its another way of jump roping i guess... just that jumping jack moves the arm up higher...
i dont think its so harmful for the calf muscles... it should be more good to your body... since it "trims" your heart each time you do it, and that way burn more fat... another word is just simply cardio exercise...
so its a good thing

how many do you do each day btw?
depends how often i pee/poop (at home) in a day. haha. i guess it varies on my mood. sometimes after i wake up I do 100. sometimes i don't. i
always do 125 before i go to sleep though. i do
at least 50 to dry my hands, but sometimes i just keep going until the toilet flushing sounds stops (LOL. that's like around 70~ish).
Just be sure to wear a good sports bra when you do them otherwise you'll end up with very saggy breasts someday.
LOL.

i thought about that once, actually. thanks for the reminder though!