No one saw this coming: England is in the World Cup semi-final. As if that wasn’t surprising enough, research conducted at the University of Huddersfield suggests that the English, after a historic series of wins that put them the closest to a World Cup since 1990, may even have a physiological advantage when they meet Croatia on Wednesday. The fact that the Croatian team played at least 30 minutes more England last weekend may just be enough to give the three lions an edge.

This potential advantage for the English team lies in the path they’ve taken to the semi-final. Despite their highly stressful shootout against Colombia on July 3, England has only gone into extra time once during the tournament. Croatia, meanwhile, had had to play extra time in back-to-back games since emerging from group play, and this is crucial to performance, says Liam Harper, Ph.D., a senior lecturer in sport, exercise and nutrition science at the University of Huddersfield and author of a suite of studies on the effect of extra time on game play. “Because glycogen is depleted further in extra time, they may have muscle damage that could have the potential to impair the recovery process,” Harper tells Inverse. Muscle glycogen is the stored form of carbohydrates, which cells primarily use for energy. “It typically takes three to four days to fully recover.” After going into extra time versus Russia on Saturday, Croatia will just be entering the fourth day of their crucial recovery window when they meet England.

It might seem obvious that playing more soccer would lead to longer recovery times. But the physiological effects of prolonged soccer matches is actually very understudied and quite complicated, Harper says. This is because soccer is a unique game with the length of an endurance event (two hours total, if extra time is added) but the explosive sprinting of a much shorter game, like basketball. To better understand how these two variables physiologically effect players, Harper conducted a series of studies on professional and amateur players from 2016 to 2018, analyzing them for signs of exhaustion that could elucidate what goes on inside the body in those crucial 30 minutes of extra time.

Harper observed that levels of blood glucose — sugar that cells metabolize to create energy — were very low at the end of matches that entered extra time. As blood-glucose decreases even more during extra time, it leads to a change in the type of fuel that muscle cells use to create energy. “We attribute some of that to a change in substrate usage, from carbohydrate to fat as a fuel source,” says Harper. “Fat is usually used for low intensity exercise and carbohydrate is used for high intensity sprinting, like you see in football.”

The results of his research suggests that the Croatian team, after playing back-to-back extra-time games, has been seriously depleting their muscle glycogen and falling back on fat-burning. This is a risky spot for Croatia, particularly because Harper’s 2015 paper suggests that there’s a [35 percent chance] they’ll enter extra time tomorrow for a third time. Harper, who has been watching the tournament with professional interest, thinks that the odds of extra time tomorrow are perhaps even higher:

“I wouldn’t be surprised to see both semi-final games go to extra time,” he says. “That would be great for my research.”