Two groups of people were chosen to text the effectiveness of chiropractic adjustments on hamstring flexibility. All people had a Straight Leg Test of 70 degrees or less.
Both groups of people were given the same specific instructions to stretch their hamstring muscles everyday. One group also received chiropractic adjustments to both sacroiliac joints (lower back region).
After three weeks, the group that received chiropractic adjustments had an average of 11 degrees greater increase in hamstring flexibility on the right leg, and 9 degrees on the left.
To read the whole study, see this link: http://www.chiropracticresearchreview.com/crr/article/php?id=1428