How do I know if I'm doing moderate or vigorous intensity exercise?

Moderate-intensity aerobic activity means you're working hard enough to raise your heart rate and break a sweat. One way to tell if you're working at a moderate intensity is if you can still talk but you can't sing the words to a song.

Vigorous-intensity aerobic activity means you're breathing hard and fast, and your heart rate has gone up quite a bit. If you're working at this level, you won't be able to say more than a few words without pausing for a breath.

* Information provided by www.nhs.uk/chq/Pages/2419.aspx?CategoryID=52&SubCategoryID=145"