Sorry, I meant "muscle confusion". Read this article from Hayhurst, it's pretty good.
http://dirkhayhurst.com/2016/06/height-doesnt-measure-heart-but-it-does-measure/
And I'm fairly confident that many of those "4SM" on Brooksbaseball were actually 2SM missing high, since most, if not all, had the same horizontal movement as the 2SM.