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It's not MAF calculation, it's experience. That 180-age MAF formula just wouldn't work for me at the moment, I'm too heavy and not fit enough. I'm 44, BMI>30 and so 136bpm would be a slow jog around 7:45/km. No point doing that when I can comfortably hold a conversation doing 6:30/km at ~150bpm.
150bpm is gentle/easy for me. 160bpm is tempo. 170bpm is getting there and 180bpm is pretty much the max I've managed for a 5k. (These are averages, not maximums.)
Greenbank-
Each to their own, and I don’t deny that running to MAF heart rate is incredibly frustrating and at times embarrassing when someone storms past you running, and I’ve failed many times, but it does seem to work if you stick at it. The reality is that you can run faster (I ran 25k in November chatting with a mate at 4:32 avg but got a bollocking from coach, because while it’s possible it’s not going to contribute to building aerobic fitness.
To add to the embarrassment of running slowly and being overtaken you also have to be prepared to positive split every long run and occasionally walk up hills!
Sainsburys_Ed
Are you sure about that 158hr suggest by MAF calculation that you are 22?
Under that calculation 158bpm is firmly in the “grey zone” of not actually doing very much for fitness and increasing risk of injury...