In vivo measurement of the passive viscoelastic properties of the human knee joint

The purpose of this investigation was to measure the net passive elastic moment of force and the angular damping coefficient of the human knee joint over the full range of motion. Seventeen males (22-31 years) participated in the study. The passive elastic moment function was determined using a specially constructed direct stiffness apparatus. The subject's leg was rotated through the full range of voluntary motion and forced slowly into the extremes of flexion and extension. The damping coefficient was determined using the small oscillation technique which is a linearization method allowing the determination of the angular damping coefficient as a function of knee angle through a simple analysis of the underdamped response curve. Measurements were made at 10 degrees, 45 degrees, 90 degrees, 110 degrees and 130 degrees of flexion (0 degrees is full extension). The hip joint was fixed at 90 degrees and the ankle was set at 0 degrees (90 degrees with leg). Surface electromyogram (EMG) activity of five major muscles crossing the knee joint was monitored to ensure a passive measurement. The passive elastic moments increased exponentially as the limits of either flexion or extension were approached. At 140 degrees of flexion, the lowest moment measured was about 5 N m and the maximum was about 86 N m; similarly, the range at full extension (00) was about 6-22 N m. The midrange of knee joint.motion was characterized as a low moment (<4 N m) region where the passive influences can be justifiably neglected. The angular damping coefficient was a nonlinear function (approximately quadratic) of the knee joint angle. In conclusion, it appears that the resistive moment and the damping offered by the passive tissues spanning the knee joint varies considerably across subjects and can reach relatively significant magnitudes. Researchers may consider including such sources in their biomechanical models to enhance their fidelity. (C) 1998 Elsevier Science B.V. All rights reserved.