I noticed an interesting effect this week on the trainer. I was riding at a steady state pace of around 151bpm and 90rpm. Then I straightened up my lower back and while maintaining the same rpm and power, noticed my HR decrease by 5bpm to 146bpm. I rounded my back to the original posture and HR went back up. Tried the same thing again the next training session so it seems this effect is totally repeatable. I'm wondering if this is a well known thing or just my own quirk. I'm also curious about the reason for reduced HR. Does straightening the back give the legs better leverage so they are more mechanically efficient, or does it somehow reduce restriction on bloodflow and let the heart work easier. Any thoughts?