PREPARATION
1.Lie on your back with your knees
  slightly bent.

2.Place your hands behind your
  head, with your elbows out to
  the side. Lightly clasp your
  fingers to support your head.

3.Tilt your pelvis slightly
  forward to press your back into
  the floor. Breathe in.

EXERCISE
1.Breathe out as you curl your
  shoulders away from the floor,
  raising your upper body into a
  sitting position. Allow your
  knees to bend to help prevent
  straining your back.

2.Breathe in as you lower your
  upper body back to the floor.

Try not to pause between
repetions.

MAINLY FOR: Rectus abdominis