A while back, I wrote about my dislike and eradication of crunches from my strength training. Part of my reasoning for dropping them from my training was because I don't think that they train the abdominals to move in a way that they're not really meant to move. They're supposed to contract to support the natural position of the spine when it's under a heavy or intense load. So, the best exercises for them are exercises that force them to contract when the spine is in a natural position, not when the spine is bent forward.
One of the exercises that work the abs in just that manner is the ab planks. Plus, these exercises are good for just about anyone, regardless of physical condition, can do. They're frequently used in rehab and physical therapy. Unfortunately, that's enough for some to disqualify them from more advanced strength trainers. Frankly, they have a bit of a point.
After a while, some would have to do these for an unreasonable amount of time to make them a challenge. So, I've heard some talk about dropping them, declaring them a worthless exercise.
The truth is that they're far from a worthless exercise. What these people suffer from is a lack of creativity that plagues BW training. Let's be frank about what a plank really is: it's a modified, isometric version of the push-up, usually the starting position. We're used to seeing people perform it on their forearms and ocasionallly on the hands. Where does it say in the book of planks that you can't do them with your hands at the BOTTOM of the push-up position? Try holding that for a minute. You could make the plank harder by moving your hands closer together. This is more unstable since you're narrowing your base. Mix it up by performing these close-hand planks in the down position.
Since we're making isometrics out of the standard push-up, why not turn other push-ups into planks? This produces a whole new level of hard fun for those who insist that these rehab exercises are strictly for beginners. In my not-so-humble, non-expert opinion, there are two prime canidates. The first one would be the superman push-up. Simply move your hands out in front of your body when performing this plank. The same principle applies here as with most other BW exercises: the narrower your base, the more difficult the exercise. So, you can make this one harder by putting your hands (or feet) closer together.
My second canidate is the one-arm push-up. Obviously, you'll need to do alternate between your hands. Still, it's brutal! If you need to make it easier, simply spread your legs wider. If you really want to have some fun, you could do this one in the down position but I don't recommend doing this one all the way down. It can put a lot of strain on your shoulder and elbow. Stop just a few inches short of rock bottom. I've even done these while on a push-up T-handle, although not in the down position (not yet, anyway).
Planks, like any other BW exercise, have a remarkable range of versatility to them. Like their half-brother, the push-up, they can range from reasonably easy and suitable for a beginner to very difficult for the most advanced strength trainer. It just requires some careful creativity when exploring their full potential.