I've had a lot of trouble understanding this idea of HR “running low” when you're tired, but have finally sorted it out. This post is more to keep it sorted for me than anything, but if it helps you, I'm happy.
The reason I was confused is that I've made the wrong assumption all this time, the one that's the complete opposite of reality. It seemed logical to me that when you're tired, your HR rises quickly and goes too high. After all, that's what it feels like right? You get out of breath in no time, your heart seems to race, and you feel like you're putting in a lot of effort.
But at these times, if you look at your HR on a monitor, you'll probably be surprised by how low it is. All that huffing and puffing and yet the ticker's clattering along at a relatively sedate 140bpm (Perhaps you were expecting 150). In other words, your perceived effort (PE) is high, but the actual effort is low (since HR is indirectly a measure of the amount of work your muscles are doing). In still other words, and back to our first words, you're tired.
At this time, it's vital to remember that it's actual muscular effort above a certain level that triggers training adaptation, not perceived muscular effort. So just because it feels like a hard workout, doesn't mean it is a hard workout. (In another post I mentioned that when you have the flu, just getting out of bed tires you, but that doesn't mean you got any fitter.)
And so, the opposite is also true: when you're well rested, your HR rises quickly and easily. You might be riding along your usual route, feeling good, look down and see your HR is 160 when you were expecting 150. If that sounds wrong, don't forget your average speed for the section will be higher. You're doing more work, but it feels like less work.
I don't know about you, but these concepts are totally counter-intuitive to me. But if you think it through, you realise they aren't counter-intuitive, it's just that it's a shifting POV.
So imagine you ride exactly the same route, same weather, same gearing. You do it on three days—the first day you're well-rested, the second moderately rested, and the third you're tired.
If you keep speed constant over the three days, you'll find that on the first day, your PE will be lowest and your HR lowest. The second, PE will be higher and HR will be higher. The third, PE will be highest, HR will be highest. That follows the intuitive pattern, that more tired you get, the harder it is to hold a certain speed, the higher your HR at that speed.
But we rarely keep speed constant. We usually ride by “feel” or PE. So assume we keep PE constant, i.e. the amount of effort we put into the ride feels the same, no matter what our level of tiredness. On the first day, speed will be highest, HR highest (we feel good, so we push). On the second day, speed will be lower, HR lower. (To feel as good as yesterday, we have to slow down.) On the third day, speed will be lowest, HR the lowest. (We have to go really slowly to feel as good as we did on the first day.)
So it's pretty much the same thing, just seen from a different point of view. When we talk about HR “running low” when we are tired, we are simply leaving out the part that we are also riding slowly, and yet feeling as if we're putting in a lot of effort.
Phew, I think I've got it. Zhu?
Here's an analogy that just struck me that makes this whole HR low when tired concept a lot clearer.
Imagine a car engine running on good fuel with clean fuel lines. You push the throttle and it revs quickly to a high RPM. That's you when well-rested. You rev up quickly and can rev really high.
Now imagine the engine is "tired" i.e. it has adulterated fuel and clogged up fuel lines. You push the throttle the same amount, but it takes some time to rev up, and doesn't get to as high an RPM as when everything was good and clean.