Some muscles burn more calories than others. Or, to put it another way, which muscles use the most calories during exercise? I'm a certified personal trainer. If you want to use up a lot of energy during exercise, there are indeed certain muscle groups to focus on.
These are the muscle groups in the legs. Actually, exercises that simultaneously recruit the buttocks muscles and those in the upper legs will use up the most calories, if done intensely, relative to routines done with equal intensity for other body parts.
In other words, strenuous weighted squatting will require more energy than strenuous bench pressing or cable triceps push-downs.
The larger the muscle, the more energy it requires; that is, the more calories are burned while this body part is being exercised. But again, realize that intensity is a factor. Certainly, a lame set of leg presses won't require as much energy as will a brutal set of chest presses.
When intensity or exertion level is a constant, it is the leg/buttocks muscles that burn the most calories.
Which begs another question: Which exercises, involving the legs and buttocks, burn the most energy? Again, when intensity is equal across the board, the one routine that gets the most votes for requiring the most metabolic work are squats with a free barbell.
However, some may disagree and argue that barbell deadlifts require the most energy output. Squats can also be done in different ways, such as holding heavy dumbbells straight at one's side. Go deep and spring back up each time, and you'll burn a ton of calories, compared to stepping up and down on a stool for the same length of time.
In summary, the muscles of the legs and buttocks burn the most calories. These groups are recruited while squatting, deadlifting, leg pressing and lunging. However, lunges will burn quite a bit of energy only when they are heavily weighted. Lunges without weights are more of an aerobic activity, in which you're able to maintain this activity for a number of minutes, and these don't burn nearly as many calories as lunges done while holding heavy dumbbells, especially deep lunges.
Next on the list are the back muscles, which is why pull-ups and wide-grip rows burn many calories. So do other back exercises like chin-ups, lat pull-downs and bent-over rows. Back muscles are the largest in the upper body.
In third place for burning calories are the chest muscles. So, it's legs/buttocks, back, and chest. Focus on these body parts if you want to burn as many calories as possible.