When I am designing a workout program for my clients, I give them a "quota" or minimum amount of time for the week they need to spend on cardio. I also specify the heart rate they need to be above for that time. I have two similar clients (age and weight) who tend to use different pieces of cardio equipment. Client A tends to use the treadmill and gets her heart rate to about 80 percent of her predicted max. Client B tends to use the crosstrainer and performs at 70 percent of her predicted max. Obviously with the crosstrainer, Client B is using more muscles (and one would assume burning more calories) but at a lower heart rate. I know that on the treadmill the muscles being used are mainly functioning only to recover, instead of getting a lot of quad extension that would be used when running outside or off of the treadmill, but the heart rate (with Client A) is still getting higher. My question is, who would typically burn more calories, Client B who is using more major muscle groups at a lower heart rate or Client A who has the higher heart rate using less muscles??
As an aside, it's not necessarily true that you're only using your muscles to recover on a treadmill. That's just the way most people tend to use it.
If you observe the average, casual jogger you see on the street, they're moving with the same style, too - plopping one foot in front of the other instead of pushing off using triple extension. Average runners are average because they don't actively propel their bodies forward - but that's another story.
Yes, it is true that a study published in the Journal of Exercise Physiology showed that you burn more calories on the crosstrainer than on the treadmill at the same RPE.
And we know intuitively that there's some correlation between RPE and heart rate - in a given person. So Client A can work at 80 percent on the treadmill and 70 percent on the crosstrainer and theoretically burn the same amount of calories.
No comparison at all can be made, however, between two different clients. They're too many other variables involved including fitness level, running (or "crosstraining") economy, experience on a given machine, etc.
Have you ever noticed that the crosstrainer arms keep moving and the calorie reading keeps clicking even when you're not using the [crosstrainer's] arms? Use the calorie readings only to gauge the relative intensity of various workouts on the same machine with the same client.