Anyone know why my legs will burn and tire out but the rest of my body feels fine when doing a full body workout?

Im doing insanity and there are workouts for example where i have to do 3 sets of jumping jacks for 3 minutes each, and stamina wise im fine im breathing well not tired but my legs are always on fire. I'd say im in good shape and have no diea what the problem is, if anyone could help.