Dopefish905

I don't have a outdoor power meter and haven't used a HRM outside yet, I've always went by the 400~cal/hr burned kinda rule, but my indoor setup is a wahoo kickr2 and wahoo tickr HRM, proper age weight and hight added into Zwift and strava..so I have accurate power and BPM, strava says I burn about 950cal a hour @ a adverage of 250watts and 145bpm. Does this sound right? Would this be correct with accurate power and BPM? I ride about this hard when outside as well and was just wondering if I'm under estimating my calories burned..

Thx!

DrIsotope

That's fairly accurate, depending on your weight. On Tuesday, I averaged 277w over a ride of exactly one hour, and total work was 998kJ. My normal endurance pace, ~225w results in ~800kJ/hr. HR won't impact calorie burn to any predictable degree. Heart rate is dependent on a laundry list of factors, while power is just power.

I don't do the Strava up-estimate for calories, I look at it as 1kJ = 1kcal.

PepeM

250 watts for an hour = 900 kJ ~ 900 kcal.

Dopefish905

Thx gents! Just checked out the watts - kj calculator will be using this more often

Machka 

Personally, I go with 100 cal for every 5 km ... and even that might be a bit high.

Interestingly, Strava matches that fairly well.

DrIsotope

The problem is, the amount of power required to increase speed isn't linear, so 20kcal/km works at one particular speed, but above that, the work required goes up a whole lot for diminishing returns.

An "average rider" (172cm, 71kg) @ 150w will indeed burn around 20kcal/km, but I make 150w walking to the kitchen.

150w = 27kph = 22kcal/km = 594kcal/hr
200w = 30kph = 26kcal/km = 780kcal/hr
250w = 33kph = 30kcal/km = 990kcal/hr
300w = 35kph = 35kcal/km = 1225kcal/hr

If I could manage 27kph (16.8mph) @ 150w, I'd have to ride a lot more miles. I averaged 818kcal/hr this morning at 28.5kph (17.7mph).

Machka 

If you're attempting to lose weight, it can be helpful to use your goal numbers instead of your current numbers and/or to estimate your calories burned on the low side and/or to eat about half your calories back.

To that end, I stick with either 100 cal/5 km or what Strava tells me, whichever is lower ... and then usually only eat a portion of those calories back.

Cyclist0108

Since 4.184 kJ = 1 kcal, is ~1/4 of the energy burned going into work, and ~3/4 into heat?

Machka 

Yeah, I wondered about that too.

Here in Australia, they list the energy on food packets as kJ. Because I don't understand kJ particularly well, I like to translate it to calories (kcal) ... and a quick and easy way to do that is to divide the kJ by 4.

For example, the soup in my drawer is 693 cal. Divide that by 4 and it comes to 173 cal. It's actually 164 cal, but when I'm doing a quick calculation in a grocery store or something dividing by 4 is close enough.


So if indeed 250 watts for an hour = 900 kJ ....

To my mind that would NOT be ~ 900 kcal, that would be ~225 kcal.

DrIsotope

1kj = 1kcal assuming metabolic efficiency ~24%. Some calculators (including the one I used above) go by 22% efficiency.

The calorie calculators already take efficiency into account. If you find a way to cycle at 100% efficiency, let me know. I'd like to do a nice summer ride without losing 2kgs in water weight. I would also be able to ride like... 400 miles a day. At least.

gregf83 

Strava calculates Calories by multiplying kJ by 1.1 hence the higher number reported by the OP.

Cyclist0108

I thought 25% efficiency was too optimistic

gregf83 

250 is the output. Because we're inefficient the 'input', or what we have to burn, is roughly 4x what we get out.

PepeM

That's the assumption. Of course, since we all probably have different efficiencies and don't really have a way of knowing ours, those 'calories burned' estimates should always be taken as such, regardless of how they are being calculated. If weight loss is the goal then I'd say that erring on the lower side would be better. If weight loss isn't the goal then I would just ignore that metric altogether.

FBinNY 

Yes and no. It's going to All purposes other than output, ie. keeping your heart and lungs going, and heat. If you were an engine, you'd be described as having a 25% fuel efficiency.

Cyclist0108

But most of the increased metabolic effort is used to produce the work (at ~25% efficiency) to drive the bike and rider forward or up the hill, so you don't want to count it twice.

At risk of appearing to be slightly pedantic a couple of hours before I run out to teach thermodynamics, the conversion factor of ~4 J/cal is nothing more than just that; the factor of 4 (or 1/4) being similar to the efficiency is purely accidental. It would probably be less confusing to use the same units of energy to describe all of the components of heat and work. (I actually do have students confused by this.)

