Infiniti exercise bikes - reported calories

[NickF]

I am sometimes training on my exercise bike and I am trying to get some perspective about what my FTP is and about what amount of food I am burning during a training session. From the reported used calories, I tried to calculate the average power I generate, in watts. And it doesn't seem right. I will explain below what I am trying to do.

The known values:
One Joule = 0.2389 calorie.
An ultimate double whopper from Hungry Jacks has 4710 kJoules or 1125 kcalories.
A cyclist FTP can be around 200 W. My understanding is that he/she can generate 200 W for an entire hour.
One watt = 1 Joule / second.
A cyclist who trains at an average power of 200 watt, produces (and consumes) an amount of energy of 200 Joules every second.
It means over 30 minutes, there is an energy consumption of 360,000 Joules or 360 kJoules or 86 kcalories.
That also means, if somebody eats an ultimate double whopper on top of their regular meal, they'd have to train for about 6.5 hours (at an average of 200 W) to consume the extra calories. Although surprising, it seems correct to me.

On my exercise bike, I train in sessions of 10 minutes (yesterday I did 2 sessions with half an hour break and I was tired enough). At the end of each session, the bike reported I used 110 calories. That is obviously incorrect, it should rather be 110 kcalories and even that appears incorrect. During the training, the reported instantaneous power varied between about 50 W to 450 W. That means the average power should be somewhere between 50 W to 450 W (which seems right to me). But if I convert 110 kcalories consumed in 10 minutes, that results in an average power of 767 W, which is simply too high.

For a new cyclist, an expected FTP is 2, while a top cyclist can have a FTP of 7. For me, I think I may have about 2.5. At 67 kg, that means I would expect to deliver 167 W for an hour. For ten minutes, I could probably reach 200 W or 120 kJoules or 28.6 kcalories. Considering my Infinity exercising bike shows 110 cal, I assume the correct unit should be 110 kJoules, not 110 (k)cal.

Does this seem right to you?

I'd be curious if any person who has an Infiniti exercise bike could report their FTP value (or level of fitness), number of calories after a session of 10 minutes and the model of bike. The model is Infiniti PG725.

NickF

[NickF]

I found out there is a definition for energy for food, and 1 Calorie = 1 kcalorie = 1000 calorie. The capital C should be used to avoid the confusion between the units, but in practice few people care about the difference. On my exercising bike, it's written all in uppercase, but it represents food Calories.
I found a table which shows that for vigorous cycling on a stationary bike, for a 140 lb (63.5 kg) person, the energy burnt in 30 min is 331 Cal, which matches my results on the Infiniti bike.

In this case, the conversion to watts will give the unexpected results I mentioned. There may be an explanation if the efficiency of conversion of food energy (= chemical energy) to mechanical energy would be around 25%.

https://i.pinimg.com/originals/e1/90/65 ... eb6875.jpg

I did a search for food to energy conversion by the human body and I found an article where the author indeed mentions an efficiency of about 25%: https://dothemath.ucsd.edu/2011/11/mpg-of-a-human/

Now everything is clear - for the calculated 767 W of average power, assuming a conversion efficiency of 25%, the mechanical power (to the pedals) would be around 192 W. A cyclist consumes 767 W of energy (food and reserves such as fat) and produces 192 W of mechanical energy to the pedals.

[Alex Simmons/RST]

To know how much energy you have metabolised while cycling you need to know:
either:
i. total mechanical energy imparted to the cranks of the bicycle, and
ii. your Gross Mechanical Efficiency
or
iii. your gas exchange data (measurements of O₂ utilisation and CO₂ output)

iii. requires specialist lab equipment and good experimental protocol. So that's out.

i. requires a means of measuring power output. There are specialist power measurement devices for bikes that do this, and some indoor bikes also have them. Many however estimate the power output based on the speed of the flywheel rather than actually measuring the forces involved. Accuracy/reliability can be quite variable (not just between bikes but also for the same bike). I've no idea how accurate or reliable your exercise bike is.

ii. For cyclists riding in mostly steady state aerobic effort levels, gross mechanical efficiency is typically in the range of 18% - 24%. To know what gross mechanical efficiency you typically ride at requires laboratory testing as per iii. above (but keep in mind it is acutely variable depending on various factors).

As a rough rule of thumb, the Calories metabolised while cycling will be roughly equivalent to the number of kilojoules of mechanical work done with a multiplication factor of ~1.1 (i.e. add 10%).

e.g. say you have accurate power measurement and the mechanical energy expended for a ride was 100kJ.

And let's assume a GME of 21%.

Then 100kJ / 21% = 476kJ of energy metabolised

476kJ / 4.18kJ/Cal = 114Cal of energy metabolised

Now many exercise machines are doing this calculation for you, and displaying an estimate of Calories metabolised.

