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。