The influence of a joint displacement on torque, angle and angular velocity during isokinetic knee extension/flexion. Theoretical considerations

In this article the influence of a knee joint displacement in relation to the axis of the device on the results of torque, angle and angular velocity during an isokinetic test is theoretically investigated, using different tibial pads. In the case where the axis of rotation of the joint is not aligned with the axis of the device the test results become incorrect. However, due to the subjects activities during an isokinetic test the position of the joint changes all the time. The results of this theoretical investigation show that the errors of torque, angle and angular velocity are in scale of the percentage displacement. This means for a lever arm of 40 cm and a displacement of 4 cm (this is a 10% shift) the error of torque and angular velocity is in scale of 10%, the error of the angle max. 10 degrees. If a test is made with an instationary cuff, a torque correction will improve the results. The maximal error of torque for a shift of 10% can be reduced below 0,5%. A correct alignment of the subject on the isokinetic device in isokinetic testing is extremely important. If this adjustment is not made with special accuracy, errors can increase considerably. A comparison of both fixation types (stationary and instationary) does not show big differences concerning the error of measurement. This article describes some theoretical considerations and gives some theoretical explanations.