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This question sort of comes to mind when hearing how efficient an internal combustion engine is turning chemical energy in mechanical energy (something like 20-40%) with lots of excess heat. As an analog, how efficient is (or potentially) the human body at turning food into energy? Please bare with me, I realise there LOTS of different variables (how much the person weighs vs mass, metabolism, diet, etc). But I would imagine that there shouldn't be much margin of error given that most people maintain the same constant temperature (98 F +/- 1 degree).

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The MET (Metabolic Equivalent Task) readout on your gym equipment is your body doing 1Kcal/kg/h = 4184 J/kg/h and can be reasonably accurately measured by how much oxygen a test victim uses.

Sitting still is roughly 1 met and cycling at 100 Watts is around 5.5 Mets.

So taking a man of 75kg, cycling at 100Watts (100J/s) he is having to do 5.5 * 4184 * 75 / 3600s = 480Watts so an efficency of 20%

Remember though that the person is spending 80-100Watts just staying alive doing nothing - unlike your car. There is an interesting experimental fit to how much energy you need to just stay alive, calculated about 100 years ago, the Harris-Benedict equation

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    $\begingroup$ WOW...great answer... today i found out that i need 100 watts just to stay alive!! :) $\endgroup$ – Martin Sep 30 '16 at 13:39
  • $\begingroup$ Yes, but the 100 watts to just stay alive include self repair and maybe just enough energy for replication. Try that with a car. $\endgroup$ – brianlmerritt Jan 18 '17 at 15:12
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The human muscle efficiency (the mechanical work divided by the total metabolic cost) when performing intense exercise is measured to be in the typical range of 18-26%. Manufacturers of fitness equipment use such results and typically show a guesstimate of burned calories based on the actual mechanical work delivered.

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    $\begingroup$ So as an example, if you're doing crunches without weights and lower roughly 100 lbs of your bodyweight 1 meter and back up, then you should be burning approximately 0.5 kCals per crunch right? (i.e., 100lbs * 4.45lbs/newton * 1meter * 0.000239joules/kilocalories /.20% efficiency) $\endgroup$ – speedplane Nov 13 '13 at 14:42
  • $\begingroup$ I meant squats, not crunches. $\endgroup$ – speedplane Nov 13 '13 at 14:59
  • $\begingroup$ @speedplane I would guess that it's on the order of 0.5 kcal indeed, but you have to be careful. When running, for example, your horizontal acceleration is zero, so the effective work is also zero. In this sense, running is incredibly energy-inefficient. Your muscles actually spend energy even when the work done is zero or negative, i.e., eccentric and isometric contractions. The 18-26% range cited is probably for concentric exercises, like cycling. Doing squats is probably less efficient. $\endgroup$ – Wood Sep 1 '19 at 18:20

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