Lying Leg raise
While lying flat on the back, raise the legs straight up until the hip is angled at 90 degree. Place the arms off to the side and keep the back as flat a possible to the ground.
Slowly lower the legs towards the ground while keeping the knees locked. Hold the abs as tight as possible to prevent to lower back from over extending in the bottom position. If you are unable to control the position of the lower back, shorten up the range of motion.
Raise the legs back up to the starting position. Squeeze the abs and take a big breath into the belly before starting the next rep.