Skiing is all about the legs, right? So there's no real point in getting strong abs is there? First, let's clear up a misconception: there's a lot more to core strengthening than just the abs. All the muscles of the torso that give you support and stability for good posture need help, especially if you sit at a desk all day.

Okay, so why bother with the core?

Two reasons:

1. Injury prevention. Most importantly, because skiing, especially mogul skiing, can really pound the lower back, you need to strengthen those muscles and antagonistic muscles as well. A Swedish study found that people who could hold an isometric back extension for about a minute were much less likely to suffer back pain in the following year. For recreational skiers, this is far and away the more important factor.
2. Power transfer. If you have strong legs, but a weak core, your torso will be like a noodle that can't remain rigid and transfer forces efficiently. This is why Austrian skiers refer to torso "thickness" as a crucial characteristic of top competitors. Using those powerful legs efficiently requires a powerful core.

So what do you do?

If you're still doing sit-ups because that's what your high school football coach taught you, please stop. If you have a weak core, these can do more harm than good. What we want is exercises that are both safe and effective. There's not much point in getting injured doing the exercises that are supposed to help prevent you from getting injured!

Planks

If you do only one exercise, this is it (in various forms). Start with front planks. Essentially, get in a position like a pushup, but resting on your elbows, rather than your hands. Try to keep your body straight as a board and hold for one minute if you can (or two if you can). Form is important. If you can only go 20 seconds before your middle sags, only do 20 seconds.

Now, add in all variations. Do them on your left and right side. If you have the shoulder flexibility, do them in reverse as well. One great thing about planks is that you can do them almost anywhere. When I travel, I commonly do them in the airport and sometimes in the aisle of the aircraft.

Back extensions

These require a bit of equipment, but you can improvise. If you own a stability ball (yoga ball, Pilates ball, Swiss ball), you can do this in your living room. Put your hips on the ball and your feet under the sofa and again hold a position where your body is straight as a board. If you have a partner, you can get a bit better position by putting your feet under a dining chair with someone sitting on it. At the gym, you might have a special apparatus for this exercise (called a Roman chair) or you can put your feet under the dumbbell rack instead of the sofa.

The key point is that studies show that isometric endurance is more important than strength in terms of injury prevention. In other words, it's more important to hold the straight out position against the clock than it is to do "reps" moving up and down.

If you don't have any equipment at all, you can do similar exercises by getting on all fours on the floor and lifting one leg and one arm straight out, or by doing a "superman" where you lie on your stomach and lift your arms and legs slightly off the floor.

Go Skiing!

Obviously, that set of exercises barely cracks the surface. There are many many more, but this is a good start and will allow you to build some basic core rigidity and help maintain good posture and keep you healthy the next time you're out banging the bumps on the ski slopes.