Hi Torid,
Not Cathe, but I've sometimes noticed the same effect in myself, especially when I was first starting Kickmax when the Hardcores came out last winter. I think that the cramping is probably due to the intense high impact with very little recovery time between high impact moves. I know that I have to do this one on a completely empty stomach and that I have to take extra care that I am bracing my core as tightly as I can, really squeezing the abdominals while I'm doing the jacks. Both of these steps, and becoming much more accustomed to the blasts, has alleviated the cramping for me. If you still cramp, just sub the lower impact jack options that Cathe demos at one point during the blasts until you work your way up.
Hope this helps!
Mattea