This report reviews data obtained from electromyographic studies of the musculature of the trunk and considers their physiologic and methodologic bases. Current knowledge regarding the relation between electrical and mechanical output of muscle is incomplete and conflicting, but supports interpreting electromyographic information based on a principle of monotonicity, in which changes in electrical output parallel changes in force. Electromyographic information is utilized to define the role of individual trunk muscle groups in the functional anatomy of the lumbar spine in various positions, motions, and conditions.