Interesting Observations - Power Meter Related (calories)

LockyHK

Hi All,

Thought I would post some observations I have noticed the last couple of weeks since having my PowerTap G3 fitted.

I am quite analytical and used to be extremely interested in sports nutrition and energy expenditures etc. It was with this in mind I thought it may be of interest to compare a route I often ride (4-5 times a week) and see the energy output recorded with and without the power meter.

The route is 58km's and I have done it over the last two weeks each time between 2hr00 and 2hr05min. The average speed is 28km/hr +/-2km/hr.

Calorie Expenditure Garmin Connect - Garmin 520 with HR Strap (first beat technology) - Garmin Speed / Cadence Sensors (no power meter)

Calories: 1775, 1652, 1772

Strava Calories - Same 3 rides - same setup (no power meter)

Calories: 1061, 1088, 1105

PowerMeter kJ Energy output (Strava and Garmin connect kJ within 5 of each other)

Calories: 1427, 1440, 1390

From this it appears, in my case anyway, Garmin over estimate expenditure, Strava is quite a bit under and the truth lies somewhere between the two.

And yes before anyone says it, I know it's not all about the numbers, this is purely out of my own interest and thought others might find it helpful.

Cheers

fat daddy

LockyHK wrote:

From this it appears, in my case anyway, Garmin over estimate expenditure, Strava is quite a bit under and the truth lies somewhere between the two.

errr, no ...... in this case, Garmin has applied a formula to guess the cals, strava has applied a different formula to guess, and the truth is, you have burnt some calories that might be either side of these 2 but not necessarily in between.

chances are they are both over reading and you have burnt quite a bit less

I use reported calories on these devices as a form of TSS score to judge how much stress I put on my body .. but even that seems random as a bitching short hill climb that kills me for the rest of the day seemingly uses less calories than a tempo paced ride over the same time but on the flats covering a lot more distance :?

LockyHK

I'm taking the power meter expenditure to be rather accurate (within a couple %) and comparing the guesstimated garmin and strava calculations to that.

Man Of Lard

You can see the effect of all the fields on the calculations by using a tool like https://www.fitfiletools.com/index.html on the same ride data and stripping out power and/or heart rate...

Calories derived from a power meter should be most accurate but there is often an assumption made that the total energy delivered in kJ as measured by the power meter is the same in Calories (kilocalories) - so they assume that you are ~23.9% efficient (typical range of values is 20-25%).

If you are 20% efficient, the ~4% deviation that the 23.9% assumption represents 15% deviation from reality. If your power meter is accurate to within ±2% the calories figure could be as much as 17% out (but still most likely a better estimate that the calculation without a power measurement source)

Alex99

Man Of Lard wrote:

You can see the effect of all the fields on the calculations by using a tool like https://www.fitfiletools.com/index.html on the same ride data and stripping out power and/or heart rate...

Calories derived from a power meter should be most accurate but there is often an assumption made that the total energy delivered in kJ as measured by the power meter is the same in Calories (kilocalories) - so they assume that you are ~23.9% efficient (typical range of values is 20-25%).

If you are 20% efficient, the ~4% deviation that the 23.9% assumption represents 15% deviation from reality. If your power meter is accurate to within ±2% the calories figure could be as much as 17% out (but still most likely a better estimate that the calculation without a power measurement source)

Unless you get lab tested to measure your gross efficiency, then it's guess work. Admittedly, educated guess work. It makes sense that the HR estimate will be the most approximate because as we know, people's tickers vary a lot.

Alex99

LockyHK wrote:

Hi All,

Thought I would post some observations I have noticed the last couple of weeks since having my PowerTap G3 fitted.

I am quite analytical and used to be extremely interested in sports nutrition and energy expenditures etc. It was with this in mind I thought it may be of interest to compare a route I often ride (4-5 times a week) and see the energy output recorded with and without the power meter.

The route is 58km's and I have done it over the last two weeks each time between 2hr00 and 2hr05min. The average speed is 28km/hr +/-2km/hr.

Calorie Expenditure Garmin Connect - Garmin 520 with HR Strap (first beat technology) - Garmin Speed / Cadence Sensors (no power meter)

Calories: 1775, 1652, 1772

Strava Calories - Same 3 rides - same setup (no power meter)

Calories: 1061, 1088, 1105

PowerMeter kJ Energy output (Strava and Garmin connect kJ within 5 of each other)

Calories: 1427, 1440, 1390

From this it appears, in my case anyway, Garmin over estimate expenditure, Strava is quite a bit under and the truth lies somewhere between the two.

