Going to primary research today! This is a nice little article on why it is so difficult to measure physical activity. There are so many factors that affect our habits, activity patterns and intensities, not one single measure can tell us the 'whole picture' when it comes to physical activity!
Exerc Sport Sci Rev. 2015 Apr;43(2):67-74. doi: 10.1249/JES.0000000000000039.