There's no link between personal physical exertion and energy used. What you find hard to do, doesn't mean you're burning more calories doing it than the person next to you who finds it easy. I could sprint 200m and burn 20 calories, in a race with Usain Bolt. He'd burn a couple more because he's got probably 3-4 stone on me. I'd be absolutely ****ed trying to catch him and he'd be ready to go again in 30 seconds. We'd still burn roughly the same amount of calories during the exercise though. Afterwards, I'm sure his body composition would have him burning many more calories than me, but that's not the discussion here.
I disagree there is a definite link between personal physical exertion and energy used. You're talking different people doing the same exercise which to a certain extent is true, different people use different amounts of energy differently, depending on their physical make up and fitness, if they were the same they would use the same. (I agree with you on that) I am saying if I sit still all day doing nothing I will burn less calories than if I walk round all day. There is a definite link between the amount of physical exertion and energy used. The discussion was about different types of physical exertion, eg boxing vs ground work. So I think maybe we are trying to both make different points.
The key word I used was 'personal' mate. You being tired has no bearing on how much energy you have used. You could be incredibly unfit (I know you're not) and ****ed off a jog round the block, but an extremely fit person would not burn any more calories during the run round the block (and that's the important part here) than the unfit person does. Fit people generally do things faster, or for longer. Body composition means a man with more muscle will burn more calories than a man built like a praying mantis, but most/a lot of those calories will be burned whilst their body is recovering. Of course you are right, energy expended is directly proportionate to calories burned but energy is measured with Joules, not effort, so we shouldn't confuse the two. We're essentially debating the same conclusion here.
