Originally Posted by Simetrical
Yeah, but you aren't. You're resting the shield on your head. This wouldn't tire you the slightest bit (although it might give you a headache if you weren't wearing a helmet, which the legionaries were). The force is basically transferred straight to the ground via your skeleton—you only get tired when you use your muscles, and the only function muscles would serve here is to balance you. Balancing with a shield on your head is, if anything, less taxing than balancing with one on your arm. If you were holding something in the air, on the other hand, your muscles would have to exert themselves constantly so that your arm wouldn't fall.
Testudo should have no effect on stamina—standing still is testudo should actually refresh you, as standing still normally does.
-Simetrical