We’ve discussed power and split. Finally it’s time to tackle the number that people are usually most concerned with, but is actually least accurate – calories.
Cals burned is calculated:
Cals = (4 x avg Watt + 0.35) x time / 4.2
Let’s break it down. Recall: power is energy put into the erg per second. It seems that since cals measure energy, we can just multiply power by time to get cals. From the formula, that is true except for 3 ‘extra’ numbers. We’ll explain the 3 numbers.
Starting on the right, dividing by 4.2 converts from SI units – Watt/second, to dietary cals (seen on food labels etc).
0.35 is an assumption by the erg manufacturer that we use 0.35W just to sit on the erg (to breathe, hold yourself upright, etc). 0.35W is based on a “representative” 80kg male.
Lastly, the factor 4 is another assumption. It’s assumed we are 25% efficient converting chemical (fat or carbs) to mechanical energy (what is measured) ie we burn 4 cals to produce 1 cal of work (the other 3 are lost as heat or actions that don’t spin the flywheel eg holding the handle, swinging the body forward during recovery, etc).
Now it should be obvious why number of cals is inaccurate. Most people are not an 80kg male (although conversions exist for different weights). More important, assuming 25% efficiency is generally inaccurate, especially for beginners since our strokes are not “perfect”. Most of us likely waste significant energy due to bad form/technique.
Hopefully, understanding how cals are calculated convinces you of its dysfunction. While it is a reference, remember that it’s not very accurate so don’t place too much importance on it.
A last note: we haven’t even discussed comparing cals across different brands of machines or different exercises. Imagine the assumptions needed to calculate calories for rowing vs running vs climbing stairs or riding a bike. What’s more, each manufacturer has an inherent incentive to make assumptions that increase their cal number to make you feel you’re getting more bang for your buck!
Hopefully, reading this post makes you question how each manufacturer arrives at their ‘magical’ number, and not be fooled!