Spartan420

Ive been doing Trainerroad for 6 weeks. Most of the workouts are high cadence with less force on the pedals. Yesterday, I switched it up and did a low cadence but with the same power. I was wondering if say 600Kj equated to the same calorie burn at at 99 RPM cadence and at a 72 RPM cadence? The power and speed were the same. The only difference was a lower heart rate with the lower cadence.

Example: Tray Mountain
High Cadence - Avg Cadence 95 RPM - Power - 128w - NP - 135w KJ - 691
Low Cadence - Ave Cadence 72 RPM - Power- 128w - NP - 135w KJ - 691

I want to build a little more power for hills is the reason I am going higher force, lower cadence. But, I want to know if the calorie burn is the same since I am getting less of an aerobic workout.

redlude97

the calorie burn assumes a metabolic efficiency of ~25% which can vary from individual to individual and within the same individual depending on various parameters like cadence/hr/temp etc. How much you won't know unless you get tested in a lab. The difference won't be huge. Concern yourself with the training adaption, not the minimal differences in calorie burn

Spartan420

So a lower cadence and lower HR results in a lower calorie burn? Am I right in saying that the KJ number is close to the calorie burn? I thought that was more accurate than HR when biking.

prathmann

A calorie is equal to just over 4 Joules (actually 4.184), so when you take into account the body's efficiency of about 25% the number of Joules of energy output is a close match to the number of calories input. But, as stated before, the 25% efficiency is only approximate and does vary with cadence. Studies I've seen indicate that efficiency is highest at a pretty low cadence (around 60 rpm), but that results in faster muscle fatigue so most cyclists do better at a higher cadence (around 90) even though it's a little less efficient. Your lower heart rate is confirmation that you are a bit more efficient at the lower cadence and therefore your body's calorie consumption would also be a little lower.

Spartan420

Thanks for this reply. This brings up something I have been curious about. When I run, I think I get lower calorie burn numbers than what I am really burning.

Running at an 8:00 minute mile pace - my HR is only around 120 bpm. If I run easy at 9:00 minutes per mile, my HR stays around 114 BPM ( I wear a chest HR monitor). Same on the bike. MY HR at 70% FTP and a 95 RPM cadence is usually around 111 BPM. My 5k pace is 6:45/mi - Half Marathon 7:14/mi... but I dont want to run that pace as a norm.

I always assumed because of my stupid low HR that I was burning less calories. 41yr Male - 137lbs, but used to be a fat filled 225lbs.

Seattle Forrest

This is absolutely true. But, for context, there's only about 5 % variation, at least among the way people ride road bikes.

DrIsotope

Active HR has essentially nothing to do with work done-- so average heart rate and power output (and therefore calorie burn) are not measurably related. HR is determined by a laundry list of factors. PM measured power is torque * RPM. I can make 200W with my HR in the 130s one day, and it might be in the 150s the next. The weather has a significant impact on HR. Power is power.

The OP is gonna have to do a lot of work to burn the same amount of fuel I do just toddling around. I'm 50% bigger, probably make (on average) about 50% more power, and therefore burn about 50% more calories in the same time period. I dunno if it's worth the trade.

Spartan420

So 600kj of work is 600kj of work regardless of HR or cadence and therefore calories burned are somewhat the same?

DrIsotope

Work is work-- a kJ is a kJ regardless of the HR or cadence. We all have different natural cadences and different resting/active heart rates. But work done is work done.

jsk

IMHO this is an oversimplification. It's true you can't make any real conclusions about work or kcal by looking at different HR's across different people. But that's not the same as saying that variations in HR for an individual have no relation to work done.

On the cadence/HR example, lab studies have shown that higher cadences result in higher oxygen consumption, which goes to a difference in metabolic efficiency (even if it's small, it can't be completely discounted).

Likewise, if you're doing 200W with HR in the 150's instead of 130's because it's hot outside and your body is struggling to cool your core temp, then yes your boding is working harder. The amount of work applied to moving the bike forward may be roughly the same, but that's not the only work your body is doing, it's just the easiest to measure.

jsk

Work is work and a kJ is a kJ, but that doesn't mean that 1 kJ equals exactly 1kcal for everybody. "kj work equals calories burned" is just an estimate; it's a pretty good estimate, and better than anything other measurement short of a gas exchange mask in a lab, but it's not an absolute truth.

DrIsotope

Splitting hairs aside, as I said in another thread, TSS/TRIMP is far more useful than a number like W/HR. Because yes, there are factors outside of just crank power than contribute to training stress. Sometimes it's very cold. Sometimes it's very hot. My PM reads zero on the fire road so steep I have to get off and walk. But I still have to walk up that hill.

But to the OP this is inconsequential. He asked if HR or cadence made a difference in power output. I said that in general, they don't.

colombo357

Close enough for all practical purposes.

DannoXYZ 

Yup, power = force * cadence. If you spin slower have to push harder. In end, it's ssme power output and same energy expenditure.

Seattle Forrest

What you say ^ is completely true. But, again, the difference between any two humans on road bikes is going to be within 5 %.

I guess it gets to be a question of what do you need to know for? Calories burnt isn't a useful metric for training, it's really a diet question. Assuming 1 kJ = 1 kCal isn't perfect, but we're talking a maximum error of a single oreo every 30 miles.