Objective measurement of knee extension force based on computer adaptive testing.

False impairment is encountered when tested subjects either unintentionally or deliberately put an artificial upper limit on their force, in which case their true capacity cannot be disclosed in a straight forward measurement. The aim of this study was to develop a computer adaptive testing (CAT) system for directing subjects into generating greater forces than they intended. The system was tested on eleven cooperative female subjects who volunteered to take part in this study. The CAT consisted of interactive testing cycles, each containing a series of isometric tasks of differing intensities. While fulfilling these tasks, the tested subjects were asked to take care not to exceed a self-selected upper force limit (F(ssl)) that they were previously trained to memorize (order of 40% of the maximal voluntary contraction). Visual feedback, displaying the applied force exertions, was provided to the tested subjects but was modified by re-scaling the display in an un-anticipated manner. To confirm the subject's ability to remember her F(ssl), repeatability of joint memory was tested one week after the CAT. The CAT results were successful in causing ten out of the eleven tested participants to exert a higher force than they intended to. Additionally, the CAT algorithm caused a statistically significant higher force than the repeatability test. These results demonstrate the potential of CAT methods in improving the clinical evaluation of muscle strength, particularly in those cases where the subject's cooperation is not sufficient.