While I couldn't find the publication where the calculation was based on, you managed to find a formula on which its likely based:
A mixed model was used to derive the following equation for predicting
physical activity energy expenditure (EE):
EE = -59.3954 + gender x (-36.3781 + 0.271 x age + 0.394 x weight +
0.404 V[O.sub.2max] + 0.634x heart rate) + (1 - gender) x (0.274 x age + 0.103x weight + 0.380x V[O.sub.2max] + 0.450 x heart rate)
where gender = 1 for males and 0 for females.
Formula reformatted for easier comparison:
EE(male) = 0.271*age + 0.394*weight + 0.404*V[O.sub.2max] + 0.634*hr - 95.7735
EE(female) = 0.274*age + 0.103*weight + 0.380*V[O.sub.2max] + 0.450*hr - 59.3954
This formula looks plausible, but it also helps point out the problems I have with these calculators. Lets pick it apart piece by piece:
- gender differences, while there are certainly differences between the physique of males and females, I'd be surprised if there would be some magical constant for converting their values.
- age, again not everybody at the same age is created equally, so its doubtful there's a constant factor that expresses this reliably.
- VO2, while I think VO2 is a very important measure (combined with heart rate) for your energy expenditure, the changes are very likely that you have no idea what these values are. So the one value that would give us an estimation of how much CO2 you ventilate (which is strongly related to burning energy) is not accessible to most who use the calculator! Clearly, trained individuals, all else being equal can expect to get much different values from the calculation than a couch potato.
- Heart rate, while I'm a big fan of tracking your heart rate, there's clearly a large variation in the heart rate between individuals given the same energy requirement. The explanation is simple: your heart rate depends on the combination of your heart, lungs and muscles (simplified), so unless those are all equal, don't expect exactly the same heart rates.
What it all boils down to: we're not all made equal! Some are more trained, have a better technique, more experience, not to mention the conditions under which we swim (temperature or flow). On top of that, the calculation ignores the speed at which we swim, probably to assume its defined by the heart rate. So clearly, its likely to be wildly inaccurate and that's to be expected, given that its based on the power of averages. Take a whole bunch of people and fit a model that fits them and you end up with a model that predicts accurately for nobody.
So what can you do?
Well we need to ask ourselves, why are we measuring calories to begin with? If its meant to calculate how many cookies you can eat to compensate for the energy you burned, I probably have to disappoint you, I believe its nearly impossible to accurately calculate it and expect it to translate equally to other people.
A more useful goal would be to get an idea of how intensive we worked out, so how do we do this? Well there are several variables which we can easily track over time:
How does your weight change over time? If you have a fairly stable/balanced diet, you can expect to consume a similar amount of calories every day/week. Keeping track of your diet and weight will help you to explain variations over time (e.g. the bad effect of a birthday party)
Keep track of your workouts, how long and how many laps did you swim? This gives you a ballpark idea of average speed, depending on your goal, perhaps you want to swim more laps in the same time or be able to swim longer. If you have kept the diet the same, then improvements in this area should have effects in other areas. You may not necessarily loose weight (because fat got replaced by muscles), but you will notice 'a difference'
Track your performance by measuring your heart rate, while this gives the same kind of values as the speed information, its good to have a validation of those assumptions. So you don't even have to measure it every workout, but just regularly to 'see' whether the speed changes are due to you swimming harder (higher heart rate) or you swimming more efficient (lower heart rate at same pace).
Combining all these variables gives you a much better view of what effect exercise has on your body and gives you actionable information.
- Is your weight going up, while you work out just the same? Perhaps you need to change your diet
- Is your weight staying the same, but you don't seem to improve your pace? Change your training program, perhaps its not well balanced and you're either not training hard enough or not getting enough rest
- Is your heart rate lower than usual? Perhaps you weren't as motivated or concentrated enough and need to try and push a bit harder next time (or take more rest).
So go forth and measure!