So I've recently built up a TT bike to help me be a bit more competitive in TTs next season. Yesterday was the first time I went out and did some proper training in the aero position - 2x20 intervals on a flat circuit. Weather was perfect, little wind and only about 27 celsius (80 fahrenheit), and I was coming off a fairly easy week of Base 2, with two easy days before the workout.
About 6 weeks ago, under almost identical conditions, I carried out a threshold heart rate test in the form of a 17km TT, as per Friel's instruction, and calculated my LTHR to be 179 bpm, which is consistent with my observations on many rides over the past 6 months. However, on the TT bike, I only averaged ~170 bpm for the 20 minutes, even though perceived exertion was at threshold levels.
So, my question is - does cycling in the aero position, when you're not used to it, result in a lowering of your threshold HR?
Also, I should note that I was faster on the TT bike over the course than I was on the road bike 6 weeks ago - 39.4km/h TT vs. 38.4km/h roadie.
Head position effects HR. However, I don't think that effect is large enough to explain away the whole 9bpm...
http://home.hia.no/~stephens/hrchngs.htm Body Position on the Bike will Influence Heart Rate. Let's say I am riding on an indoor bicycle trainer with my upper body parallel to the ground (Hands on the drops) at a heart rate of 145. Raising upright while continuing to cycle at the exact same workload will result in an increase in heart rate of about 5 beats per minute. Trust me I have experimented with this effect on many a winter evening! This is due to decreased venous return in the more upright position. Heart rate increases to compensate for the slightly decreased venous return and stroke volume, keeping cardiac output constant. Whe I return to the drops, the heart rate drops again.