I can't speak to the Apple watch specifically, but wrist based HRMs seem to really suck for exercise that involves gripping (lifting, rowing, etc). I've tried to use a Garmin Vivofit while rowing, and it rarely reads higher than 70 bpm. Meanwhile my Polar chest strap monitor shows 130 or whatever.
Some googling reveals that this is common with wrist HRMs, something to do with the muscles being flexed all the time.
ETA: the point I didn't actually make - I'd bet the Rogue bike would have the same issue, since you're constantly pushing/pulling on the handles.
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