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Watts output, calories burned, and that sort of thing...(9 posts)

Watts output, calories burned, and that sort of thing...mk_42
Sep 10, 2001 11:40 AM
I've seen from several sources (this month's Bicicling most recently) that a ~150lb rider going at about 18 miles per hour burns about 850 calories an hour. But that converted into power is (850*1000*4.1868)/3600 = 988.55W. That seems like a crazy amount or energy burned.

But going from the other angle (analytic cycling model) for a ~80 Kilo rider/bike, going 8m/s we get a more believable power output around 100 W. Using this figure and going the other way we get around 85.98 (food)calories per hour (=(100*3600)/(4.1868*1000)). That seems ridiculously little.

Is it possible that the calories burned by the body equal 850 while the calories output by the rider/bike system are a measly 85 (meaning that the body as a machine is 10% efficient)? I thought pros output was on the order of 500W for an they get this by raising the efficiency or do they burn 5000 calories an hour?

What gives?


My conversions:
1 food calories = 1000 physics calories
1 hour = 3600 second
1 calorie = 4.1868 J
1 J/s = 1 W
The efficiency of the human motor is not so great ...Humma Hah
Sep 10, 2001 12:05 PM
We've got a lot of overhead. You probably burn close to 300 calories per hour just sitting around. The circularory system is draggy, and as you get the blood pumping, the heart itself becomes a pretty big sink of energy. The lungs are also not particularly efficient. The rest of the muscles are generating quite a bit of heat.

There's also the aerobic/anaerobic problem. There are two stages to sugar metabolism, the EMP, and the Krebs cycle. EMP is the fermentation process, produces little energy per sugar molecule, and a lot of noxious waste (things like lactic acid). The Krebs cycle oxidizes these byproducts and gleans much more energy. But strenuous muscle contractions use mostly the EMP, with the Krebs cycle being a recovery process.

The result, humans typically lose efficiency as power output increases toward anaerobic work. Athletes become more efficient in this regard as they train.

Exercise physiologist rarely try to draw the kind of equality you are proposing. They measure watts output as "work" by the physics standards of force and distance, and usually measure calories burned by O2 utilization. The relationships between these can be quite complex, with O2 uptake frequently lagging behind work output by some minutes, and big non-linearities as different biochemical machinery comes into play (fat burn, carb burn, anaerobic, etc.)

Automobile engines are usually around 20% efficient.
Interesting site...SamDC
Sep 10, 2001 12:19 PM
I don't know the approximate number for the efficiency of a human body (I, however, always that humans were actually very efficient, but I think it's a bit harder to measure than, say, an internal combustible engine). Anyway, found an interesting site if you're interested in more info:
Sep 10, 2001 3:06 PM
you burn 40-60Cal in sleep and ~90-120 awake. Surprisingly brain consumes a lot (40Cal/hr).

Yes humans not efficient, the only reason why man on the bike is more efficient then car is because we don't carry around 2tons of dead weight, and we travel at slower speeds.
I used to see the claim ...Humma Hah
Sep 10, 2001 3:17 PM
... that the bicycle is the most efficient machine on Earth. I've since concluded that the person who came up with that either didn't ride, or didn't ride very HARD.

The truth is, the roller chain is about the most efficient form of power transmission, slightly better than spur gears in oil, around 98% efficient. On a bike with nice, high-pressure tires and a good surface, at low speeds, a bike is pretty efficient.

But ridden correctly, they're inefficient as all get-out. I've drag-tested mine, which is a fast downhiller. It'll max out at 42 mph on a hill on which my draggy pickup truck will coast at 70 and I have to hit the brakes to keep it that low.

I'm sure bikes are more efficient than walking or running, but anyone who claims some kind of ultimate efficiency is riding slower than even I do!

But that two tons of dead weight is a real issue. Kar R Koffins, as we say on the singlespeed forum.
I'm too lazyJon Billheimer
Sep 10, 2001 3:54 PM look up some tables in my old files, but going from memory the human machine is about 25%
efficient. The often quoted efficiency of the bike refers to its own mechanical efficiency and does not figure
in the inefficiency of the power source, e.g. you and me. HH's analysis is spot on about the complex
relationship between watts and calories burned. The number I seem to remember from an old Jim
Martin article in Bicyclist is about 600 Kcal per hour for your 80kg rider/bike combo on flat terrain in
calm conditions @ 30 kph. And I would think the wattage output would be in the neighbourhood of
160 or so. Just a guess.
Bingo!!!!!!!!!Kerry Irons
Sep 10, 2001 5:28 PM
Lazy, maybe, but a pretty good memory. A 150 lb cyclist on flat ground burns about 610 calories per hour at 20 mph, and this translates to about 165 watts delivered to the pedals - 0.276 watts delivered per calorie burned. This assumes the generally accepted 24% efficiency of the human body. As usual, Bicycling can't even simple references right. The numbers I list have been published over and over again in several high quality references. That huge 850 calorie number is derived from some old studies on baloon tired bikes with untrained riders. Bicycling suffers another cranial/rectal inversion, and appears to be enjoying the view.
this I assume excludes recumbents and/or aerobars?cyclopathic
Sep 10, 2001 6:05 PM
when I was riding in Vermont 3 weeks ago I was going 50mph on downhill and guy passed me still on his recumbent with flairs. He was doing ~70mph. Of cause he was ~60lbs more then I.

Lots of bicycle inefficiency at higher speeds comes from aerodrag. Larger frontal area vs lower weight, higher drag coef (.5 vs .3) and your car is rolling way faster on that downhill ;-)
regarding efficiencyDuane Gran
Sep 11, 2001 3:41 AM
Just the other day I was reading something from the makers of Power Cranks about this subject:

Scroll down to the "theoretical maximum efficiency" section and to paraphrase, the upper limit of muscular efficiency would be around 50% of calories burned. Heat as a byproduct of the chemical reaction uses up a lot of your efficiency. Errant movements that don't translate into the drivetrain consume the remainder. According to this group, we typically are only 16-22% efficient.

Incidentally, the idea of Power Cranks is appealing, however their research fails to demonstrate that their product can bridge much of the delta and bring you closer to 50% efficiency. They also rest upon the assumption that the most efficient movement is circular, which is a controversial statement at best.