This is exactly what makes me nervous about starting the shred. On Amazon's reviews, almost all the negative ones mention a knee injury...

I ended up wrenching my knee..... Although not because of 30DS. I wrenched it getting into my low sedan with heels (with heels I'm 6'+). So I had to stop. I finally started feeling better with the knee and now I'm sick lol.

I'm going to start reincorporating 30ds soon. I enjoyed the workout and did not feel any knee pain - I tried to focus on the quality of the movements and not the quantity because of what people said about the knees. I modified the jumping jacks a bit because they do tend to hurt my knees of I do them for more than one day.