The recent admission of steroid use by Mark McGwire brings another black chapter in baseball. This news report cited a paper by Roger Tobin, which I mentioned already a while back. The AJP paper examines the correlation between increase in batted speed (presumably due to increase in muscle mass) and the ability to hit long balls.
Note also that statistical evidence has also been used to show that there's a likelihood of the use of performance enhancing drugs in baseball.
 R. Tobin, Am. J. Phys. v.76, p. 15 (2008).