I'm an amateur golfer trying to improve my swing. I notice at the top of my backswing I get twinges of low back pain. What could be causing this?

Low back pain is the most common complaint among amateur and professional golfers. The modern swing puts a lot of torque on the leading hip. If you don't have enough rotation in that hip the force of the motion gets translated through the hip to the sacroiliac
joint or the low back.

Several studies show golfers with more outward hip rotation than inward rotation are more likely to have back pain. Experts in preventing and managing golf injuries advise stretching the hip and lumbar spine. There are no studies to prove this works yet, but until proven otherwise, it's considered good advice.

Vijay B. Vad, MD, et al. Low Back Pain in Professional Golfers: The Role of Associated Hip and Low Back Range-of-Motion Deficits. In The American Journal of Sports Medicine. January/February 2004. Vol. 32. No. 1. Pp. 494-497.