Max Verstappen has hit back at claims about how easy driving a modern Formula 1 car is.
Recently, Lewis Hamilton said he could do three race distances on the trot, while Romain Grosjean says go-karting is more physically demanding than F1.
Red Bull's Verstappen said he does not agree.
"If I go karting now, I would be completely exhausted while it was easy when I was 15," he told Dutch media at Paul Ricard.
"But that doesn't mean I am less fit now. If you put friends of mine who go karting in an F1 car, they will be in severe neck pain the next day. It makes absolutely no sense to compare.
"You use different muscle groups for different activities and prepare differently. You see that in every sport," Verstappen added.
Honda has prepared another engine update for Red Bull in France, but Verstappen and the other Honda-powered drivers claim the step is small.
"It's nothing ridiculously big, but all the little bits help," he said, also referring to car upgrades.
"We can still take major steps on both fronts. And we need to do that to stay with Ferrari.
"Our car is better than theirs, but if you lose a second on the straight, it's difficult."
Verstappen also played down recent speculation about his future, including that if Red Bull cannot win in 2019, he may switch teams.
"I have no stress about my future," he said. "I have always said that I find this an interesting project that I enjoy a lot. Time will tell how it goes."