在体育比赛里经常会听到 to be under starter's orders 这个短语，意思是待发起跑令。

例句

The athletes have been waiting for this moment for years. They're under starter's orders. And they're off!

It was only a second or two but the runners seemed to be under starter's orders for hours before the pistol was fired.

请注意

Under arms 意思是出于备战状态中，随时准备作战。

Example: The rebels were under arms and an attack was just a matter of time.