spivey44

I find this a bit curious.

Just did a 15 miles road ride. kept my HR between 120-130 . Garmin shows caloric output of 1,300 cal. I was not really tired following the road ride.
No power meter, just Garmin 500 uploaded to Garmin connect.

Yesterday did an 8 mile mountain bike loop, climbs,drops etc. Heart rate at 138-150 . Exhausted, sweating like a pig. Caloric output was only 786 calories. Again no power meter.

Judging by my "perceived level of exertion" i would have expected the opposite in terms of caloric output. Both rides took right at 1 hour give or take a minute or 2.

Yes the road ride was almost twice as far but like i mentioned not really tired.

Any thoughts on this?

Sixty Fiver

Don't trust the Garmin.

vesteroid

All I can say is there is no way you burned 1300 calories in an hour of easy riding. No way.

spivey44

Thats what I was thinking. I have no clue how Garmin "estimates" this.

If it were accurate I'd be weighing in at about 100 pounds.

Looigi

Garmin calorie calculations are primitive and notoriously far off, usually under by 50% or more. I have a Garmin but use an equation plugging in gender, age, weight, Avg HR, and duration. I checked the eqn against calories determined by a power meter and found that for me I needed to correct the equation downward by multiplying by 0.7.

Calories Burned = [[(years x 0.2017) + (lbs x 0.09036) + (BPM x 0.6309) - 55.0969] x minutes / 4.18] x 0.7

On my typical ride I burn 600-700/hr.

DaveWC

I know you said you don't have a power meter, but for those that do, here's a formula to estimate calories burned:

(Avg Watts X 0.8604) / 0.24 X # of hours or

(Avg Watts X 0.8604) / 0.24 / 60 X # of minutes


If you typically average 250 watts that works out to 896 calories per hour. If I use Looigi's formula I get 734 calories per hour.

jsutkeepspining

To do a quick kj check you can just do 60/(1000/avg power) times 60=hourly kj exertion. then you multiply by 1.1 and you get your caloric expenditure for the ride.

mtalinm

Put half your weight in the Garmin and it will be close...ish. That's what I do.

gregf83 

I just assume Calories = kJ so multiply avg power by 3.6.

bikerjp

I wouldn't put a ton of faith in it, but with a HRM I think it's not too far off. Here are three different days of the same ride and very similar conditions. Calories are very consistent. Consistently high maybe but for a 90 min ride with a fair bit of climbing it isn't that far off what would be expected. I've seen 500-600 per hour on flat roads thrown around a bit. A little more than 1,000 for this ride seems reasonable.

https://connect.garmin.com/activity/266813939
https://connect.garmin.com/activity/266350584
https://connect.garmin.com/activity/264743565

OP, did you use a HRM? If not, all bets are off. If so, something is wonky.

Drew Eckhardt 

You're neglecting that people aren't very efficient. If we were cyclists wouldn't get hot after a hard ride at 40 degrees F wearing the lightest wind stopping jacket and skiers wouldn't be opening their shells on 20 degree F days to cool off.

1 Joule = .24 calories from the unit conversion (1 calorie is the energy it takes to raise the temperature of 1 gram of water 1 degree C; 1 Joule is 1 Newton applied for 1 meter; there's intermediate arithmetic I don't care enough to look up).

With gross metabolic efficiency at a high of 25%, you can multiply by 1/.25 or 4 to find 1 Joule = .96 calories in or 1kj = .96 Calories down the hatch.

At a low of 20% you can multiply by 1/.20 or 5 to find 1 joule = 1.2 calories or 1kj = 1.2 Calories.

Assuming 1kj = 1 Calorie is a reasonable approximation unlikely to have you eating too much food to balance what you're spending.

Revisiting the toasty scenario a rider delivering 200W to the cranks is producing 600-800W of heat.

With approximately half a space heater inside your jacket it's no surprise you sweat.

gregf83 

I'm nor sure what you mean by 1:1. My formula assumes about 24% efficiency which is conservative. Most likely you'd burn a few more calories but most people underestimate how much they eat.

Drew Eckhardt 

Your formula is wrong and would only work for some sort of machine or alien that's 86% efficient.

1 kj = 1 Calorie is almost exact at 24% efficiency which is close to the high end for cyclists and therefore a reasonable approximation.

gregf83 

That's exactly what I said. If I ride with an average power of 200W I do 3.6x200=720kJ of work which I assume equates to 720Cals.

edit: My original response was for calculating the number of Calories burned per hour, hence the 3.6. I can see I might not have been too clear.

benajah

GPS monitors for calorie burn are way off...but we simply don't have a meter of any sort that can measure calories burnt in a workout.
Compare watts to joules, etc, it doesn't account for energy your body uses to keep you warm, or cool, doesn't account for daily metobolic fluctuations that make your body very efficient one day, very wasteful the next.
end of the day, any electronic gadget that says it can measure calories burned, is lying to you.
The human body is the greatest endurance machine nature has ever built, and is way, way too complex for the level of technology we are at today to measure beyond simple things like watts expended, speed, hr, and the like.
Twenty years from now, with sensors implanted in the body, maybe.

ivan_yulaev

Are you in running or biking mode? The numbers you throw out are pretty consistent for running (~100-150 cal / mile) but way off for cycling. That could explain it.

spivey44

?? I have Garmin 500 set up for my road bike. Using HR monitor that came with cadence sensor bundle. The HRM is spot on.

Drew Eckhardt 

The HRM is fine. The math converting it into calories burned is not.

Drew Eckhardt 

You used the wrong units.

Calories and joules are quantities of energy with the conversion I mentioned.

1 Watt is by definition 1 joule of energy per second.

As you note multiplying Watts by 3600 seconds per hour yields Joules per hour. Divide by 1000 and you have kj/hour where 1kj is a good approximation for 1 Calorie at the high end of measured efficiencies; or Watts * 3.6 = Calories/hour.

gregf83 

Yes, we're in violent agreement. I just left out the per hour part in my original comment.

bikerjp

Without a PM I'm just guessing here, but would an average of 200w be reasonable for the rides I linked above? I'm heavy and there are a number of steepish climbs. If so,

200 x 3.6 x 1.5 (because it was a 90 min ride) = 1,080 calories which is nearly spot on to what my Garmin says. So either my power is pathetically low (a possibility) or the Garmin +HRM is not hugely off.

All this really makes me want a PM. Maybe I should try and find a used powertap wheel for kicks.

Drew Eckhardt 

As a point of reference 200W might net a 200 pound rider approaching 6' high atop a road bike on the hoods about 21 MPH solo.

You can use the analyticcycling.com calculator to estimate climbs up known grades

https://www.analyticcycling.com/ForcesPower_Page.html

Down-hill and on flat ground most of your power is going into overcoming aerodynamic drag and accuracy requires numbers which accurately describe how slippery you are and your frontal area.

The .5 Cd default is much slicker than what's been measured using professional cyclists in wind tunnels (.760 Cd on the hoods, higher in the drops).

gregf83 

That seems a little optimistic for speed. Kreuzotter.de indicates 200W will get a 200lb rider 17.7mph when riding on the tops or 19.9mph in the drops. Add corners, the odd stop and crosswinds and there is no way 200W will get you a 21mph avg.

bikerjp

Using the resources I have at hand (namely Strava segments and other rides with PMs) I can see that on various long segments (such as this one) of the rides posted above people who have a similar speed to me are averaging between 180 and 220w or more. As I don't know the specifics of all these individuals I don't know who I would be most similar to, but seems I'm in the ballpark.

#42, for example, only netted 16.6mph at 250w and that is apparently data from a PM.