thorfinn

"High-intensity" does not necessarily mean "anaerobic". The term literally means "without air" - i.e. your body is burning glucose without using oxygen. Sprinting is so energy intensive that after the first second or so, more energy is needed than there is oxygen to burn, resulting in anaerobic exercise. "Aerobics" is about the level of exercise where almost all the exercise is "aerobic", hence the name. It does teeter a bit on the border between - but that's the idea. The point of "aerobics" is to maximise the amount of exercise that you're doing without getting into anaerobic metabolism. That's not necessarily a point that your particular instructor actually understands, but it's certainly the original reason for the name.

The way to tell the difference? Listen to your muscles... if they're feeling "crunchy" afterwards, that's bad. "Crunchy" means that you have lactic acid buildup, and lactic acid is the direct byproduct of anaerobic metabolism.

The other thing is, your muscle "tiredness" is fairly strongly correlated with muscle glycogen content... the more "tired" your muscle feels, the less glycogen is left in that muscle, and thus the more likely that muscle is to have had to burn proteins. Overall tiredness is a summation of overall body glycogen content.

Running completely out of glycogen is likely to lower your metabolic rate, as is doing anaerobic exercise.