Turning Pedal Work Into the Food Energy It Cost
An ergometer, a power meter or a rowing monitor measures one thing honestly: the mechanical work you pushed into the machine, counted in joules. What a rider actually wants to know is the other number — how much chemical energy the body had to release to produce that work, which nutrition counts in kilocalories. The two are not the same quantity, and the gap between them is the whole of exercise physiology's efficiency story.
Why the Two Numbers Drift Apart
Work Is What the Crank Measures
The 23.9 Per Cent Coincidence
Three Quarters Leaves as Heat
Reading a Session's Work Total as a Metabolic Cost
The workflow always has the same shape: get the mechanical work in joules first, convert it, then divide by an efficiency you are willing to defend.
Get the work in joules first
Average watts multiplied by seconds for anything with a power display; m·g·h for stairs, step-ups or a lifted mass, with the height in metres and 9.81 for g. Keep to kilograms, metres and seconds and the total lands in joules with nothing to correct afterwards.
Type that total into the joule side
The kilocalorie figure resolves beside it while you are still typing. Session totals run to six digits, and the thousands are set apart so 720 000 stays a readable number rather than a run of characters you have to count.
Divide by the efficiency you are assuming
The converted value is the work only. Dividing it by 0.20 to 0.25 brackets the gross metabolic cost, and stating which figure you used matters more than the third decimal place — two labs testing the same rider will not agree past the first.
Work backwards from a kilocalorie target
Coming the other way — from a nutritionist's figure, or a machine that only reports Calories — the swap arrows put kilocalories on the input side, and typing straight into the kcal box does the same thing without the extra click. Multiplying that by the efficiency then gives the joules of work it implies.
Session Work Output and What It Cost to Produce
Every row starts from a mechanical calculation — watts times seconds, or mass times gravity times height. The third column is that work expressed in kilocalories; the fourth divides it by 0.23 to give the gross metabolic cost at a typical trained efficiency.
| Effort | Mechanical work (J) | Work as kcal | Gross cost at 23 % |
|---|---|---|---|
| One 20 kg lift to 1.5 m | 294 | 0.07 | 0.3 kcal |
| 50 reps, 60 kg through 0.5 m | 14 715 | 3.52 | 15.3 kcal |
| 2 000 m row at 2:00/500 m (203 W) | 97 222 | 23.24 | 101 kcal |
| 20 minutes at 150 W | 180 000 | 43.02 | 187 kcal |
| 100 stair flights, 75 kg, 3 m each | 220 725 | 52.75 | 229 kcal |
| One hour easy at 100 W | 360 000 | 86.04 | 374 kcal |
| One hour at 200 W | 720 000 | 172.08 | 748 kcal |
| 40 km time trial, 250 W for 58 min | 870 000 | 207.93 | 904 kcal |
Read the second and fourth columns against each other and the coach's rule of thumb falls out: the joule total divided by a thousand is within a few per cent of the gross kilocalories. The 50-rep row is the exception that proves the point — 15 kcal is plainly not what a heavy set costs, because almost none of that effort shows up as a mass moving upward.
What This Page Adds to a Training Calculation
Wattage Trials Recomputed on the Spot
Retype the joule total for a slightly harder interval and the kilocalorie side follows the keystroke, which makes it quick to see what twenty extra watts over an hour is really worth before you commit to the session.
A Bare Figure for the Training Log
The copy control above either field, and Ctrl+C inside it, hand over the digits alone — no unit, no separators — ready for the cell where you then divide by an efficiency.
Rep-Sized and Ride-Sized Totals Together
A single lift is a few hundred joules and a long ride approaches a million, and both stay legible: thousands are spaced apart, and anything past ten billion drops into exponent form instead of sprawling across the field.
The Units a Head Unit Reports Are All Listed
Search either dropdown for kilojoules, watt-hours or plain calories, since ride files and machine displays rarely agree on which of them to print, and the same pair of fields handles whichever one you were handed.
Efficiency Questions From the Ergometer and the Weight Room
Why does a kilojoule of pedalling happen to cost about a kilocalorie of food?
Pure arithmetic meeting pure biology. A kilocalorie is 4.184 kJ, so if the body were perfectly efficient one kilojoule of work would cost only 0.239 kcal. Human gross efficiency on a bicycle is close to a quarter of that ideal, and at exactly 23.9 % the two effects cancel: 0.239 kcal ÷ 0.239 = 1.00 kcal per kilojoule. Trained riders happen to sit within a couple of points of that figure, so the shortcut holds to roughly ±10 % — good enough for planning a feed, not good enough for a research paper.
Where does the energy that never reaches the pedals actually go?
Three main places. Roughly half the loss happens upstream of the muscle, in the oxidative chain that turns substrate into ATP — that machinery is thermodynamically leaky by design. A further slice disappears inside the contraction itself, where cross-bridges cycle and release heat whether or not the limb is moving. The rest goes on work you are not paid for: ventilation, cardiac output, postural muscles and the leg coming back round. All of it ends up as heat, which is why core temperature climbs during a steady effort that produces no extra external work at all.
Why is m·g·h such a poor estimate of what a set of lifts costs?
Because in resistance training the raised mass is only a fraction of the metabolic story. The table's 50-rep set gives 14 715 J, about 15 kcal gross — an absurd figure for the effort involved. The lowering phase does negative work, so the equation scores it as zero even though eccentric contraction burns energy. Stabilising muscles, grip and bracing consume energy without displacing anything. Isometric holds at the sticking point do no mechanical work by definition. And the recovery between sets, plus the raised metabolism for hours afterwards, is invisible to the formula. Cycling suits this conversion precisely because the crank captures nearly all of the useful output; a barbell does not.
My treadmill and my bike disagree about the same perceived effort — why?
They measure different things and report them under the same word. A cycle ergometer knows the resistance it is applying, so its joule figure is close to true external work. A treadmill knows only your speed, its incline and the body mass you typed in, and estimates energy from published tables rather than measuring anything about you. Running is also a less tidy way of producing measurable external work, because a large share goes into vertical oscillation and elastic loading of the tendons. Two machines can therefore report Calorie counts differing by a third for efforts that feel identical, and neither is lying — they are answering different questions.
Gross, net or delta efficiency — which one is a published figure using?
Gross efficiency divides work by the total energy consumed during the effort, resting metabolism included; it is the lowest of the three and the one that matches a converted joule figure most directly. Net efficiency subtracts resting metabolism from the denominator first, so the same rider scores a few points higher. Delta efficiency compares the change in work with the change in energy consumed between two workloads, which strips out the fixed overhead and typically lands around 25 %. That is why one paper reports 20 % and another 25 % for comparable athletes. Some machines build the choice in silently: a Concept2 rowing monitor assumes 4 kcal per watt-hour, which implies about 21.5 % efficiency, then adds roughly 300 kcal per hour of resting metabolism on top of it.
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