There are four distinct muscle-groups to the abdominals: Rectus Abdominis (top and bottom), Internal Obliques, External Obliques, Transverse Abdominis (core). Each performs a different task and Sitting Twists work all four of them, to some degree.
What's involved: Rectus Abdominis (upper and lower abs), Internal Obliques, External Obliques, Transverse Abdominis (core), front hip flexors.
Powered by: Lower abs, front hip flexors, obliques, core.
Tip: Breathe out sharply as you twist from side to side, tightening up your lower abs and aligning the muscles for even better results.