Using heart rate to calculate intensity is
not as accurate, because increases in heart
rate are not always attributable solely to increases
in exercise intensity or energy demand;
heart rate can be affected by other
factors, such as stress,medications, caffeine,
dehydration, etc. Subjective measures also
have their limitations, since it requires
some experience with RPE and the talk test
to monitor intensity accurately with these
methods. In essence,VO2 is the best indicator
of exercise intensity because it is tied
closely to energy expenditure. The higher
the intensity, the more oxygen clients consume
and the more calories they burn.