Abstract
During strength measurement using isokinetic dynamometers great care is usually taken to align the anatomical joint axis to the rotation axis of the machine's lever arm. This ensures agreement between joint angular position/velocity and the corresponding measurements from the machine's potentiometer. We examined the accuracy of this correspondence during concentric knee extension with subjects sitting upright and strapped to the machine's chair. Extensions were performed at 30 dg, 120 dg and 240 dg with range of movement from 90 dgz knee flexion to full extension. We observed that due to compression of the soft tissue at the back of the thigh and elevation of the pelvis (despite strapping) the anatomical joint axis moved downward relative to the rotation axis of the lever arm. This caused discrepancies between joint angle measurements from the machine and from a knee-mounted goniometer of up to 33 dgz. These circumstances should be taken into account when muscle force-length-velocity relation studies are conducted using isokinetic machines.