[tom brady]Mike Ehrmann/Getty At age 38, Tom Brady just put together one of the most efficient seasons of his career. Brady's incredible 2015 stood in stark contrast to his other older quarterback peers, like Peyton Manning, whose bodies had clearly betrayed them. Brady has structured his life in a way that allows him keep playing football at an age when everyone else retires. He goes to sleep at 8:30, he doesn't party, and he has a strict diet, which he details in his new $200 "nutrition manual." Brady spoke to GQ's Andrew Goble about the book, and explained his biggest secret to maintaining his body to stay on the field. "The biggest issue is muscle pliability," Brady said. "That’s what I think the biggest secret to me is." As Brady explained, muscle pliability is keeping muscles soft and long. It stands in contrast to lifting heavy weights, which shortens the muscles and makes them denser. Brady then went into a long, rambling answer about why he views this as such an important part of his routine and his longevity. We do so many exercises in the gym through all our strength training that make our muscles short and dense. So now you get up and you’re doing all these active things where you’re running and cutting, and you're asking these muscles to expand and contract. Well, you basically taught your muscles just to stay contracted. ... There’s a lot of components to every muscle function, to every type of exercise that you do. For me, I focus on dropping and throwing. That’s what my job is. For a runner, you’re going to need to be incredibly biomechanically efficient and balanced in order to run that 5K, and if something is out of balance, you’re going to need to get it back in balance. And the only way traditional physical therapy does it, they think that muscle soreness is muscle weakness. So they take muscle soreness and they go, well, it must be weak. Let’s do more strengthening. My point is, it’s not weak, it’s actually working too hard. We need to lengthen it and soften it, so that the other muscles can work, too. Brady continued, saying that when an athlete then tries to go out and perform, their muscles are essentially confused and unable to expand and contract as needed. Brady said this tricks an athlete's brain into trying to compensate by "going to recruit fibers" from other parts of the body, thus weakening those parts and throwing the system off-balance. Brady told Goble that this type of understanding of the body is what pushed him to publish his nutritional manual. Watching Brady play football is not like watching, say, Cam Newton. Brady's arm is not the strongest, and he's certainly not the quickest when it comes to scrambling. He isn't built like a Greek God. But his longevity and health can't be argued with, so clearly Brady is onto something. NOW WATCH: STEPH CURRY: Inside the spectacular life of the the NBA’s first unanimous MVP