Doing sit upsWatch
so for general exercise you burn fat, but when fat is deemed too 'slow' (ie the body needs a fast energy source) it switches to carbohydrates.
but then if you work harder, that will improve your cv system too ? like strengthen the heart etc?
But have you seen how slowly some people swim? Their hearts are probably barely above their resting rate!
it doesn't matter if it is slow, they are still burning calories not as much as if they were doing it fast but at least some.