Identifying feigned isokinetic trunk extension effort-in normal subjects: An efficiency study of the DEC

Study Design. A test-retest comparison of maximal and feigned trunk extension effort in normal subjects was performed. Objective. To test the robustness of an index termed DEC for differentiating maximal from feigned effort in normal subjects. Summary of Background Data. Previous studies have indicated that the DEC, which is the difference between the eccentric to concentric strength ratio at two test velocities, is a powerful identifier of feigned effort. This has been found for various muscle groups including the trunk extensors. However, whether this efficiency is affected by exposure to the protocol, learning, or other factors had not been examined. Methods. Using a KinCom isokinetic dynamometer, 18 men and 17 women were tested twice within 1 to 2 weeks. Concentric and eccentric efforts of the trunk extensors were exerted using a short range of motion (20°) and two test velocities (10° and 40° per second). In the first part of each test, participants exerted maximal force against the lever arm, whereas in the second part they were to feign their maximal capability, pretending the presence of injury. Results. In both Tests I and II, the feigned DEC scores (DEC_f) of all the participants were greater than the maximal DEC (DEC_m) scores (P = 0.0001). Repeated measurement analysis showed that neither DEC_m nor DEC_f varied systematically from Test 1 to Test 2. On the basis of the individual male scores, the DEC cutoff score of 0.41 was 100% efficient in Test 1 at differentiating feigned from maximal effort, with neither false-positive nor false-negative cases. Test 2 had a single false-positive case (efficiency 95%), and the optimal cutoff score was 0.275. In the female group, the corresponding efficiency of the DEC was 82% (optimal cutoff, 0.35) for Test 1 and 70% (optimal cutoff, 0.25-0.35) for Test 2. A statistical model for tolerance intervals at 90%, 95% and 99% indicated higher cutoff values for women than men. Conclusion. In healthy subjects, the DEC effectively differentiates feigned from maximal performance.