The critical part of this movement is to pull the lumbar arch from the back so that the entire back is rounded from shoulders to butt. This exercise is a perfect measure of both a weakness in and inability to innervate the lower abs.
The role of the hip flexors is fairly insignificant in the hollow rock but the role of the lower rectus (lower abs) is crucial.
For many people the hollow rock is so hard that no matter how hard they try they crash on each rocking as they come to level and the flat spot caused by insufficient lumbar flexion smacks the floor. This crashing is an ideal measure of ones lack of lower ab recruitment.
Lower ab recruitment is the toughest part of ab training and is never well developed by most athletes. Activation, full recruitment, and development of the lower abs require massive concentration and focus over months or even years. The hollow rock is a near perfect tool to both test and develop low ab capacity.
hollow rock should be practiced even if it gives you colossal difficulties. Start by trying to rock continuously for two minutes regardless of the quality of the movement. Avoid raising the hands and feet to maintain the rocking motion as best you can.
When mastered, the body is dished out flat, the hands and feet are low, and the impetus for the rocking is almost undetectable. When you can do this smoothly – no flat spot – for two minutes you’ll have absolutely great abs.