Lie on your back and hold the abs in a crunched position to press the lower back into the floor. Raise one straight leg to a vertical position while the other remains just off the floor, then alternate legs while keeping the lower back pressed into the floor and not allowing the heel to touch the floor at the bottom.
 
Purpose
The alternating lying leg raise is a more accessible variation of more intense hip flexor exercises like the hanging leg raise, and like the dead bug, the purpose is to both strengthen the hip flexors independently for balance, and the ability to stabilize the pelvis against unwanted movement.
 
Programming
3-4 sets of 10-50 reps or doing max reps in set periods of time such as 20-60 seconds are good prescriptions.