The Side Plank is one of my favorite exercises that challenge the stabilizers of the spine. The muscle of focus is the Quadratus Lumborum. Notice it's location in reference to the lumbar spine and how it is located compared to the other muscles. We spend too much time focusing on the Prime Movers and not enough time focusing on the stabilizers. Side Plank. Don't get me wrong, we must improve strength, power, speed and explosiveness, but what if movement is compromised? While spinal load is occurring and shear force is increased, if we are unable to stabilized, in this case, through the lumbar spine then performance will be compromized. Think of it like and Orchestra(McGill 2007). The muscles work in complete harmony to make music or stabilization in this case. If the underlying stability and control is lost, it doesn't matter how much strength you have. You may be able to compensate for a while, but at some point things will break down.