Watts, Cadence and HR

Training and Performance Fundamentals
#1

I’m a bit confused… Watt output is a function of power x cadence. So : if I want to do 200W, I can either push hard at a lower cadence, or pedal faster for a lower power output. Outdoors this works exactly like this.

When using the brilliant indoor HR based training feature, I’m confused. I put my target HR at 120, which is my zone 2 target HR. I clearly have the natural tendency to cycle at a higher cadence as time moves on. Which to be honest is another great side effect of this form or training. The trainer is adjusting perfectly in the recent version of the app. When the HR goes up, it adjusts the power down and the other way round. that’s the power of the trainer. The part that I don’t understand : when I start cycling faster, this gives a higher load on the cardio system. Which means : the power of the trainer should adjust down. Which is does. But : the show wattage is also going down. I played with it and indeed : when I lower my candence, the watts shown go up… I’m probably missing something. In my understanding, the watts show should be the same, it should only be the torque power of the trainer that is adjusted …

Trying to understand, the training model works, that’s the most important part :slight_smile:

#2

I think this is what you meant but to clarify
Watts != power X cadence :slight_smile:
Watts = torque X cadence (this is correct)

Let’s say normally in ERG mode (example below)
@80 rpm 120watts = 120bpm
@90 rpm 120watts = 130bpm (same watts more HR strain)

So to reverse that for HR mode 120bpm = 110watts. This is how it should work. That is exact definition of “higher load on the cardio system.” It pushes HR higher but reduces the muscular tension/torque for the same watt output.

So you said everything correctly (other than torque/watts), so I am not sure where your confusion is :slight_smile:

#3

If you cycle with higher cadence, your heart has to do more work, because you are spinning faster, and therefor the muscle need more oxygen from blood. (=> higher heart rate at same power)

If you cycle with lower cadence, then your muscle use more fast twitch fibres and they use more glycogen stored in the muscle, and as a result your heart rate is going down.

I think you are confused with power and torque and watts.
Your muscle do put some force on the pedals (Unit Newton). To measure the torque you have to multiply force x crank length (=Nm). To get power = torque x cadence (=Nm/s=W)
If you change cadence, you have to change torque for same power.

2 Likes
#4

Exactly, low cadence puts more strain on the peripheral system (muscles), high cadence puts more strain on the central system (cardiovascular). Your body sort of automatically balances both to get you as far as possible. That’s why almost all cyclists start to grind when fatigued, except for those that have weak peripheral system and strong cardiovascular from x-training.

#5

Hey Alex, correct, I should have said torque, but that’s what I meant. Let me take the time to do the math :slight_smile:

#6

What math? I was just making up numbers. We have no idea of what RPM is going to cause what additional strain on someone’s cardiovascular system.

I am sure that is extremely individual and if it is not we would need to look at thousands of people doing steady state workouts to see if there is any consistent relationship. I guess I have this data :slight_smile:

So I think everything you are experiencing is totally normal and while interesting, especially if you are trying to maximize your performance I am not sure if it needs a lot of thought. You could do two workouts and different RPM average but same HR and see how you feel at the end of both of them.

#7

It’s not important. The important part is : it’s working. But I do believe something is wrong with the WATT number shown.

Let me try to explain.

Imagine I do 150W at 80RPM. That means I do 1.875Nm torque. Say I do that at 120BPM HR.
I get it, when I start doing 90RPM, my HR will go up. What that actually happens is that torque is adapted to 1.667Nm, so that my HR stays at 120. In both cases, the Wattage stays 150W. What happens in the app now is that the displayed wattage is shown at a lower number. Unless I’m totally wrong in the logic, I believe this is not correct. But I can be totally wrong …

#8

When you do a HR Structured Workout or a HR based workout, what happens is the app will not care what is the current wattage shown. The logic will only be to either Increase wattage or lower wattage to get your HR to be at the desired setpoint.

If your HR goes up, the wattage will go down in an effort to get your HR to go back down and vice versa.

No need to think about torque or such for HR Based workout.

1 Like
#9

That would be the case if you have a dumb trainer with a speed/power curve and you keep the same gearing. Higher cadence = higher speed = higher power. Smart trainers will simply show the result of torque x cadence and the power number will not change.

#10

The power goes down because your heart rate goes up. So your torque settle more to 1.61 Nm in your example.

But actually the app controls only the power (watts). If your heart rate is above a certain limit, it will decrease the power. If your heart rate is under a certain limit, it will increase the power.
No matter of your cadence, It will only look to heart rate, and will set the power.

#11

That’s exactly what I’m thinking, the Wattage should not change in the display. Since it does, it could be the smart trainer that is not smart. I have an Elite, don’t know by hard the exact version. Was assuming it would be smart :slight_smile: maybe it’s not. It’s working, with or without a correct wattage number. My confusion is that I was anticipating I should see the same wattage, regardless of cadence. Which is what you are saying as well.

#12

Yeah, but if your heart rate goes up, if you do higher cadence, then it will decrease the power, and then you will see less watts.
If your heart rate would stay at the same level with higher cadence, then the watts should also stay at same level. But it depends, whether your heart rate goes up or not.

If you would have a dumb trainer , it would show more (!) watts with higher cadence at the same gearing, if it uses a speed/power curve.

#13

True again :wink:
Most riders will have a slightly higher HR when cadence goes up. And it will be more if you are not used to riding higher cadences.
For very low cadences as in the SFR workouts, you will put out considerably more power at the same HR. But your legs will fatigue much faster.
Within your ‘comfortable’ cadence range, the HR difference will be minimal.
So you probably see a lower power number because your HR goes up with the higher cadence.
If you would do similar in an ERG workout with constant power, you will see changes in HR. Higher HR with higher cadence, lower HR with lower cadence.
Like I said, it’s a balance of cardiovascular vs muscular load. And what you see in the numbers depends on which variable is being kept constant by the loading system (app + trainer).

1 Like
#14

HR goes up, we tell the trainer to lower the wattage, this is exactly how it works and should work. If we did not lower the power target then your HR would stay too high. If you want power to stay constant and HR to go up, then use ERG mode :slight_smile:

As others said torque does not matter in that story it is very simple just HR and power. Your torque goes down when RPM goes up. The trainer controls this automatically, we just provide power target. Your Elite is working perfectly :slight_smile: