AKA inverted plank
The back plank is a simple exercise for strengthening isometric back extension.
Place the upper back and lower legs on elevated surfaces such as boxes or stacked plates so that the body in between these points is unsupported. Use the back and glutes to raise the hips up until the body is held in a straight line, and maintain this straight position rigidly for the prescribed period of time.
Weight can be added to the plank to increase difficulty by placing a plate or plates over the hips.
The back plank is a simple but effective exercise to strengthen the glutes and the ability of spinal extensors to maintain back extension against resistance.
The back plank should generally be held for 20-60 seconds per set, but shorter (8-15 seconds) weighted planks are also effective.
Rather than holding the plank position continuously for a given period of time, repetitions of hip extension can be performed by lowering the hips slightly and pushing them back up into the plank position. This can be done for reps or time. In either case, the tempo should be controlled to maintain continuous tension.