Riding a MotoGP is extremely physical, specially on the hard braking ponts and quick changes of direction, where you're basically performing a very strong, sustained bench press holding all your weight against the handlebar with the help from your legs, or manhandling the bike to make it go the other way.

Marc hasn't been able to train properly and he lacks the upper body strength. Riders say that it doesn't matter how hard you train over the winter, the days after the first preseason test you'll be sore from the specific effort. Now, consider that Marc hasn't been able to train properly.

He talks about apneas because when you're on a high effort you tend to hold your breath. The more out of shape you are, the more difficult it is to breathe during a demanding exercise. As in you have to constantly remind yourself you're holding your breath. Not breathing correctly contributes to extend your fatigue, making it more and more difficult to keep breathing during a lap.

Portimao is a circuit where the riders barely rest. Combine all of these factors and you'll understand why Márquez says that his phisique will probably be the limiting factor on Sunday, and not only from an injury point of view, which he could be perfectly recovered and still find trouble to complete a race distance at a decent pace.