I tend to go OK just on HR for the half IM distance. out of curiosity, how much does going too hard for the first 15 minutes of an IM and then settling in to your "right" pace affect your run, if you have several hours to go.
My thought process to this is that if you are having a bad day (it happens), power output will be down for a given level of exertion. So if you try to ride at the prescribed power output, then you are actually going too hard, no?
Where I do see power measurment being beneficial in a race situation is on hills, since there is a lag between changes in effort and the HR response. The PM can help you throttle back at the start of a climb, if your tendency is to ride hills too hard.
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2020 National Masters Champion - M50-54 - 50m Butterfly