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Computing Calories Burned using a Cycling Power Meter

Energy expenditure is relevant to weight loss and diet— how much to eat? Here the SRM power meter helps track one’s exercise with a high degree of accuracy as to caloric expenditure.

For more about the particular graph below, see my Training Results page for the Everest Challenge, and do continue reading below.

Physiological efficiency

To make the bike go, the crankset has to be turned. Like any living creature, the human body cannot do this at 100% efficiency.

An untrained “couch potato” might turn the cranks at, say, 18% efficiency. That is, only 18% of calories burned goes into mechanical energy to turn the crank.

A highly trained athlete can achieve 25% efficiency, operating aerobically at high output levels as well. Years of measurement suggest I hit at least 25% and as much as 28% when lean and in top condition at aerobic effort levels—that would explain in part the increasingly difficult challenge of dropping body fat below 10%.

Why does this matter? Because the SRM power meter measures kilojoules at the crank. To convert that into the useful metric of calories needed in food, this efficiency factor must come into play. This is why it is idiotic to say someone burns X calories cycling or running or walking for an hour—the potential error is very large—as much as 55%.

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Calories burned

In terms of computing “calories burned” for weight loss purposes, power measurement is much more accurate than an approximation based on heart rate, because heart rate varies with fitness, hydration, temperature, illness, etc. Only measuring actual power produced is consistently accurate and some power meters still suck—SRM is the gold standard uses by pros.

Suppose a cyclist rides for one hour at an average of 250 watts. A watt is one joule per second, so 250 watts is (250 * 60 * 60) = 900,000 joules = 900 kilojoules. Which is about the level I train at for everyday rides when lean and in my top condition.

The SRM PowerControl 7 displays kilojoules for total energy, not directly useful for those looking to track Calories burned:

To calculate “cereal box” calories, take the Kj figure from the SRM PC7 or PC8 and multiply by 0.956, e.g. 1000 Kj = 956 calories. Or just ignore the small difference and don’t multiply at all, since additional post-ride energy is consumed by the body for repair and recovery.

To calculate calories burned, convert kilojoules to calories, then divide by the assumed physiological effciency factor. I use 25% for myself, which is very high (for a highly trained athlete), with the limit around 26% (after some years of intensive training which has raised my aerobic capability to very high levels, I now assume 26%).

# Kj to Kcal = Kj * 0.239006

Calories burned = (kilojoules at crank as per SRM) * 0.239006 / (efficiency factor) = depends on physiological efficiency e.g.:

Calories burned = kJ_SRM * 0.239006 / 0.25 = kJ_SRM * 0.956

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Aerobic vs anaerobic

Aerobic means “with oxygen”, anaerobic means “without oxygen”—for the latter, the body must expend additional effort to clean up the mess, so to speak.

Aerobic vs anaerobic influences overall efficiency, and watts produced at the crank incurs additional downstream energy requirements for recovery and repair and muscle growth. If you’ve ever trained hard and later remained hot for hours while trying to sleep, this is some deferred energy consumption going on as the body repairs and grows, which generates body heat.

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