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

In this article we investigated the influence of a displacement of the knee joint in relation to the axis of the isokinetic device on the measured torque, angle and angular velocity during isokinetic knee extension/flexion. We analysed the motion of the knee joint of four young men with the CCD-camera system KINEMETRIX (ORTHODATA GmbH, Ludenscheid, Germany) at different isokinetic speed (60 degrees/s and 180 degrees/s), using a device with a stationary (CYBEX 6000, LUMEN Inc., Ronkonkoma, NY) and a device with an instationary (LIDO ACTIVE, LOREDAN Biomedical Inc., Davies, California) tibial pad. The results show that the displacement of the knee joint in relation to the axis of the device using the instationary (sliding) input cuff is greater than using the stationary cuff. The torque errors range from 0% to -15% using the stationary pad and from +10% to -20% using the instationary cuff. While it is impossible to correct the torque values using the stationary pad (the displacement cannot be measured), the sliding cuff allows a mathematical correction of the torque values after the exercise. The max, torque error can be reduced below 2%. The largest deviations in angle occur at the higher speed (180 degrees/s). At the moment the knee is in full extension, the max. error amounts approximately 20 degrees. At lower speed (60 degrees/s) the max. error amounts around 10 degrees. The device with the stationary input cuff produces more accurate results. The measurement errors are below 10 degrees. Using the instationary cuff, the selected isokinetic speed of 60 degrees/s and 180 degrees/s had been kept well. Considering the angular velocity of the knee joint during a test session, no real isokinetic state could be found. The device with the stationary input cuff has no real isokinetic state of the knee joint during the ROM too. In addition, the speed control during the motion is insufficient. Analysing the different factors which can influence the test results, it must be said that a comparison of test results, receiving from different isokinetic devices, is not possible resp. very questionable at today's state of the art.