The effect of preload on variability in dynamometric measurements of knee extension

The unwanted variability in dynamometric measurements limits accurate interpretation of these measurements. Although unpredictable variability in measurements has been identified repeatedly, few studies have investigated strategies for reducing measurement error. This study investigated the effect of preload on variability in dynamometry. Preload is the force that must be overcome before the lever arm begins to move at a preset speed. On the basis of related research, it was hypothesised that under higher preloads, subjects would perform more consistently. Twenty subjects (ten male and ten female), aged 19-24 years, performed repeated right concentric knee extension tests. Using the Kin-Com dynamometer and a lever arm speed of 60 degrees x s(-1), extension torques were recorded for each subject under four different preload conditions. Preloads were 5%, 25%, 45% and 65% of the torque achieved during each subject's maximal voluntary contraction (MVC). Each subject performed six test repetitions under each preload. Variability in peak torque was significantly smaller under the 65% preload compared to the 5% preload. In addition, the regression of preload against group mean variability indicated that as preload increased, the variability in peak torque decreased in a highly predictable way. The results from this study suggest that subjects perform more consistently under conditions of higher preload than under lower preloads. By adopting higher preloads for dynamometry testing, measurement accuracy and the potential clinical utility of measurements is improved.