This makes sense based on my experience; like fewer and fewer kids these days, I stayed in the same school system for all 12 years of my early education and remember that the kids who were good at all the competitive sports in gym class at age six became the fittest varsity athletes in high school.  From day one in first-grade gym class to high school graduation, there was no major reshuffling of athletic prowess among the 200 kids in my school.  

No real surprise:  if you're one of the school standouts at catching and throwing and kicking as a kid, you keep doing it.  You join sports teams.  You get fired up at the thought of competing, and working on your fitness feels like fun and games to you and becomes an affirming, joyous experience.  So you get fitter, just as smart kids rise to the challenge of tough academics and thereby get smarter than their less academically-inclined classmates.  The fit get fitter, the smart get smarter.

To some extent, this is fine, of course; there's no sense in trying to force high-level musical or artistic competence (for instance) on every student.  At some point you cut your losses and work on developing your strengths.

But gym class is a different animal:  it helps set the stage for how kids will approach fitness throughout their lives, and while one could argue that it's no tragedy for a kid to make it through grade school without knowing how to play a G major scale on the harpsichord,* it's a rather more significant loss if kids graduate hating all activities that could enhance their fitness. 

Given this equation, the challenge for educators--and parents--may be to base gym classes not on competitive sports (which require coordination and skills that some kids clearly lack) but on fitness-building activities that are "Personal Best" in nature.

There's no reason that a kid who can't throw or catch can't still have a decent VO2 max, good upper-body strength, or excellent body composition, except that, in the current model, the one is a prerequisite for the other.  

[*Eds. note:  I have nothing against music class!]