rack one`s brains
to try hard to think of something or remember something
I have been racking my brains all day trying to remember the man's name.
Best Bodyweight Exercises for a Strong Core
Lying Side Crunch
Lie on on your side with knees bent at a right angle and twisted to the left. Curl upper body, lifting shoulders off the floor a few inches. Pause at the top of the contraction and slowly lower back down. Switch sides and repeat.