Romain Grosjean says even go-karting is now more physically difficult than racing in Formula 1.
In Canada last weekend, five time world champion Lewis Hamilton said F1 used to be "a man's sport".
"Sometimes you do these races now and you could get up and probably do two or three races more. Formula 1 should not be like that," he said.
Grosjean, who drives for Haas, agrees.
"Recently I did a go-kart race with friends. 125cc with gearshift. And I was more tired than after a grand prix," he is quoted by Ekstra Bladet newspaper.
"Why? Because you are pushing all the time. But in F1, we are saving fuel and also the tyres. In Barcelona it felt like we were driving at 50 per cent of the car's capacity," Grosjean added.
"If the race was like qualifying then our necks would be totally gone in the end. We would be tired and our focus would disappear. But constantly saving petrol and tyres means it is not so difficult for us.
"Also, going back to refuelling would mean we don't have to run around with 100kg at the start but only 30-40kg. Then we would be a few seconds faster, if not more, and it would be harder," he said.
