Last night as I was subbing for Dana (lucky girl, off in the Bahamas), some questions came up about heart rate, calories burned, fat burned, and exercise intensity. This is one of those areas where lots of people are, unfortunately, misinformed. Working harder won't necessarily help you achieve your fitness goals, especially if those goals include weight loss.
Check out these brief articles from Polar for some easy-to-understand explanations of how things work during your workout. They should also be helpful if you've been considering buying a heart rate monitor, to understand why they're so important to have.
Benefits of Heart Rate MonitoringDifferent Training Heart Rates are Used for Different PurposesAbout Exercise Zones (intensity)
Calorie Burning (includes explanation of fat vs carbohydrate burning at different intensities)
Indoor Cycling (discusses aerobic vs anaerobic training)