In one book I read, the target heart rate for a cardiovascular workout for a person my age (36) was somewhat lower (10-15bpm) than the rate listed on some of the machines I'm familiar with at the gym. I've also seen the range differ slightly from one brand of machine to another. So, I'm slightly confused about this. I don't want to overdo it, but I also don't want to underdo it.
Is there definitive information available (e.g. a study or paper) that details the correct approach to estimating one's target heart rate range?