Calories Burned changing as you get into better shape...

Probably due to the fact that you are getting into a physically better shape. Not using as much effort to do the same amount of work will translate to lower calorie burning.

No clue as to your current shape, but your body has to work harder to move 300 lbs than 100 lbs. That's all it is.

bump for later

If you lose weight, you will burn less calories. Also, your cadiovascular strength is growing stronger as you do more exercises that work your heart (cardio). Two people with the same weight can burn different calories if one has greater cardiovascular strength than the other, so if you haven't lost a lot of weight - don't be worried.

I burn a LOT less now than I used to.

Where it is true that it takes more effort and therefore typically burns more calories the more you weigh, everyone's body works differently. That is one of the reasons a heart rate monitor is handy to have around (I personally don't have one, but that is the very reason I want one). Some people will burn less, some will burn more as they become more fit. If you think of it this way, your body may weigh less but you've also got more muscle and using those muscles can burn more. Plus you may be getting more out of your workouts being able to complete them with the correct form and focus on intensity as opposed to just surviving when you were not in as good of shape...
Clear as mud?
Moral of the story - everyone's body runs differently. A good heart rate monitor can help you get some exacts....
Keep at it!

While weight decrease affects calories burned, improved condition also decreases calories. Coaches recommend athletes switch to different exercises as the current exercise become easier. Each switch will use different muscles or use muscles differently, initially harder and burning more calories.

I use an HRM as well. I do find that I can reach good calorie burn if I continue an exercise longer than I did previously. It takes me longer to get to a heart exercise zone but continuing after that point longer gives me good burn. Also, I switch between cardio machines to rev up the heart and burn with changes.

The good news is that this problem means you are healthier. That's good, isn't it?.

That's not really true. It can be a little confusing because people are using their HRMs as the only indicator of what is going on with their bodies and, in this case, the HRM can often be misleading.

It's true that, as weight decreases, caloric expenditure for any comparable intensity will decrease. The math is simple: calories burned is the product of intensity x body wt.

The confusion comes from the fact that, as conditioning improves, the heart rate for any given intensity will decrease also. Because the HRM shows a lower number, most people think that they are burning fewer calories, but, in fact, it just means that the HRM is undercounting.

Steady-state aerobic exercises (e.g. running a 10:00 mile) have relatively fixed energy costs. The energy cost (and the subsequent caloric burn) does not change significantly as your fitness level improves. Heart rate decreases because the effort is now a LOWER PERCENTAGE of your maximum (because max fitness level has improved with training), but the energy cost has not. So in this case, the lowered heart rate does NOT indicate that you are burning fewer calories--it just means you can burn the same calories with less effort. An HRM does not know your fitness level has improved--it just counts heart rate and references the initial set up data.

Over time, there may be a slight increase in mechanical efficiency, but that is relatively trivial.

For more complex, less structured activities such as mixed-movement classes, the effect of mechanical efficiency might be greater. However, as efficiency improves, one is likely to work HARDER, so you would expect actual caloric expenditure to increase.

The concept that "Each switch will use different muscles or use muscles differently, initially harder and burning more calories" is physiologically not true. Changing exercises changes the training stimulus, which can be helpful or sometimes necessary, but any hypothetical increase in calories burned due to mechanical inefficiency will be more than offset by the fact that, during that period of inefficiency, you cannot exert yourself at the same level.

As you suggest, it should not be seen as a "problem", since the whole conundrum comes from the fact that fitness level has improved. And since fitness level has improved, people can work harder and burn more calories anyway, so it's not something anyone should lose sleep over.

For more detail, anyone can read this:
http://www.myfitnesspal.com/blog/Azdak/view/calories-burned-during-exercise-it-s-the-intensity-not-the-heart-rate-that-counts-26524

Great information! Thanks! Now I want to read more about it.

The explanatin helps put the HRM's role in exercise in perspective. I had dropped one pleasant exercise, fast-walking hillside trails in a local park, because I thought it was no longer efficient exercise for me, based on HRM calorie data (decreasing). But I love being outside in a lovely place with the sounds of woodpeckers and sighting the occasional coyote, enjoyment that I can't get in the gym. Now I'll add it back as an exercise option. But I don't regret learning the new exercise options I've gained in the meantime. .

This is probably somewhat repetitive, but a few additional thoughts. Yes, as you lose weight, you're moving less weight, which means you're burning fewer calories. But, as your cardiovascular condition improves, your heart rate will also stay lower for the same kinds of exercise and won't entirely reflect the calories burned if you use your original settings. If your HRM has fitness options (eg: my HRM has a "fitness test" that checks for resting heart rate, estimates or lets you enter VO2 max, and has a setting for exercise level), you might just need to adjust them.