One of the most important muscle group as a highland dancer!
Russian Twist: While engaging your core, lift your bent legs off of the ground. With or without a weight, twist and tap the object or your hands to the ground to the side slightly behind you.
Dead Bugs: With your core engaged, and all of your back touching the ground with your spine aligned, bend your knees and lift them one at a time into the air. Straighten one leg out straight to the height that is manageable for you while bending the other knee even more to bring your leg in closer to you. Make sure your core is completely engaged and that you do this exercise slowly.
Bird Dog: Go onto your hands and knees. Make sure your back is not arched either upward or downward. Keep your head down. Extend opposite arm and leg and hold for at least ten seconds.
Plank variations: Keep your back flat and head down and aligned. You can also do this on your elbows as demonstrated in the side plank. For the side plank, keep your spine aligned and the hand that is not supporting you, on your waist or in the air. In the normal plank, you can bend your knee to touch your elbow. This can be done with opposite knee touching opposite elbow (may need to shift forward).