Foam muscle suit question

For those who have made foam muscle suits for predator type costumes, what type of undershirt works best? Does it need to be spandex?

 

Any compression shirt will do the trick. I wear one under all my costumes. For a predator suit I would also wear compression pants. Check out eastbay.com

 

the ones underarmour do are supposed to be good no experiance with them though http://www.underarmour.co.uk