Posted: 04/07/2014 at 10:12
Why max HR should depend on posture is a complicated story. the bottom line is: you need to determine your HR max for running, not cycling, then base your running training zones (and race effort if you are using % of max) around that.
People using intuition want to say the heart can ultimately only beat x beats/min so that's the end of the story. Unfortunately, the pump is not in isolation and one big factor is rate rate of filling, which in turn is affected by venous return pressure, which is affected by posture. When erect, there's immediately two big factors: hydrostatic head, and muscle-pumping effect that are different to cycling (not to mention all the physiological feedback loops like venous stretch).
Within any one sport, how easy it is to reach the max can be limited by how much muscle is engaged, ie you wont hit it playing darts. This confuses the issue: even if you were lying on a recumbent bike pedalling with both foot and hand cranks to engage max muscle, you still wouldn't hit as high a max as running upright, because of this venous return and filling issue. Same applies to a rowing machine where you are sitting: you'd think that legs, back and arms all together would guarantee you hit the highest number ever, but you don't, compared with running.
This doesn't mean one sport is tougher than the other. I just means that one system set-up hits a higher number than the other, the max is set-up specific (ie, sport-specific). So your zones and anything else set relative to HRmax (ie race effort) must be sport-specific.