I find my HRM slows me down. This is a good thing when I'm trying to do a long slow run or a recovery run.
I don't know my max HR, but experience tells me what is the right HR for a recovery run. In the end I still do it by feel, the HRM just reminds me when I lose concentration and speed up.
They can be useful without having to build your training program around them.
I get no speed signal on mine when I turn the light on (mounted on the handlebars next to the wrist unit). The dynamo light doesn't interfere at all, just the battery one.
denotes Subscriber-only content | 
RossB
