The term "adjustment" refers to the specific manipulation chiropractors apply to vertebrae that have abnormal movement patterns or fail to function normally. The objective of the chiropractic treatment is to improve vertebral motion (to the extent possible), which results in increased ranges of motion in all directions, reduced nerve irritability and improved function.

The adjustment made by the chiropractor consists of a high velocity, low amplitude thrust applied to a vertebra, which is often accompanied by an audible "pop" (joint cavitation). The audible sound is caused by the release of oxygen, nitrogen, and carbon dioxide, which releases joint pressure. The sensation is usually relieving, though minor discomfort has been reported (that usually lasts for only a short time duration) if the surrounding muscles are in spasm or the patient tenses up during the chiropractic procedure.

Positive effects of a chiropractic adjustment from a chiropractor
Objective effects of a chiropractic adjustment have been investigated, reported, and include increased joint range of motion, pain reduction, increased skin pain tolerance levels, increased muscle pressure pain tolerance and reduced muscle electrical activity and tension.

Sympathetic nervous system effects of a chiropractic manipulation include increased blood flow and distal skin temperature as well as decrease in blood pressure.

Blood chemistry changes after chiropractic manipulations include increased melatonin levels, increased plasma endorphin levels, elevation of substance P (pain reduction), and pupillary diameter changes.

Negative effects of chiropractic adjustment from a chiropractor
The most common complaints are local discomfort, headache, tiredness, or radiating discomfort. Reactions are usually mild lasting less than 24 hours. Uncommon reactions are dizziness, nausea, hot skin, or "other" complaints, which occur in less than 5% of cases. Symptoms usually occur within a few hours of the adjustment and rarely last longer than the day following treatment. It is also rare that they result in reduced activities of daily living.

Types of chiropractic manipulation
There are many different techniques a chiropractor can choose from including the high velocity technique and other more gentle forms of manipulation.

Treatment frequency
The number of chiropractic treatments required for the particular patient varies significantly due to the degree of the injury, variability between patients, and other associated factors of chronicity (anxiety, depression, poor coping strategies, extensive arthritis, muscle deconditioning, and others). For example, the chiropractic treatment plan of a grade 1, lumbar sprain/strain (with no complicating factors) rarely demands greater than 4-6 weeks of care. Most cases begin 3 visits per week and reduce over time based on measurable improvement.