I am looking for an implementation (or a document that describes the actual algorithm) for the energy expenditure/VO2/EPOC calculation from the heart rate variability.

A few white papers about

• Oxygen consumption estimation
• Energy expenditure estimation
• EPOC (Excess post-exercise oxygen consumption)
• Training effect
• Athlete's recovery analysis

can be downloaded here.

According to Garmin's webpage, the error is about 50% smaller than for the generic method.

While these are fascinating to read, they don't tell us much about how to actually calculate these values from heart rate data. All it says:

Neural networks were used to construct a model that derives VO2 from R-R intervals (time between successive heart beats), using respiration rate and on/off-response information.

Even though the normal heart rate monitors don't make the data available, there are cheap receivers for polar and ANT+ around that allow beat by beat monitoring, like this one.

In case this question is too specific, other interesting algorithms related to fitness are welcome, too!

The conventional estimation of caloric expenditure as a function of the heart rate can be found in this paper: Prediction of Energy Expenditure from Heart Rate Monitoring During Submaximal Exercise.