A method was developed to obtain static and dynamic measures of trunk flexor and extensor strength and endurance. The method was evaluated using 32 normal subjects. Variables of trunk strength and endurance were used to compare 24 normals (12 men and 12 women) and 24 patients (16 men and eight women) with chronic low-back dysfunction. The Iowa Trunk Dynamometer is acceptably reliable and provides for assessment of isolated function of the abdominal and back muscles. For peak abdominal and back extensor strength, the range of superiority of men over women was 39-57%, and the range of superiority of normals over patients with chronic low-back dysfunction was 48-82%. Using time to percent decrement of peak strength as a criterion, the abdominals were more susceptible to fatigue than the back extensors, women demonstrated more endurance than men, and the endurance for normals was less than those patients who were able to perform dynamic reciprocal trunk movements.