30DS - level 3 rock star jumps

leappleton

Hi guys, A bit of advice or suggestions please if anyone can help.
I'm on day 3 of level 3 of Jillian Michael's 30 days shred and I'm loving it, have lost inches all over so far and can tell that level 3 is going to make me even more 'shredded'.
However, I'm physically struggling with the 'rock star jumps' in level 3, not 'wow I'm not fit enough or will have to work really hard to do these' type struggling just plain and simple 'I can't do these!' struggle!! Some of the other moves throughout the DVD I struggled with at first but as I got stronger I mastered them but after 3 days of level 3 I just feel I can't do these, they just don't feel right. I have had 2 operations on my right knee to replace my cruciate ligament and I think this is contributing as well, I had a graft taken from my hamstring to replace the ligament and so I struggle to 'pull' this heel up in that movement.
I wondered what is similar that people may suggest I substitute these for?
Thanks :-)