When this happened to me, I was advised (by my golf coach, former Mr. California so knows lots about muscles) to get myofascial release therapy from a physican therapist or massage therapist. I tried both, and the massage therapist (make sure they have been trained on this) was better at it, but the physical therapist gave me ab exercises that helped too. They will pull on your legs and find trigger points where your fascia (sp?) is bound up, because what you describe is sciatic nerve pain (and it will stop you in your tracks!!).
My bone density was fine, and x-ray showed nothing. I got an MRI which showed a mildly bulging disk, which is something that can resolve itself in time (but still cause much pain and spasms). I had an epidural cortisone shot per doc's recommendation, but the M-R massage therapy had already worked pretty well so I didn't really know if that had any effect. Some seats irritate it, also standing for a long time or walking on hard surfaces.
In the meantime, ICE!! Aleve helps when I feel pain developing, and the oxycodone works really well but I'm conservative about using what I have left of the prescription. I'm starting to feel this again, and have another appointment with the massage therapist.
BTW, I get a throbbing feeling in my lower back when I'm on prednisone, it's different and goes away when the prednisone ends. I worried it was a kidney issue, but all always tested OK.
49 year old female attorney, diagnosed UC/pancolitis 1985, no surgery but much suffering.
Asacol/5ASA 6x400 mg maintenance for 20+ years; use prednisone & Rowasa for flares.
August 2008 sought care of naturopathic doctor. Food sensitivity test showed gluten, other intolerances; started diet without gluten, soy, dairy, tomato etc. Replete probiotic first, now Forte pills.
Achieved remission for the first time in many, many years!? Seemed so, but colonoscopy Oct. 6 proved not so, heavy inflammation in ascending colon & will be using Rowasa again. But I AM better.