Old school abdominal crunches are not en vogue anymore in the fitness industry, with good reason.
Our clients sit all day in a flexed position, so there would seem to be little sense in having them come to the gym and repeatedly flex their spine again. Renowned spine expert Stuart McGill makes a pretty water tight argument against training spinal flexion is his book ultimate low back performance. Add to that the limited time most clients have to spend with their trainer (we see our clients on average 8 hours per month) and its not hard to see why crunches tend to get dumped in favour of the big bang-for-your-buck exercises like deadlifts.
I still agree with all of the above arguments so it is still quite rare that I will include flexion in a clients program. But as with all training modalities I feel there is a time and a place where it can be effective.
When a client is already lean , circa 12% bodyfat, and is looking to step things up for an event such as a beach holiday, I will now have them perform direct ab work in the last two weeks on a 3 days on, 1 day off basis. Naturally, this is in conjunction with a number of other rapid fat loss strategies including additional short home workouts , diet and supplementation.
The benefits of this approach are as follows
1- The abdominals are muscles , they are capable of hypertrophy (muscle growth). Their capacity is pretty limited, but as a short term strategy for "peaking", a little bit of ab work can help show more definition.
2- it reinforces good client behaviour- the easiest part of my job as a trainer is the training session. The hard part, the art and the science of achieving real world results, is in everything the client does in the other 166 hours of the week when I'm not seeing them. Anything I can make the client do outside of the session to keep them on track is beneficial. Short ab sessions done at home most days help keep the client focussed on the outcome they want and therefore more likely to stick to the other behaviours that support that goal. If every night you're spending 15 minutes focussing solely on developing your abs, don't you think you'll be that little bit less likely to cheat on your diet or miss other training sessions?
As you can see the psychological benefit is as important as the physiological one, probably more so.
As for how I advise training the abs when using this strategy, I don't think exercise selection is as important as how the exercise is done. I advise the client to simply choose two flexion exercises, for example, crunches on the floor and reverse crunches, and do 3 sets each of about 15 reps. I tell them to focus on flexing the abs as hard as possible throughout the sets. Squeeze hard to bring the rib cage toward the pelvis, and maintain that tension when lowering the body so the low back almost has to pull the upper body back down.
That's it. Again, this is not a long term strategy, and is a very small part of the overall approach I use to get clients into the elusive "six pack abs" club. So if you have an already solid fat loss plan in place to take you to single digit bodyfat give it a try, but if you still have an appreciable amount of fat to lose, your effort should be spent elsewhere.