You can do a jumping jack in plank, so then you work not only legs but core. I don't know if that will hurt or not but you will be on your toes not the sides of your feet. You start in plank and keep your core tight, then you jump your feet out like a jumping jack. You can also add a push-up in, when you go down for the push-up jump your feet out. You can do a lot of things in plank, running, jumping the feet in and out, etc. Jumping jacks used to kill my ankles and calves, now I can do them like mad.
125 whenever I get there
135 by May 24th (2012)~~~~Accomplished!