Some good questions. you also need to keep in mind that it is easier for the body to turn muscle/muscle proteins into energy than it is to burn fat. I may be wrong....maybe someone can shed more light. The other thing is the slow and fast twitch muscle fibres as already mentioned. your big body builder will never be able to last a high intensity spinning class...just as the cyclist or runner will not last a high intensity body building session. Been there and done both. keep training
