This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
From: "David LaPorte (Biochem)" <email@example.com>
I'm not a medical expert, but I've had my share of low back pain and I've
learned a few things. When in doubt, go see a medical professional.
Low back pain is one of the most common problems afflicting humans. It's
been estimated that about 80% of these problems arise because of poor
posture. These posture problems occur when we stand but are even more
significant when we sit or ride a bike. We tend to round up our low
backs, stressing the ligaments and tendons which lie along the spine. It is
the irritation and inflammation of these ligaments and tendons which leads
to most low back problems.
It is important to remember that back pain results from the sum total of
ALL the stresses your back experiences. Even if you only experience pain
when you're riding, poor riding posture may not be your only problem.
For example, you may be sitting poorly at a desk all day or lifting boxes