Dr. Kirkaldy-Willis first conceptualized and published theories regarding the Biomechanics and Biology of the Spinal Degenerative Cascade. He defined the HVLA technique as "a skilled, passive manual therapeutic maneuver during which a synovial joint is beyond the normal physiological range of movement (in the direction of the restriction) without exceeding the boundaries of anatomical integrity." High-velocity low amplitude techniques employ a rapid use of force over a short duration, distance, and/or rotational area within the anatomical range of motion of a joint to engage the restrictive barrier in one or more planes of motion to elicit the release of restriction. This manipulation technique uses high velocity and low amplitude thrusts to manipulate joints. Osteopathic physicians and chiropractors are trained in HVLA and commonly perform these techniques. Physical therapists in the USA typically perform non-thrusting joint manipulative techniques. Both therapies are used in cases of acute musculoskeletal back pain. The physician positions the person at the barrier of limited movement and then gives a rapid thrust in the isolated barrier's direction to resolve the restriction and improve motion. This method is among the oldest and most frequently used, and it is among the most common types of complementary medicine for children. Most clinical research has focused on evaluating the efficacy of this form of manipulation, particularly for low back, mid-back, and neck pain.