The plank is the most basic isometric trunk stability exercise, but can be very effective.
 
With the feet together, toes on the floor, and elbows bent and resting on the floor, crunch the abs and hold the body in a straight position for the desired length of time.
 
The abs should be contracted forcefully and the pelvis slightly posteriorly rotated (the trunk slightly “crunched” or hollow). The lower back should never be hyperextended, and the upper back should be as straight as possible rather than rounded forward.
 
Difficulty can be increased by adding weight to the back. Place plates or similar centered over the lower back.
 
Notes
Weight should be positioned over the lower back—it should not be on the upper back, or they’re being supported overwhelmingly by the arms rather than requiring more work for the trunk.
 
Purpose
The plank is a simple but effective exercise to strengthen the ability of the abs to maintain the proper relationship of the pelvis and spine, in particular preventing hyperextension of the lumbar spine.  
 
Programming
The plank should generally be held for 20-60 seconds per set, but shorter (8-15 seconds) weighted planks are also effective. Generally more than 60 seconds is unnecessary and unhelpful—add weight to reduce time.