She had a ligament removed from her knee that had started to become troublesome (if I remember correctly). This happened right after the presale for Drill Max, Low Impact Circuit, Butts 'n Guts, & Body Max 2 (not it's own series, but she released them together) had begun. It delayed the filming by a significant amount of time. However, you'd never know she'd just had major knee surgery from the videos themselves.