http://AskTheTrainer.com Personal Trainer Michael Behnken, MS, NASM-CPT-PES, CSCS BOSU plank is around the same difficulty as the floor plank. There are many progressions which can increase the difficulty. The key is proper form. The stomach must be drawn with contracted glutes. If the neutral alignment of the spine cannot be maintained, you should regress to learning how to control your body properly http://AskTheTrainer.com