First I'd get a history; i.e.,did you injure yourself recently, surgeries, etc., blood work and then you need a lumbar spine MRI. I'd also do a neuromuscular check of your lower extremeities. Any weakness or sensory loss (numbness, "pins & needles") of the leg affected?
It sounds like you have a radiculopathy, an inflammation of a lumbar nerve root and there are basically four on each side that give innervation and affect the muscles to each leg.
The history, physical exam, and MRI are usually 100% diagnostic; since you have acute pain (it started very recently-or is this a reoccurence) I'd probably try some pain pills, anti-seizure med's (yes, they can help), and NSAIDS (non-steroidal antiiflammatory drugs-celebrex) first with physical therapy or a good chiropracter who specializes in pure spinal manipulation.
Failing a course of that or both there are certain injections such as a lumbar epidural steroid injection or a selective nerve root block I'd do if it sounds like what I think it is and doesn't respond to the more conservative therapy.
edited to add as the poster above mentioned if it progresses to numbness, loss of bladder or bowel control, you skip most of the above (you will get an MRI anyway)and go straight to the OR. Once nerves are severely compromised they need to be relieved pronto as their capability and time to heal would make a snail crawling on a Dec Iowa sidewalk look like lightening.
Gdv
Last edited by goodnews; 12/28/08.
