Lie on your back with the legs extended straight and your arms extended overhead. Perform a sit-up motion while lifting one straight leg to meet the opposite hand overhead. Return to flat while maintaining tension in the abs, and repeat with the opposite leg and arm.
 
Purpose
 The single-leg V-up trains both strength and stamina of the ab and hip flexors, and provides a more challenging variation of the sit-up and jack knife, and improves symmetry and control of the trunk and hips.  
 
Programming
The single-leg V-up should be performed for sets of 10-30+ and will generally be more of a stamina exercise with higher reps.
 
Variations
The single-leg V-up can be performed without allowing the heels to touch the floor in the bottom, requiring more strength, stamina and control.