If you’ve never worked out with a heart rate monitor, you’re really missing out. They’re a fantastic way to completely monitor how you work out, and even turn otherwise mundane activities into workouts.

Here’s the thing: your heart doesn’t know what you’re doing, at any given time. You could be reading a book, running a marathon, making love — it doesn’t matter. When it comes to cardiac fitness, all your heart knows or cares about it is how hard its working, no matter what you’re doing.

A heart rate monitor lets you know what your heart is doing, which enables you to turn anything into a workout. Don’t have time to get to the gym? Put on your monitor, and start cleaning the house. Clean faster until your heart gets into the right zone, and — voila! — you’re working out.

Polar makes the most popular heart rate monitors around. You can get a cheap one for about $50. They usually have some standard features, one of which is the ability to alarm when you enter the “cardiac zone” and alarm again when you drop out.

I have a Polar F4, and it has two things I really like:

1. The chest strap is compatible with the heart rate monitors on most treadmills. So the treadmill will detect the chest strap and use it to display your heart rate on the screen.
2. If you run or walk at night, the watch will detect its proximity to the chest strap to turn on the light. So if you want to see your time or heart rate, just move the watch up against your chest and the light goes on.

About $80.