Lying flat on your back, crunch your abs to push your lower back flat into the floor and lift your straight legs so both heels are off the floor. Alternate lifting and lowering the legs in a short range of motion while preventing the pelvis from moving—keep the lower back pressed into the floor.
 
Purpose
Flutter kicks are a low-intensity hip flexor exercise and a pelvis and back stability exercise—training the ability of the abs to stabilize the pelvis and back against unwanted movement.
 
Programming
Do 3-4 sets of 20-100 reps, or do max reps in specified periods of time, such as 20-60 seconds.