I touched on the topic of functional strength in an earlier post. I mentioned my misgivings about what "functional strength" training has become. Many who speak on the topic have no business even discussing the matter since they don't work for a living. I remember someone on a forum who discussed what he thought was functional movements once. I believe he's some kind of software programmer and he believed that the farmer's walk and certain deadlifts were among the most functional movements since they mimic real life movements.
Certainly he has a point. If you're weak in those movements, then it makes some sense to train that movement. Certainly practicing a movement that you do on a regular basis will help you get stronger in that movement. Still, I don't think that's good enough. You train to get strong but you should also train to stay healthy. If you aren't healthy, eventually, you'll get weak. So, you need to do things that will strengthen your body to withstand the rigors that your body endures.
This past year, if I had to point to one exercise that has kept me functionally strong this year above all other, I wouldn't hesitate to say that ab wheel rollouts. It's not functional in the sense that it mimics any movement that do on a regular basis like a deadlift does. However, I can say that prevented me from getting lower back pain from the constant, repetitive lifting that I did on my digester cleaning job.
This exercise works so well because one of the many things that the abs do is stabilize the spine under heavy loads. Sit-ups are kind of worthless because your abs are supposed to contract powerfully in a straight line. That's why Pavel stated in, "The Naked Warrior" that flat abs are strong abs. When you do ab wheel rollouts, the entire core must tense powerfully to stabilize the movement.
Now, I'm not saying that this is the ultimate, functional exercise (although, for me, I personally think and feel like it might be). I'm just trying to illustrate that functional strength training has another element to it that many people miss. Keep that in mind when you're training. It's not always about getting stronger. If anything, if you do a movement at work long enough, you'll get strong without an exercise to affirm it. You need to make sure you're bomb-proofing your body while you're at it.