I went back to 2005 (PED testing did notbreallybbegun until 2005) and looked at all players who had hit 10 or more HR in the contests as a proxy for how much effort was expended as opposed to someone one and done who did not do much in the contestis there any credence in the HR Derby theory? I know people have said that it screws with a hitter's swing, and his 1st half/2nd half split is pretty stark.
I found there was a 23% drop in HR/G in the 2nd half. The same players had a 6% drop in the 2nd half for the career. The net effect was a 17% drop after going somewhat deep in the HRD.
About 30% of these players had no significant drop in HR/G output in the 2nd half and were more likely to repeat the HRD.
Only a handful of cases where HR/G output increased out of 46 player seasons.
Whether its the HRD itself or simply regression or some combination its hard to say one way or the other, and it may be some players are more susceptible to a HRD effect than others.