Why is Russian twist so hard?

Russian twists target the following muscles:

The Russian twist is a simple and effective exercise to tone the core, shoulders, and hips . It’s an ideal exercise for those trying to burn fat and strengthen their midsection and get rid of love handles.

The Russian Twist got its name because it was used by the Russian Army as a conditioning exercise for soldiers . The Turkish Get Up. Another abdominal exercise, the Turkish Get Up was originally used by Turkish wrestlers as part of their conditioning training. ?