Cardiovascular machines, such as the treadmill, rowing machine and elliptical, are designed to help your body burn a high number of calories.
Often times, these machines will include a calorie expenditure display on them. This is a great way to estimate your caloric loss during a workout.
What is important to realize is that this is only an estimated value. The number of calories expended by your body during the workout depends on several factors, including previous adaptation to the machine itself. In other words, if you keep using the same machine the same way, you will burn fewer calories over time. To get the most out of your cardio workout, you must change the cardio machines that you use periodically.
The reason your calorie expenditure on a particular machine decreases over time is because your body is very good at adaptation. For example, let’s say it is your first day on a new cardio machine and you work out for 30 minutes at a level 1 and burn 400 calories. Two days later, you do the same thing and keep repeating this for weeks on end.
The machine will probably tell you that you have been burning the same amount of calories every time, but in reality, your muscles have been adapting to this machine and therefore become more efficient. This usually results in burning fewer calories during the same workout.
Hopefully, you were increasing the level of difficulty on the machine during these few weeks, which would help keep the number of calories burned the same or higher, but eventually, your body will become “bored.” This is called a plateau.
I suggest switching machines every four to six weeks in order to prevent this plateau effect. Beginners don’t have to switch as often because they are slower to adapt than advanced users. You want to give your body enough time to adapt to the exercise but not enough time to decrease the effectiveness of the exercise.