A sample of 508 male and female white-collar and blue-collar employees aged 35 to 54 years was evaluated clinically to determine the reliability of repetitive sit-ups, repetitive arch-ups, repetitive squatting, and static back endurance tests, to determine the normal values of these tests and to detect determinants for trunk muscle performance. All of the given tests had fairly good or even excellent test-retest reliability. Muscular performance capacity decreased with advancing age, particularly among blue-collar workers. Men showed greater muscle endurance in all the muscle tests, and blue-collar workers lower values in all tests. The repetitive tests, in particular, showed reduced values in those with previous low-back pain. Normative values of back endurance, repetitive squatting, sit-up and arch-up tests for different age, sex and occupational groups are presented.