Polar S720i calorie measurement seems way off

Hi All,

I'm talking about the "OwnCal" feature of the S720i, which
gives you an estimate of how many calories you've burned
during a workout. I'm using it on a trainer at the moment,
but the results aren't very different on real terrain.

Anyway, the thing tells me I'm burning almost 9 calories/minute,
on a 75-minute ride that starts off fairly gently for the first
ten minutes or so, and picks up intensity only as I go along.
So for today's session, e.g., the gadget tells me I burned 650
calories. The problem is, this corresponds to about 600 Watts,
and I seriously doubt that I'm putting out that much power.
I've told the HRM my weight, age, and other user parameters,
so this should not be a cause for error.

I'm in decent shape, but I'm almost 60 years old, with typical
HR ranging between 115 and 150 on these "rides." As another
point of reference, I'm averaging about 20 km/hr on a CycleOps
magneto trainer (new model). According to the chart for this
trainer at http://www.geocities.com/almost_fast/trainerpower/,
20 km/hr corresponds to a measly 100 Watts, but in this case
the calories burned should be closer to 110, rather than 650.
Thus, there is a serious discrepancy between the OwnCal report
and what seems reasonable.

I have two questions:

1. Is there someone out there who has the power attachment, who
would be kind enough to report the two measurments -- average
power and calories burned -- at the end of a comparable ride?
Failing that, even just a result from OwnCal would be helpful
in figuring out where the problem lies.

2. I did the conversion like this:

650 cal X 4.2 X 1000 = 2730000 Joules / 4500 sec = 607 Watts

(The factor of 1000 is in there because one dietary calorie is
actually a kilocalorie, but the power is given in Watts, not
kilowatts. The Polar software knows about the factor.)

Did I miss something?

Thanks!

AMG
--------------------------------

amg001(at)earthlink.net

--------------------------------

AMG wrote:

2. I did the conversion like this:

650 cal X 4.2 X 1000 = 2730000 Joules / 4500 sec = 607 Watts

Did I miss something?

Yes. Efficiency in turning food calories into work. Depending on the
person, gross efficiency runs in the neighborhood of maybe 20% or so.
Thus, you're overestimating average power by a factor of about 5. Your
average wattage for that workout should be somewhere around 120ish watts.

I wrote:
AMG wrote:

2. I did the conversion like this:

650 cal X 4.2 X 1000 = 2730000 Joules / 4500 sec = 607 Watts

Did I miss something?

Yes. Efficiency in turning food calories into work. Depending on the
person, gross efficiency runs in the neighborhood of maybe 20% or so.
Thus, you're overestimating average power by a factor of about 5. Your
average wattage for that workout should be somewhere around 120ish
watts.

That's 120ish watts based on the Polar OwnCal estimate. The speed-power
curves from http://www.geocities.com/almost_fast/trainerpower/ are taken
from actual measurements with a PowerTap and, if the tire-roller interface
is consistently maintained (same tire, same tire pressure, same
tire-roller pressure), should be more accurate and consistent than
estimates based on OwnCal.

AMG wrote:

I've told the HRM my weight, age, and other user parameters,
so this should not be a cause for error.

The Polar OwnCal measurement is sensitive to your entered max. heart
rate and VO2max. If the numbers look high, you may have set your
VO2max too high (or your max. heart rate too low). Then again, about
500 kcal/hr seems about right for a peppy pace. I see up to 750
kcal/hr on a fairly high intensity ride, and I've read that pros
ride at 1000 kcal/hr.

--
terry morse Palo Alto, CA http://www.terrymorse.com/bike/

Terry Morse wrote:
AMG wrote:


I've told the HRM my weight, age, and other user parameters,
so this should not be a cause for error.


The Polar OwnCal measurement is sensitive to your entered max. heart
rate and VO2max. If the numbers look high, you may have set your
VO2max too high (or your max. heart rate too low). Then again, about
500 kcal/hr seems about right for a peppy pace. I see up to 750
kcal/hr on a fairly high intensity ride, and I've read that pros
ride at 1000 kcal/hr.


kcal? Channel 9 in LA?

Greg

Hi Robert, and thanks for the response.

To clarify, you say that I am in fact burning the 650 calories, as
indicated by OwnCal -- but only 120 or so is going into pushing the
wheels around? In this case, both OwnCal and the power curves could be
more or less correct, but a power attachment should then show the lower
number.

Anyone out there to verify?

Thanks a lot!

AMG


Robert Chung wrote:
AMG wrote:

2. I did the conversion like this:

650 cal X 4.2 X 1000 = 2730000 Joules / 4500 sec = 607 Watts

Did I miss something?


