Lie on your back and crunch your abs to create a “hollow” position—this should push your lower back flat into the floor and elevate your feet and upper back. While holding this position forcefully, press a dumbbell or similar straight up with one arm for the prescribed reps, then switch and repeat on the other arm, without allowing the body position to change.
 
Purpose
The single-arm press in hollow increases the difficulty of the hollow hold both by increasing the resistance and adding an anti-rotational stability component.
 
Programming
The single-arm press in hollow can be performed for reps of 10-20/per arm—a rep range that will create similar hold times to a plank, i.e. 20-60 seconds.
 
Variations
Difficulty can be increased further by holding weight on the shins.