A Canadian study involving 129 sedentary adults (aged 18-64) found that they tended to underestimate how hard they should be working to achieve levels of moderate and vigorous intensity while moving on a treadmill. This is despite being given commonly used exercise intensity descriptors.
The finding suggests that, while considerable thought has been given in developing physical activity guidelines, most people don't understand them well enough to use them.
For adults to achieve a moderate intensity, their heart rates should be within the range of 64-76% of their maximum heart rate and between 77-83% for vigorous intensity, according to the Canadian and global physical activity guidelines.
Canning KL, Brown RE, Jamnik VK, Salmon A, Ardern CI, Kuk JL (2014) Individuals Underestimate Moderate and Vigorous Intensity Physical Activity. PLoS ONE 9(5): e97927. doi:10.1371/journal.pone.0097927