This is a great question. But I think you need to clarify if you are referring to a steady rate of exercise (such as walking or running at the same speed or biking at the same RPM’s) or HIIT workouts (such as Crossfit type training). In the case of the first one, your rate of calories burned will eventually drop because you will become more efficient and you will require less energy to do the work. This will lead to a plateau. In HIIT I believe the calories burned will increase for two reasons:
-You will have to increase your weight and/or repetitions when doing the exercise because you will become used to the same weight and reps and in order to keep the same intensity level you will have to adjust these two variables.
-Your BF% will drop, which means your lean mass will increase so you will be burning more calories doing the same work. More muscle tissue means higher metabolism which equals to more calories burned.
It would be likely that someone would burn fewer Calories if they continue to do the same exercise duration and intensity for prolonged periods of time, particularly if they are simultaneously losing weight. The oxygen demands of the muscle not likely increase, but would probably decrease due to improvements in efficiency of the movement. This improvement in efficiency may or may not be substantial.
This is definitely part of the problem with people who plateau during weight loss, or even regress and gain weight back. As they lose weight, their metabolism will decrease. This is rarely accounted for in their diet. On top of this, some trainees will fail to increase the intensity of their cardiovascular exercise over time.
This is one reason why I like interval training. While it does not need to be performed every session, I believe that it allows people to practice training at a higher intensity. I have found that many people enjoy interval training and it will carry over to when they perform lower intensity cardiovascular training.
Steve Teahan, BS, CSCS