Let's say I'm riding my bike on an ordinary flat road and hold my arms and wrists in exactly the same position for 2-3 minutes every now and then. That wouldn't create much movement. No more than my breathing in and out would under a chest belt heart rate monitor.
Maybe I should just stick to measuring my pulse like with a Polar CS400. If those gadgets are also made for real runners, any arm movement could apparently create no harm.
Mike