Ultimate Back Fitness and Performance, 4th Edition.  By Stuart McGill

According to Dr. Stuart McGill who is a Professor of spine biomechanics at the University of Waterloo and the foremost researcher and is recognized as the foremost expert in back injury and rehabilitation:

“The science on disc Herniation shows that:

1. Repeated flexion-bending of the spine is necessary to cause herniation (Adams and Hutton, 1982).  In fact, herniation of the disc seems impossible without full flexion.  This has implications for exercise prescription particularly for flexion stretching and sit-ups, or for activities such as prolonged sitting, all of which are characterized by a flexed spine.  Some resistance exercise machines that take the spine to full flexion repeatedly must be reconsidered for those interested in sparing the posterior annulus portions of their discs.  (while some appear to be helped by this approach, it is very problematic for others)

2. Thousands of cycles of flexion are needed (Gordon et al., 1991; King, 1993; Callaghan and McGill, 2001) to herniate a healthy disc.

3. Prolonged sitting exacerbates the risk (Videman et al., 1990; Wilder er al, 1988).

4. Herniations tend to occur in younger spines (Adams and Hutton, 1985), because of the higher water content (Adams and Muir, 1976).”

Sooooo, what this means to Jiu-Jitsu is if you want to build a Jiu-Jitsu game around the guard, specifically inverting yourself with your knees pulled close to your body you are drastically increasing your risk of hurting your back.  I have strongly recommended to many people not to engage in these popular movements that have gained traction within the current sport Jiu-Jitsu scene.  A youthful body can buffer tremendous dysfunction and injury.  However, as you age it will catch up to you.  So the upside down guards, the stacking positions, the berimbolas are cool and beautiful movements but they will exact a price on your body and your ability to train for a lifetime.  The choice is your whether you would like to build your game around these movements or not.  These movements should be and will be studied, even in our schools.  However, it will never be considered a fundamental or even important technique/skill in any curriculum that Josh Johnson is in charge of.