Lie on your back with the knees and hips bent to place the feet flat on the floor—the feet can be anchored under or around a weight or rack. Hold the fingers at the sides of the head. Crunch the trunk and then lift it to bring it upright while rotating one shoulder toward the opposite knee. Return to lying flat while maintaining tension in the abs and repeat, rotating to the other knee.
 
Purpose
The rotating sit-up is a basic and accessible but effective trunk and hip flexion exercise for trunk stability; the added rotation is an easy way to train rotational strength and stamina.
 
Programming
Perform sets of 10-30+ reps; if adding weight, perform sets of 8-15 reps.
 
Variations
The rotating sit-up can be done with or without the feet anchored, with additional weight (ideally held behind the head/neck), with slow tempos, on a decline bench and with pauses in a partially-sitting position.