Alecia asked how she would know when her heart rate is at it's maximum when working out. Remember this blog post about intervals? When you are in the "high intensity" portion of your interval workout your heart rate should be at leaset 90% of it's capablitity. You can figure out your exact rate for you if you plan on wearing a heart rate monitor at this link. However, the easiest and actually most effective way for me to tell if a someone is pushing themselves to their maximum heart rate capacity is a method called the "talk test".
Basically, if you are at 90-100% maximum heart rate you should not be able to talk while working out. It's seriously that simple. See the chart below:
During interval workouts, after the high intensity portion, you would move on to the lower intensity portion. In my classes this would consist of strength training. So when we switch to strength training you should concentrate on breathing deeply in through your nose and out through your mouth to "catch" your breath. You are aiming for 70% of your maximum heart rate by the end of the lower intensity segment. This would mean that you are able to talk, but it's in short phrases.
Have fun figuring out your maximum heart rate!