I have the same problem, the first thing to seize up is my calfs and they cramp really easily, especially when Im swimming. I dont really know the answer except to concentrate on stretching your calves as much as possible. A stretch I do is - stand on the edge of a step or kerb with your toes on the step and your heels off, drop the heel of one foot down (supporting yourself with the other foot) and you should feel the stretch right down the back of the calf. To concentrate on the lower part of the calf bend the knee of the leg you are stretching then drop the heel.
Hope these are of some help, I would be interested in what others have to say.
