This is more of a thought experiment or rather just curiosity. But what is the energy expenditure of a heartbeat? And does it matter over the course of a ride?
As I was riding on the trainer, I let my mind wander, then did some quick calculations. Is the energy expenditure that different for a ride with a high HR vs a lower HR? I went back over the past couple weeks and calculated the Cal/W*min for almost every ride to be 0.06. So regardless of the power or HR, that how many calories I “burned” which I took to mean the energy required solely to put the power through the pedals. But it doesn’t take into account HR at all. So it got me thinking if this matters.
For the sake of an example, let’s say I do a 3 hour ride at 200W. So 180 minutes at 200W should be 2160 calories. But if my HR is 110BPM, that’s 19,800 total beats. If my HR is 140BPM, that’s 25,200 total beats, a difference of 5400 beats. Does this put extra stress on my body? I would think yes, but is that relevant? What is the energy cost of this?
Like I said, this is more just curiosity, but I was thinking about riding with vs without a fan. With a fan my HR is around 20BPM lower than if I ride without a fan. The difference is very noticeable, not just the HR but the heat I feel. And I would think this is more stress on the body. Is it quantifiable? For the same power, my HR is higher and my body is trying to deal with the extra heat. There’s gotta be a way to measure the extra stress I’m putting on my body for the no fan workout, even if the TSS is exactly the same. Is there a way to track this? Like, if I look at my training calendar, and see 40 TSS each day for a simple Z2 ride, those workouts are not the same if some days I use a fan and some days I don’t. So my week TSS can be the same, but I would think that the real stress on my body is different. Just curious if there’s a way to track this. Something like a combined power and HR TSS.
I don’t think the additional beats would be very accurate because there are MANY reasons your HR may be higher from, heat, sleep, caffeine, stress, prior workload, time of day, etc. The actual beats themselves aren’t a significant “energy expenditure” themselves, but the overall effect could create more “stress” on the body. I think my understanding is, you want your heart rate to be as slow as possible for as high an amount of work you can. The higher stress on the body of riding without a fan may not matter for that ride, but it can accumulate and affect other rides, which could constrain your training. As far as quantifying that, I think RPE is possibly the simplest metric. Intervals.icu has a TRIMP (Training Impulse) metric that sounds similar to what you are looking for.
“average power consumed by a human heart (0.12 Watts per beat)”
so a pretty small contribution to the total energy required.
Edit: It seems like the original article I googled may be off, this is more complicated than I care to understand (I hope someone else will summarize it though). Here’s one other source:
The heart uses an average of ~6 W of chemical power input to generate 1.3 W of mechanical power output.
I have seen quotes for the chemical energy consumed by the heart to be ~5 W. Using a my crude model, I compute about 6 W in Figure 4 assuming a chemical-t0-mechanical conversion efficiency of 20% (reference). If you want to know more about this efficiency figure, see this post for more details.
You could do a simple average of TSS and hrTSS if you want a way to track it. What would you do differently with that info though?
I’ve thought about this a lot because I mix indoor, road, and MTB, then different types of terrain on the MTB, trips to the mountains, sometimes for the day, a weekend of camping, or a full week, riding at elevation, etc, etc, etc. I’ve come to the conclusion that in a steady state, meaning you’re doing more or less the same types of rides in a given 6 week period, that it doesn’t really matter. And when it does matter, like taking a week long trip to ride, I’m going to ride what I want to ride and adjust on the fly anyway. I’m no going to worry about TSS. The absolute value of your ATL/CTL may be a little different if you figure out the absolute perfect way to measure all of these little things, but in the scheme of things it’s not going to affect how you train.