And yes before anyone says it, I know it's not all about the numbers, this is purely out of my own interest and thought others might find it helpful.

Cheers

Thanks for putting this together BTW.

bompington

fat daddy wrote:

chances are they are both over reading and you have burnt quite a bit less

Meaning that his power meter is reporting more than 50% over the true figure? :?

TBH though, I have always assumed that Strava is overstating my calorie count.

drwae

You burned 1400 calories riding 58km / 2 hours? :shock: Do you live in the Alps? For that length of ride Strava tells me I only burned 1000 calories.

Man Of Lard

drwae wrote:

You burned 1400 calories riding 58km / 2 hours? :shock: Do you live in the Alps? For that length of ride Strava tells me I only burned 1000 calories.

There is the possibility that he weighs 1.4x as much as you...

Alex_Simmons/RST

LockyHK wrote:

Hi All,

Thought I would post some observations I have noticed the last couple of weeks since having my PowerTap G3 fitted.

I am quite analytical and used to be extremely interested in sports nutrition and energy expenditures etc. It was with this in mind I thought it may be of interest to compare a route I often ride (4-5 times a week) and see the energy output recorded with and without the power meter.

The route is 58km's and I have done it over the last two weeks each time between 2hr00 and 2hr05min. The average speed is 28km/hr +/-2km/hr.

Calorie Expenditure Garmin Connect - Garmin 520 with HR Strap (first beat technology) - Garmin Speed / Cadence Sensors (no power meter)

Calories: 1775, 1652, 1772

Strava Calories - Same 3 rides - same setup (no power meter)

Calories: 1061, 1088, 1105

PowerMeter kJ Energy output (Strava and Garmin connect kJ within 5 of each other)

Calories: 1427, 1440, 1390

From this it appears, in my case anyway, Garmin over estimate expenditure, Strava is quite a bit under and the truth lies somewhere between the two.

And yes before anyone says it, I know it's not all about the numbers, this is purely out of my own interest and thought others might find it helpful.

Cheers

Some comments:

1. Energy metabolised is not the same as mechanical energy delivered to the bicycle. Strava and Garmin (sans power meter) are attempting to estimate the former, while the numbers when using a power meter are measuring the latter (i.e. mechanical energy delivered via the hub in your case).

2. Typically cyclists are in the range of 19-24% efficient (and it's individually variable, as well as chronically and acutely variable as well), meaning that energy metabolised = mechanical energy / efficiency.

So for example, if we assumed 22% efficiency for your first ride using the power meter, then mechanical energy of 1427kJ / 0.22 = 6486kJ energy metabolised. There are 4.184kJ per kcal (or Cal), so that's 1550 Cal metabolised, although depending on your efficiency it might have been anywhere from 1400 to 1800 Calories.

Nailing that down to within a tighter range typically applicable for you (but recognising there are still other factors that will affect GE) would require, as has been said, a laboratory test to establish your gross efficiency.

3. Estimates of energy metabolised (or power output) by devices and websites are generally not a lot more than light comic relief. A good power meter at least gives a decent sanity check.

4. As a rule of thumb, energy metabolised in units of Cal will be ~1.05 to 1.15 x mechanical energy in units of kJ, or say 1.1 x +/- 5%.
e.g. 1427kJ mechanical energy converts to 1427 x 1.1 = 1570Cal +/- 80Cal

buckmulligan

Following on from Man of Lard's and Alex's comments, in fairness to Garmin I believe they employ different values for gross efficiency based on your level of power output.

On some very boring turbo efforts I've had both "Calories" and "kJ" fields displayed next to each other and I noticed (IIRC) that at high power outputs the Calories ticked up slightly faster than kJs and at lower efforts the kJs ticked up slightly faster than Calories (although this could be vice versa, it was a while ago!). Seeing as the two were usually quite close together it was interesting to watch one overtake the other in work-periods and then fall back again during the recovery-periods. This was with a power meter, so I can only assume that the Garmin firmware was applying some kind of logic to the calculation of Calories Spent rather than a simple kJ multiplied by fixed gross efficiency percentage calculation.

That being said, it's still an estimate that's not based on your personal physiology at all so definitely take it with a pinch of salt.