The February 2, 2009 issue of Sports Illustrated (with LeBron James on the cover) includes a sometimes humorous, sometimes serious article by Richard Hoffer on the elusiveness of sports momentum as a documentable phenomenon (at this point, I've not been able to find an online version of the article). As the article's subtitle puts it: "Broadcasters and coaches love to cite it, but what exactly is the Big Mo?"

After citing some academic concepts and articles, Hoffer draws some conclusions:

If [momentum] were going to be that easy to explain, we'd have found a way to duplicate it, coach it and write best-selling books about it...

...at these upper levels of performance, the talent is so evenly distributed... that random events -- pieces of luck, an unlikely and inspired effort here or there, a boneheaded decision, a blown call -- are often the difference in a game, a season or even a career.