Yes, your body gets more efficient doing an activity. Aerobic activity will essentially make all the muscles involved stronger and therefore reduce their requirements when performing their duties. For instance your heart will become stronger and so pumping blood requires less energy, lungs etc are the same. It means you’ll burn less calories for the same amount of activity.
I’m not sure exactly how much less, I’d hazard a guess that the increased work you’re able to do well outweighs the efficiency loss. Also don’t forget you’ll be doing it quicker so reduced time working will reduce calories too, though there will be an increase in calories burned per unit of time to compensate sightly.
I really wouldn’t worry about this stuff. The benefits to exercise are so numerous. Just think about how much longer you’ll live and how much better you feel [/quote]
Wait… what? A stronger muscle burns less calories for the same amount of work? That simply does not make sense. It would make more sense to say that a more efficient muscle would burn less calories, but energy efficiency usually doesn’t correlate with strength. Distance runners are highly efficient, does that mean they’re very strong?
Also, to suggest that doing the SAME circuit faster than before means that you will burn less calories is akin to saying walking up a hill burns more calories than sprinting up it.