limber up
phrasal verb with limber verb uk/ˈlɪm.bər/ us/ˈlɪm.bɚ/
（运动前）做伸展运动，做准备活动
to do gentle exercises to stretch the muscles in order to prepare the body for more active physical exercise
The substitutes are beginning to limber up on the sidelines. 替补队员开始在场边做准备活动了。