Meh, I think this impression people have that they always look fresh and play high energy football is just a myth built up by people who don't watch them closely.
Klopp has been rotating this season; they played a second string in a CL quarter final just this week. And they have had a lot of games over the last couple of months where they have looked the opposite of fresh but they still manage to grind out results.
It's lazy analysis. People could have said the same thing about us under SAF. "How do Ronaldo, Rooney, Tevez, Rio, Evra etc. look so fresh week in week out despite never being rested?".