The cause of restricted intervertebral mobility, or joint fixation, has generated much speculation. A number of etiologies have been proposed in the literature by authors addressing this question. The four possible theories that merit further investigation include meniscoid entrapment, displaced intervertebral disc fragments, segmental or intersegmental muscle spasm and periarticular connective tissue adhesions. The two questions to be addressed for each theory are how does each mechanism acutely restrict intervertebral joint movement and how does a high-velocity manipulation restore movement and relieve symptoms? The pros and cons of each theory are presented, citing the relevant literature. In addition, research possibilities to further investigate each mechanism are presented.