Halfway into Friday's first practice session for the Red Bull U.S. Grand Prix, Casey Stoner knew he was in trouble.The Ducati Marlboro rider began to feel the same symptoms that have afflicted him the past two races and derailed his championship trajectory. The Australian is suddenly no longer able to challenge for the victory. Rather he's had to settle for third place, while Fiat Yamaha's Valentino Rossi and Jorge Lorenzo battle for victory. And in today's first hour at Mazda Raceway Laguna Seca, he began to suffer arm pump, something which he hadn't suffered since his days racing motocross."After probably around the middle of the session I started getting arm pump," he said after ending the first day of practice third fastest to Rossi and Lorenzo. "I've never had arm pump in my life. On motocross yes, on road racing never. And I was getting arm pump and towards the end of the session it was just so hard to change direction and be really in control and be aggressive with it. It was just impossible. So I was having to brake early already. And that last run, I just tried to push it out and see how far I could go, but it was just my limit and that was it."But the more troubling question is why he's been physically exhausted at the end of the past two races? He said that it "doesn't happen immediately. I don't have the cramps as an early problem. The cramps come because of exhaustion. The reason I'm getting exhaustion, I have no idea. We still have no insight into what's going on and there's just no reason for it. I feel good, alright during the week and then as soon as anything happens, I'm just dead. So until we find this out, I think we're going to just keep running into the same issue."Among the possible illnesses, he acknowledged, is Epstein-Barr, a fatiguing that causes weakness and exhaustion. Australian Mat Mladin suffered with Epstein-Barr in the middle of the 2003 season, though he was still able to win the AMA Superbike Championship. The symptoms can be treated, but not the virus. The most basic course of treatment is rest."This is already a symptom which I've spoken to a doctor that I trust in Australia and they say this is quite a possible thing," Stoner said. "I know another football player in Australia that had exactly this and he struggled with it a lot. Didn't realize what it was. Took a long time to diagnose, same sort of situation, so hopefully with a lot of different ideas running around we can take the right tests and they can come up with exactly what it is. And it'd be nice that we could figure out if it's something we can figure out, it's great, but at the moment we're really struggling with it."Stoner said that the Australian footballer was working with "keeping up the right amount of fuel in a certain situation. Your body...basically, they run a heap of tests. He's come up with it himself with a few other doctors and they run a heap of tests to see where your balance is out a little bit. And they give you the right amount every time to keep you in that balance if they know that you're always low on something. And it's supposedly helped him a little bit. And if it is something like that, we might be able to go for that situation, but at the same time, considering I've been training and doing everything as normal, I don't see how rest much more than what I'm getting is going to really improve it. I don't like sleeping that much."Stoner wasn't helped by the lack of recovery time between races. The day after last Saturday's Dutch TT, he flew from Amsterdam to Los Angeles for a media event at Universal Studios on Monday. Then he traveled north to Monterey."We haven't had any time to recover here, so we've gone immediately back to where we were basically on Sunday in Assen and my energy just went immediately, so there wasn't a lot I could do," he said. "I was doing my best the last few laps, but I was already getting too tired to improve anything."While he was at speed, he felt the Desmosedici was working well. They tried settings similar to last year, "but we've gone with something already we changed from this year. We have a little bit different things on the bike and it's working really well. We still need a little bit more traction, of course, and maybe a little bit more stability on the brakes. But the bike was working really well and it's capable of going a lot faster. We just, we can't do it today. So tomorrow we'll change a couple of things, but I don't think we're too far from the race setting now. We're already on the hardest tires and they're working really well, so I think everyone else is in a pretty similar situation by the look of it."Stoner's next step will be to make a decision about his course of treatment straight after the race "about which way to go first, which test to do first and then wait for those results or go straight into the next tests. I don't know. Like I said, there's so many ideas floating around, nobody really knows what it is, because it is so difficult to diagnose and if we could figure it out sooner rather than later, it'd be nice, real nice."We'll see how the weekend goes. We're taking a few different things this weekend and we'll just see if we can keep my energy levels up a little bit higher for longer during the race. I feel that these last three tracks we've had the pace to at least fight for the win, even if I eventually get the same result. But I'm just watching them ride off into the distance and struggling to get around the track myself and keep the rest of the riders behind me. So it's kind of boring races for me. I'm just sitting there trying to survive."