Optimal sensor to measure physical energy expenditure

I'm trying to find an optimal sensor, but I have difficulties.
The criteria of choosing it is:

  • form factor: small, to be placed on an Adafruit Flora
  • non-invasive (placed on the wrist)
  • work fine while in motion

Currently I have a climate sensor, which could help.

Options I found so far:

  • heart rate monitor: depends on finger tips
  • skin conductivity: not enough accurate on wrist
  • oxymeter (level of oxygen in blood): not accurate in motion
  • CO2 sensor: not accurate due to sampling method (breath out on it)

Any suggestions?