In 2010 The Journal of Orthopedic and Sports Physical Therapy put out a research article on muscle activity levels with different movements. The goal was to identify which movements resulted in the greatest muscle recruitment through the trunk musculature. A standard plank, and more challenging variations showed a high level of activity in rectus abdominus, external obliques, transverse abdominus, and erector spinae. In addition, when performed with a neutral spine, they also resulted in the least amount of lower spine compression. Check these out and add them to your training.