I'm 115 kg. Today I took two bicycle rides:

  • 42:41 time, 12.1 km distance, 72 m elevation, 17.1 km/h avg. speed, 33.1 km/h max. speed,
  • 50:50 time, 11.7 km distance, 17 m elevation, 13.9 km/h avg. speed, 32.8 km/h max. speed.

Strava calcualted folowing figures about energy in each of these rides:

  • first: 405 calories / 363 kJ energy output,
  • second: 353 calories / 317 kJ energy output.

Ever since I remember I was always told, that 1 kcal = 4,1855 kJ. So these numbers are weird or incorrect. What is energy output in Strava or in general?

  • 2
    "Calories" specifies the amount of food "burned". "Energy" is what is produced and delivered to the crank/wheels. The human body is a very inefficient motor. – Daniel R Hicks Jul 18 '15 at 19:40
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    "calories" are also most likely "food calories" or kilo calories. – Jasper Jul 18 '15 at 21:03
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    To add to Daniel R Hicks comment: Of course both are units of energy (and Joule is the official one). Strava just seems to use them to measure/estimate different things. I wonder how they estimate the amount of food energy burned. I remember an estimate of about 30% efficiency for muscles but that only gives me 1089kJ (259kcal) for 363kJ of mechanical energy. – Michael Jul 18 '15 at 21:23

I think your question is based on two possible confusing relationships. First a "calorie" is different from a "Calorie"; and, second, there is a missing variable that Strava uses but you can't see: the assumed gross metabolic efficiency (or GME).

Strava's energy output measure is Joules or kilojoules. A joule is a watt-second, so if your first ride lasted 42m41s, that is 2561 seconds. If you expended 363 kJ in 2561 seconds, that is an average output of 363000/2561 = 141 watts over your ride. If you do not have a power meter on your bike this is an approximation that Strava makes from your information about the route you took, your total weight, your total time, and assumptions about your aerodynamic and rolling drag. Strava estimated your second ride to have expended 317 kJ over 3050 seconds, or an average output of 104 watts. If you weigh 115kg, that means Strava estimates your average output for your first ride at 141/115 = 1.22 watts/kg, and your second ride at around 0.9 watts/kg.

In American English, dietary guidelines often label kilocalories as "Calories" with an uppercase "C." That is, a "calorie" (lowercase "c") is the amount of energy needed to raise the temperature of 1 g. of water 1 deg Celsius, and a kilocalorie (or "Calorie" or "food Calorie") is the amount of energy needed to raise the temperature of 1 kg of water (=1 liter) by 1 deg. Celsius. There are 4 kilocalories (=4 food Calories) of energy in a gram of carbohydrate or protein and 9 kilocalories per gram of fat.

Your memory is correct: in a straight energy conversion there are about 4.185 joules per calorie, or 4185 joules per kilocalorie (="Calorie"). However, the human body is not 100% efficient in converting food Calories into energy. In fact, gross metabolic efficiency (GME) in humans is generally in the range of 19-24%. That is, only about 19-24% of the food Calories we consume are converted into energy we can use -- the rest is converted into heat.

Strava is assuming a value for GME of 21.4%, about the mid-point of that observed range of gross efficiency.

Thus, 363kj of energy output = 86.7 kilocalories of energy output, but at a GME of 21.4% a rider would need to consume about 86.7/.214 = 405 food Calories to maintain energy balance.

Likewise, for your second ride, 317 kj of energy output = 75.7 kilocalories of energy output, but a rider would need to consume 75.7/.214 = 353 food Calories to maintain energy balance.

If your own personal gross metabolic efficiency differed from 21.4%, you would need to consume slightly different amounts of food Calories to maintain equilibrium in weight.

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From Strava's Help page :

Energy Output

Energy Output measures the amount of work you've done during a ride, expressed in kilojoules (KJ). It is a factor of how much you're pedaling, how fast you're pedaling and how much force you're exerting on the pedals (measured in W). Power output is most accurately taken from a power meter, but if you don't have a power meter we give a rough approximation through our power estimator.

This means how much energy was delivered to make the wheels turn. As Daniel R Hicks mentioned, calories in that case mean how much "food" energy your body burned, which will always be higher than the energy you give to the wheels.

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  • 4
    "force [...] measured in W" makes my head hurt -.- – Jasper Jul 18 '15 at 21:03
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    @Jasper I took my last physics course in 2000, that is 15 years ago. But, it hurts my as well. It should be "how much power you're exerting on the pedals (measured in W)", am I right? – trejder Jul 20 '15 at 6:32
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    Yes, it has to be "power". – Jasper Jul 20 '15 at 11:23
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    Sorry about that, but that's how it's written on the Strava website... – Bibz Jul 20 '15 at 12:36
  • @Bibz And that is why our hearts are in pain for Strava website, not for your quite good answer. – trejder Nov 3 '15 at 9:17

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