loosen (something) up
phrasal verb with loosen verb uk/ˈluː.sən/ us/ˈluː.sən/
（体育活动前）进行准备活动，热身
to prepare your muscles for a physical activity by stretching and doing simple exercises
I do a few stretches to loosen up before I run. 我跑步前伸展四肢热了热身。