The cornerstone of chiropractic health treatment is spinal manipulation, often called the "adjustment". It may involve either gentle pressure, or a quick thrust to restore normal motion to joints that have become restricted or "locked". While manipulation may be performed by other physicians, Doctors of Chiropractic have the most extensive experience and education in manipulation, performing 94% of the spinal manipulations in the United States. Numerous scientific studies have proven the efficacy, safety,cost-effectiveness and high levels of patient satisfaction of spinal manipulation. As a result, it is one of only two treatment protocols given the highest recommendation for the treatment of low back pain by the Agency for Health Care and Policy Research (AHCPR), an arm of the US Department of Health and Human Services.