The plank is the most basic static trunk stability exercise, but is also very effective.
With the feet together and toes on the floor, and elbows bent and resting on the floor, 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.
Weight can be added to the plank to increase difficulty. Plates should be stacked over the lower back—they should not be on the upper back, or they’re being supported overwhelmingly by the arms rather than requiring more work for the trunk.
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.
The plank should generally be held for 20-60 seconds per set, but shorter (8-15 seconds) weighted planks are also effective.