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Exercise to strengthen your "core" is big right now, and has been for a few years. Core strengthening is the basis for pilates exercise, and is a benefit of yoga, although the practitioners don't talk about it so much. Personal trainers at gyms can be seen having clients do many ordinary exercises while sitting on an exercise ball because this is thought to work the core.
The "core" in this case is the part of your body between the bottom of your rib cage and your hips, both front and back. Strength and stability in this area is indeed important to posture and health, especially to your back. Most of the muscles involved are deep postural muscles. The only one that is seen very much is the rectus abdominus--the source of the elusive "six pack." Products or programs that promise you "a sexy core" are to be viewed with suspicion. Definition in the rectus abdominus depends somewhat on genetics and a lot on having low body fat, no matter how many sit-ups or similar exercises you do.
Nordic walking is good for your core. Because Nordic walking uses most of your muscles and most of your joints, and does it while you are upright and using good posture, the core area is strengthened as those muscles work together. This can help prevent or correct muscle imbalances. The core area is important in any full body exercise. Nordic walking does more for your core, and is more of a functional exercise, than doing dumbbell biceps curls while sitting on an exercise ball, where you are probably using your legs to keep from falling off. So you can add core training to the benefits of Nordic walking.