The accuracy of this estimate is dependent upon the accuracy of the bike's power measurement (which can be quite variable) and the assumption used for the individual's Gross mechanical Efficiency, which is variable by individual.

[NickF]

Thank you very much, Alex. You explained it very nicely. I don't have power measurement devices on my bike, the price of them is pretty high. I've seen some Garmin pedals starting from around USD 750 or so. Since my exercising bike reports the instantaneous power and calories, I thought I can estimate my FTP using the bike.

I am quite sure that the power measurement of my bike is based on the flywheel speed - otherwise the bike price would be significantly higher. But I would say it is pretty consistent, if I train on different days I get similar results. Perhaps, as you mentioned, the number of calories is calculated based on the average GME of 21%. By reversing the calculus, I can get the measured mechanical energy and deduct a rough FTP for myself.

[NickF]

I found an online calculator that estimates the speed based on power, position on bike, wind speed, etc.
http://bikecalculator.com/.

In my case, I estimate I have an average speed of 35 km/h for no longer than one minute. That means I can apply 210 W for one minute and there is no way I could maintain this power for over one minute.

If I have to ride on horizontal surface and without wind for 20 minutes, I think could reach perhaps an average speed of 27 km/h (I may check that in the weekend). In that case, the average power would be 104 W, or an FTP of 98.8 (obtained by multiplying by 0.95).
If all this is correct, then I have a lousy power to weight ratio of 1.47.

I found a table in here: https://cyclingtips.com/2017/06/just-ho ... -cyclists/
This chart says that an average male cyclist could obtain a power to weight ratio of 6.10 for one minute. In my case, at 67 kg, it means I could generate a power of 408.7 W for an entire minute. If my exercising bike power reading has any accuracy, that would be impossible for me, since I cannot keep 400 W for over three seconds . My average speed, if I could apply 408.7 W of power to pedals for one minute, would be 45 km/h.

Based on my race results, for my best average speed of about 35 km/h for one minute or less, the average power is 210 W and the maximum power to weight ratio is 3.13 - about half of the performance of an average untrained cyclist. It seems I'd have to put a lot of work to reach the level of "average untrained". Maybe they need to extend their table, to make room for myself as well

If the above table is correct, then my short segments I am racing should have significantly higher speeds than they do now. Some of the riders on the top places have tens of thousands of kilometers under their belts, I wouldn't call them "average untrained" cyclists. Maybe none of them is trying to get a good time on those segments?

[NickF]

I've done two rides today, about 40 minutes each, 18 km each. On the first one I obtained an average speed of 26.9 km/h and on the return one an average speed of 26.1 km/h. The elevation was 137 m and 149 m. Strava reported an average power of 125 W on first run and 118 W on second run. If I take the average speed of 26.5 km/h, for my weight of 67 kg and assuming a bike weight of 10 kg, the average power estimated by the website http://www.bikecalculator.com is 125 W, which is in pretty good agreement with Strava. That is, assuming one rides on the hoods and not on the drops (which is significantly more efficient). In fact, I mixed both styles.

Since instead of 1 hour, I've done two sessions of 40 minutes, with about 10 minutes of break in between, I consider this would be the power I could maintain for an entire hour. I would say I gave about 95% of my best effort. I also neglect the elevation of the road. As a result, my power to weight ratio is about 1.87. Had I put more effort into it, perhaps I could get a 2.0. According to the table I referred before, that puts me in a category slightly below "average untrained". I am pretty confident in this evaluation.

Considering my age (51) and the fact I haven't practiced any sports or done any regular intense activities for at least ten years, this is an expected result. It is an eye opener about the performance of world class athletes, who can generate over 3 times the amount of power per kilo, compared to "normal" people. The good news is that I can notice an improvement in my fitness level, since I started cycling 3 months ago.

There is one point which is puzzling to me. Based on my FTP and the table https://cyclingtips.com/2017/06/just-ho ... -cyclists/, I should expect a 1 minute sustainable power of 388 W. Going back to the calculator, for that power I'm expected to reach and maintain a speed of 43.63 km/h on a flat surface. I don't think I can even reach this speed for any amount of time, rather than have it as average speed. Maybe those guys are over-estimating the capacity of an "average untrained" cyclist? Or perhaps a 20 yo average untrained cyclist is capable of generating this much power for one minute.

In my case, assuming an average speed of 36 km/h for one minute, riding on the drops, that results in a ratio of only 3.23 (for a power of 217 W), which is not even covered in the table (lowest value is 4.92). It is possible that due to the clothes I'm wearing, the drag coefficient is higher than normal, but even compensating, the result would still be probably under 3.5.

In the next weeks, I will try to maintain on the exercise bike a power above 210 W for one minute, then 220, 230, etc to find my 1 min power. Then I will compare if the one minute power results correlate with the racing results.