At that moment, he writes, “My brain is thinking only lengthen and soften and disperse.”

Mr. Brady attributes much of his unusually long N.F.L. career and the relatively little playing time he has lost to injuries to this kind of gnomic self-talk and his muscular pliability. But most of us are not pliable, he tells us, and our workouts are not helping. Standard weight training and conditioning exercises, such as running and sprints, tighten and harden our muscles, he writes, opening us to injury.

Instead, we should do less weight training or conditioning and more of… something else. It involves “targeted, deep-force muscle work,” Mr. Brady writes. “Think of a deep, rigorous massage, but much more focused, and in my case using complex techniques based on an understanding of the biomechanics of what it takes for me to throw a football and function at peak levels as an athlete who accelerates, decelerates, runs, cuts, and more, as well as the daily acts of living that complement my off-field life.”

Got it? Me, neither. Thankfully, Mr. Brady and Mr. Guerrero amplify and simplify their instructions as the book continues. Mere massage does not produce pliability, they write. You must contract muscles while also stretching and pummeling them, preferably with high-tech, vibrating foam rollers or vibrating spheres, Mr. Brady writes. These are available at The TB12 Method online store for $150 and up.

The problem with this notion is that exercise science has never heard of muscle pliability.

“It’s balderdash,” says Stuart Phillips, a professor at McMaster University in Hamilton, Ontario, and an expert in muscle physiology.

In scientific terms, he says, muscles that are soft tend to be muscles that are sick. “When folks do little or nothing, as, for instance, during bed rest, then their muscles get very soft,” he says.