'I rest after effort' sounds odd in English. I understand what it means, but it's clunky.
"I am resting after exerting myself" sounds much better, more idiomatic, than ".... after the effort."
In the states we would say, I rest after exercise or exercising, we would seldom or never use the word effort
Polish sentence does not assume that one was excercising. One could just work.
- wysilić się
- zadać sobie trud/wysiłek
- włożyć wiele wysiłku/trudu w coś
- włożyć wysiłek w coś
I go with the exertion, it just does not sound proper and I doubt if it's used.