For example, if you're a kick-ass runner and you run all the time and you run easily and without pain, your body is probably very adapted to the tissue strengths necessary to run. But say you help a friend move and, in lifting a few boxes, you are thrown out of commission for a month with a major back spasm. Weren't you strong? Weren't you in shape? Weren't you a regular mover, and doesn't that mean you were conditioned to move? The answer to all of these questions has to do with the Law of Specificity, which dictates that you get better at what you do and not better at what you don't. A particular way of moving creates loads and adaptations in only the tissues pulled and pushed and compressed by that activity.
It’s funny to see the law of specificity used as a negative to running. Context, as a runner (and coach) we love the law of specificity. “To be a good runner you need to run” is a common phrase.
But in a book about general health, this specific nature of well-trained runners can work against general health. As demonstrated by the infamous weak hips and glutes of runners, since running does little to properly work these areas - requiring us to do additional exercises to target what running neglects (lower leg work, hip work, glutes, core, etc.)