It's important to find the right balance between working out too hard and not hard enough. For maximum efficiency when trying to burn fat or increase your cardiovascular ability, you should work out while maintaining a percentage of your maximum heart rate (number of beats per minute).
Your optimum target heart rate for exercise - which is like the speed you drive your car at for optimum fuel economy - can be found by:
- 220 minus your age
- Then exercise at 60 - 75% of this number
A lot of fitness machines have this feature built-in. This is also about the level of exertion where you can still talk half-decently well.
Your heart rate is constantly reset by your body depending upon how in shape you are. The less in shape you are, the less you have to do to hit this target. The more in shape you get, the more you have to do to hit this target. Your body will always be moving this target to keep you motivated.