Yes. Efficiency in turning food calories into work. Depending on the
person, gross efficiency runs in the neighborhood of maybe 20% or so.
Thus, you're overestimating average power by a factor of about 5. Your
average wattage for that workout should be somewhere around 120ish watts.

I wrote:

I see up to 750 kcal/hr on a fairly high intensity ride,
and I've read that pros ride at 1000 kcal/hr.

I did some more googling and found that an easy pace burns about 300
kcal/hr, while Lance battling up Luz Ardiden burns about 1800
kcal/hr. The rest of us fall within that range, mostly in the bottom
half.

A Tour de France rider burns about 8600 kcal/day, a RAAM rider about
8000 kcal/day. Pretty similar, no? No. The TdF rider spends 4-5
hours per day riding, the RAAM rider about 19.
--
terry morse Palo Alto, CA http://www.terrymorse.com/bike/

"AMG" wrote in message
news
Hi All,

I'm talking about the "OwnCal" feature of the S720i, which
gives you an estimate of how many calories you've burned
during a workout. I'm using it on a trainer at the moment,
but the results aren't very different on real terrain.

Anyway, the thing tells me I'm burning almost 9 calories/minute,
on a 75-minute ride that starts off fairly gently for the first
ten minutes or so, and picks up intensity only as I go along.
So for today's session, e.g., the gadget tells me I burned 650
calories. The problem is, this corresponds to about 600 Watts,
and I seriously doubt that I'm putting out that much power.
I've told the HRM my weight, age, and other user parameters,
so this should not be a cause for error.


Although I use a different model Polar, I get similar results in terms of
calories used. I've always felt this probably wasn't accurate, but it was
useful in a relative sense comparing one workout to another. Is this a fair
assumption? Also use it as a reminder to consume water and food (on longer
rides) as I see the calories click by.

Thanks all for responding.

To summarize, it appears that OwnCal does estimate the calories burned
during a workout, but the power actually going to the wheels is only a
fraction of that amount. A power meter should provide a measure of the
latter.

Interestingly, there is an article in the March issue of Natural History
magazine (should be online soon, at www.naturalhistorymag.com. Click on
Table of Contents, then scroll down and look for the Biomechanics
feature, titled "Meddling with Pedaling.") The author discusses the
efficiency of bicycles over the years, and in doing so mentions two
numbers that come to bear on this discussion: (1) Walking at a brisk
pace burns about 350 Watts, and (2) modern bicycles are about 27%
efficient, meaning that 27% of the total energy cost of cycling actually
goes into turning the wheels.

The first of the these numbers agrees with my perception that 600 Watts
on the trainer seemed too high (since 350 Watts for walking also seems
high!) In fact, it is probably correct, but refers to the total caloric
expenditure involved in pedaling. The second number indicates that of
this amount, perhaps 160 Watts is actually going to the wheels; this is
roughly in line with the power curve of the trainer, as well as what I
have read about the power output of TdF cyclists, as in 400 Watts
continuous power on long rides, twice that for sprints.

Well, time for the afternoon "ride." This time it will be with deeper
understanding of what the HRM is telling me :-)

AMG

--------------------------

amg001(at)earthlink.net

--------------------------

"User X" wrote in message
...
Thanks all for responding.

To summarize, it appears that OwnCal does estimate the calories burned
during a workout, but the power actually going to the wheels is only a
fraction of that amount. A power meter should provide a measure of the
latter.

Interestingly, there is an article in the March issue of Natural History
magazine (should be online soon, at www.naturalhistorymag.com. Click on
Table of Contents, then scroll down and look for the Biomechanics
feature, titled "Meddling with Pedaling.") The author discusses the
efficiency of bicycles over the years, and in doing so mentions two
numbers that come to bear on this discussion: (1) Walking at a brisk
pace burns about 350 Watts, and (2) modern bicycles are about 27%
efficient, meaning that 27% of the total energy cost of cycling actually
goes into turning the wheels.

The first of the these numbers agrees with my perception that 600 Watts
on the trainer seemed too high (since 350 Watts for walking also seems
high!) In fact, it is probably correct, but refers to the total caloric
expenditure involved in pedaling. The second number indicates that of
this amount, perhaps 160 Watts is actually going to the wheels; this is
roughly in line with the power curve of the trainer, as well as what I
have read about the power output of TdF cyclists, as in 400 Watts
continuous power on long rides, twice that for sprints.

Try at least 4 times that for guys like Cipollini or Pettachi...and 5-6
times that for an elite match sprinter.

Andy Coggan