Lie on your back with your arms overhead 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. Hold this position for the intended time.
 
Purpose
The hollow hold is a simple isometric trunk exercise that develops stamina and control for general trunk stability.  
 
Programming
The hollow hold should be performed similarly to a plank with holds of 20-60 seconds. Difficulty can be increased by placing a plate or dumbbell on the shins and holding weight in the hands.