DrIsotope

Which in an odd coincidence, is about the thermal efficiency of a typical gasoline-powered internal combustion engine.

gregf83 

Except in North America no one tracks their food intake in kJ nor measures their power output in Cals so we're stuck with two units. Fortunately, because of the coincidence you pointed out they end up having about the same values for most people.

FBinNY 

I was trying to point out that some of the increased fuel consumed went to the increased efforts other than what reached the pedals. For example the heart and lungs are working harder than when at rest, as are other muscles besides those moving the pedals. My point was simply that it wasn't all heat.

IMO it's OK to accept and work with rule of thumb numbers, and sometimes that's all we have. But, we have to keep in mind that these are only approximations, and accept the limitations.

For example, it may be reasonable for someone to use 20cal/km, and that may produce fairly accurate results because that person tends to ride at a similar average speed, over similar terrain. However it won't translate to another person who rides faster or slower, is heavier, rides over very different terrain.

So to all those who do so, continue calculating calories by whatever method seems to be working for you, but understand that the answer is an approximation, so you burned 350 or 400 calories, not 373.

Heathpack 

I think this all just gets a little confusing because you hear such wildly different numbers for cal burn per hour from people.

The trick is that it really depends on body size to a tremendous extent. For me at 130 pounds to put out 250 watts for an hour would be incredible, it would make be competitive in the highest levels of racing. For a 220 pound male to put out 250 watts in an hour is pretty much a moderate effort for a good recreational cyclist. So a calorie burn per hour that seems incredible to us is pretty typically for a larger guy.

For me, a moderate everyday type of ride burns about 515 cal per hour (based on power meter data and using Strava's estimate for metabolic efficency) whereas a 40k TT (ie max effort for an hour) is around 725-765 cal. So 50% more cal burned for a max effort. The intensity really does make a huge difference to your final calorie count.

Not that you need a power meter to figure this stuff out. If you are doing endurance stuff, you might mostly ride at say 65-75% intensity, which keeps you at a fairly constant cal/hr burn and means you can use your current rule of thumb and be pretty accurate. In the end, if the goal is weight loss, the bottom line thing is if you achieve your goal, not how exactly accurate your calorie estimate is.

Cyclist0108

I think part of the problem also comes from what is actually being measured by a power-meter, eg:
P = work/time so even within North America, what you are really measuring is energy turned into work per unit time, so to get to Cal or kcal or Joules, you need to multiply the power in Watts by the ride time in Seconds (a Watt is 1J/sec, or 1 kg (m/sec)^2/sec), so that I assume is why you wind up with Joules from a power-meter-based estimate.

Cyclist0108

This confuses me.

250 W = 250 J/sec

1hr = 60^2 sec

so 250 / 60^2 = 900 kJ = 215 kcal = 215 Cals.

Assuming 25% efficiency, you burned 860 Cals in that one hour. So did the 220 pound male.

If you neglect wind resistance and stuff like that, the only differences between a heavy and a light rider should come into play on climbing hills, since the amount of energy the heavier person has to expend to climb the same hill scales with his mass (as does power, if he manages to do it in the same amount of time -- which, based on my own lard-laden personal experience -- he won't).

Seattle Forrest

But not by very much on a bike.

When I run, my Garmin says I waste too much energy bouncing up and down. I could run faster or at least easier, if I did it with less vertical oscillation. That's energy I'm burning up, that isn't contributing to forward motion. My stride length is generally too long, too.

On the bike, I sit on a saddle and turn the pedals in 350 mm circles. When the pedal comes up, I push it back down. So do you. And everyone else.

Cyclist0108

Another interesting distinction is between state and path functions. Energy is a state function. All that matters are the end-points. Work is a path function, so how you get from point A to point B is relevant. So if you measure work, the path you take to climb the hill matters (and the path with a 10% grade differs from the path with a 2% grade). Yet the gravitational potential energy you have to overcome is the same for both paths, so presumably (a good part of) the difference in power is due to the speed at which you climb.

Heathpack 

I'm not exactly sure what you're asking? But I think perhaps what you're confused about is that most ordinary 130 pound women can't generate 250 watts for an hour. If you could, you'd burn the same calories as anyone else.

I just have less muscle mass than a 220 pound male, so my power output will always be less. My speed might actually be faster on less watts. But I'm burning less calories.

Is that what you were asking about?