From recent sessions on my trainer on the TT bike, I could see my HR was around 5 - 10 bpm higher when in the full TT position compared to a similar road bike position or a more upright position on the TT bike. I think this is due to the extra muscle use through turtling and bringing in the shoulders in, compression on the lungs from the position and the closed off hip angle.
It probably means my FTP is down by 10 - 20 watts, and I could negate this by a less aggressive position, but from the aero testing I have done the difference gained by these compromises are way more than 20 watts.
Very un scientific but my 90mins on the turbo in the TT position (all but circa 5mins , I went up and down in the last 10 min interval) saw me average at 74% Max HR (it will be heavily polluted by 5 mins recoveries) and build more sustainably to 95% Max HR for an average of 190w and my 85mins on the road bike the day before (after a 20min ride to the start) saw me average 84% Max HR and saw me peak at 94% Max HR for 178w (different PM though). 
Lol, another unscientific comparison. On my road bike back in February on an outdoor work out (Round Bald, 3x20 SS 4.9) for 2h25min at an IF = 0.71 I averaged 166w and 75% Max HR
Later in the year (July) on the TT for an outdoors work out (Glassy, 4x15 SS 4.7) for 1h40min at IF 0.66 I averaged 166w and 72% Max HR. Which again suggests my HR is higher on the road bike for a similar sort of workout and power but not significantly so.
So when in a TT-position, the distance between the heart and legs would be shorter? Isn’t that like folding a map and saying that it is now shorter distance between New York and London?! 
It’s not necessarily the distance but more gravity related, half (or less or more depending on your ratio’s) of your body is basically level so it takes less effort to get blood there.
Iirc its a few beats higher sitting than in aero. I switch for long periods holding the same power.
Comparing different rides at different times wouldn’t be fair.
I think the more apt analogy would be that it would be easier to dunk on a basketball hoop with a 10 foot pole, if you bent the pole so the hoop was only 8 feet, as gravity wouldn’t be as much of an issue.
Of course I am just relaying what I heard…because anecdotally, my threshold HR is based off of an effort on my TT bike!
