A great antagonist movement for abdominal exercises like Ab 45s and the Reclining Field Goal is the Superman Back Extension. Do it on a back hyperextension or glute-ham raise machine.
Extend your arms fully overhead and keep them in-line with your torso throughout the set. Now, raise your upper body up until it’s parallel to the ground and hold that position for a 5-count, then lower back down to the original position.
Work up to 10-12 reps holding each contraction for 5-10 seconds.