Flutter kicks are a basic and relatively low-intensity pelvic and back stability exercise for the abs and hip flexors.  
 
 
Execution
 
Lying flat on your back, crunch your abs to rotate your pelvis posteriorly 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 and back from moving.
 
 
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.