So my yoga teacher at work (she is a head designer in wovens but is a certified Ashtanga teacher as well) sent me this today. Friday I was complaining that after my runs my lower back always hurts, and gets super tight. I know some other people that this happens to, so I presumed that this was normal. She sent me the below which is super interesting. I am for sure going to look further into this, but I can’t get over how amazing she is to go out of her way to do this for me!
Hi NoSauce-
Hope your back is better today.
How was your weekend?
I did a little research for you over the weekend.
I think that for a runner your issue with your lower back may be the psoas muscle …
Check it out.
Here ‘s some poses and info for lower back pain.
http://www.youtube.com/watch?v=LodlEH6TluM
http://www.yogajournal.com/health/125
see attached image.

this shows how your psoas muscle connects from your lower back through your pelvis and attaches to the back of your hamstrings.
When the hamstring is tight it pulls on the psoas constricting spaciousness in lower back http://img.tfd.com/MosbyMD/thumb/psoas_major_and_min.jpg
this is one of the causes of lower back pain. it’s always important to stretch after running!
piriformis muscle tightness usually causes sciatica pain and pain in butt. You had mentioned this but I wonder if that is the problem?
always consult your physician!!
namaste,
RB
Gotta love your yoga teacher! ;-)