The person expresses her subjective feeling of fatigue after jogging.
What is "heavy knees"? I've never heard this expression before in my life.
heavy legs or heavy arms would be more usual - it indicates they feel heavy and hence hard to lift after exertion or due to illness even. I am guessing in Romania they might more often complain of their knees than their legs more generally? WHat i would ask is why is it the definite article heavy knees in Romanian and why do we need a 'ce'?
You use "după ce " for activities that are performed ("după ce alerg/ beau / mănânc/ se termină filmul") and "după " when naming activities ( "bea un pahar de apă după alergare / masă/ terminarea filmului")
I confirm that Romanians would also use this expression for legs and arms rather than knees.
You use the definite article for parts of YOUR body. I think this comes from Latin because I have seen it in French and Italian also