This is an interesting Q&A that I came across today at http://sportscenteraustin.blogs.com/the_view/spine/
"Reader Eric asks, " I notice all of your workouts begin and end with a speed interval session. Is there some training advantage to the second speed interval session at the end?"
Great question. And, there are two parts to my answer. The first one is that I just like it. I like the feeling and especially on days where I've really stressed my muscles. The second one is that with my history of spine disease and injury, I need all the help I can get.
The disc has a poor blood supply even in healthy individuals. The outer third of the annulus has blood vessels but as you move toward the center of the disc, you have less and less direct blood supply. So, the nutrient exchange comes from the subchondral bone from a very fine capillary network woven into it. As you apply and relieve pressure - cycling, walking, elliptical etc. - the bone oozes nutrients and absorbs metabolic waste into and out of the disc through this network (Rajasekaran, S., J. N. Babu, et al. (2004)). Both the outer rim of the annulus and the inner aspect of the disc respond to movement and as your blood pressure and heart rate climb, blood is pumped into those areas.
This dynamic relationship between nutrient exchange and movement is one reason why sitting so much during the day is so detrimental to your spine. One of my teachers liked to say that,"sitting does to the spine what placing a plastic bag over your head does for your breathing."
So, I basically think of speed intervals as "nourishing" my spine and letting it "breathe".