Training & Nutrition - HR rises when I do

So I've been using an HRM on the trainer for a while now, and noticed something. With my hands on the bars, I'll be spinning along at, say, a nice steady 160 bpm. If I sit up and take my hands off the bars, and hold the same cadence and resistance level, my HR immediately rises about 5 or 6 bpm and stays there as long as I stay upright. When I drop back to the bars, the HR drops back to its original level. Has anyone else noticed this effect, or know the reason for it?

Two possibilities I've thought of:
- pedaling with hands off bars uses different muscle groups that aren't as efficient
- upright posture forces heart to work harder to get blood to head

I just bought a HRM and I noticed the same thing. I don't know why it happens either.

When you sit up you have to expend effort to maintain your balance on the saddle. This uses muscles that would be resting when you support your upper body on the handlebar.

Try this: Sit down for a few minutes and note your heart rate. Then stand up and remain motionless until your heart rate stabilizes. Why is your standing heart rate higher than your sitting heart rate? Because you are expending energy maintaining your balance.

When you sit up you have to expend effort to maintain your balance on the saddle. This uses muscles that would be resting when you support your upper body on the handlebar.

Try this: Sit down for a few minutes and note your heart rate. Then stand up and remain motionless until your heart rate stabilizes. Why is your standing heart rate higher than your sitting heart rate? Because you are expending energy maintaining your balance.

An a long while back I had a treadmill stress test done on my heart. While sitting there relaxing after the test, still hooked up to the electrocardiogram, I noticed that my heart rate went up every time I spoke. The Doctor said it was normal. I guess talking expends more energy also.

The heart has to pump (work harder) the blood more vertically when you sit up. easy as that. When you talk you hold and use breath to speak.
Velocity