there are problems with the fat burning option on the cardio machines and it really ought to be eliminated. the idea behind the fat-burning option is this: because fat is denser than carbohydrate it requires more oxygen to burn. so to maximize the percentage of fat you burn compared to carbohydrate the fat-burning mode would have you work out at a pace at which your body can deliver lots of oxygen to your muscles. that generally means a slow pace to keep you from getting breathless. the problem is that when you exercise at a slower speed you burn fewer total calories -- from both carbohydrate and fat -- because you simply don't do as much work. further the way to get aerobically fit is to get your heart rate into the training range (usually 60% to 85% of your maximum heart rate) which is hard to do at slower speeds. and fitness is ultimately what you're after whether your goals are better health burning calories or improving heart and lung capacity. the bottom line is that the fat-burning mode probably won't be intense enough to maximize total calorie- or fat-burning or to help you increase or maintain optimum fitness levels. use the cardio mode to maximize your exercise benefits.
what's the difference between the fat-burning mode and the cardio mode on the machines at the gym?