Normal joint motions vary from one person to another and depend on multiple factors, which raises issues when evaluators attempt to address issues of causation and apportionment. Although the causation and apportionment of joint motion deficits were not addressed in the fourth and earlier editions of the AMA Guides to the Evaluation of Permanent Impairment (AMA Guides), the fifth and sixth editions took three possibilities into consideration when they provided instructions about how to evaluate impairment due to diminished mobility of upper and lower limb joints. Evaluating physicians must consider whether: 1) the motion deficit existed before the injury or illness in question or if the deficit was caused by individual variation, aging, disease, trauma, and/or other cause; 2) the motion deficit was caused by the injury or illness that is at issue or is in question; and 3) a lesser, pre-existing motion deficit existed before the current injury or illness was aggravated (permanently increased). Because of variations in normal joint motions among individuals, examiners should measure the motions of the uninvolved as well as the involved joints, using the former to define normal. Any motion impairment of the uninvolved joint is subtracted from that for the involved joint to determine the net impairment. Examples from the fifth and sixth editions show applications of the AMA Guides to two sample cases.