I've spent the last couple of years working much harder in the gym with weights and am now in possession of a much perkier posterior than I've ever had in my life! i'm 49 btw.
Whilst I love (and am feeling very chuffed about achieving) my 'new' shape, I have noticed that my normal mid rise knickers now spend a lot of time riding up in between my bum cheeks which is uncomfortable and embarrassing !
My darling daughter, herself an 18 year old semi professional sportswoman with her own pert derriere (!) said (somewhat sarcastically in my opinion 
) "why do you think I wear thongs mum?".
So my downside of getting fitter is the need to purchase some thongs rather than granny pants!
Anyone else discovered their own downside?