IN-VIVO MEASUREMENT OF THE PASSIVE VISCOELASTIC PROPERTIES OF THE HUMAN KNEE-JOINT

The purpose of this investigation was to measure the net passive elast ic moment of force and the angular damping coefficient of the human kn ee joint over the full range of motion. Seventeen males (22-31 years) participated in the study. The passive elastic moment function was det ermined using a specially constructed direct stiffness apparatus. The subject's leg was rotated through the full range of voluntary motion a nd forced slowly into the extremes of flexion and extension. The dampi ng coefficient was determined using the small oscillation technique wh ich is a linearization method allowing the determination of the angula r damping coefficient as a function of knee angle through a simple ana lysis of the underdamped response curve. Measurements were made at 10 degrees, 45 degrees, 90 degrees, 110 degrees and 130 degrees of flexio n (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 ele ctromyogram (EMG) activity of five major muscles crossing the knee joi nt was monitored to ensure a passive measurement. The passive elastic moments increased exponentially as the limits of either flexion or ext ension were approached. At 140 degrees of flexion, the lowest moment m easured was about 5 N m and the maximum was about 86 N m; similarly, t he range at full extension (00) was about 6-22 N m. The midrange of kn ee joint.motion was characterized as a low moment (<4 N m) region wher e the passive influences can be justifiably neglected. The angular dam ping coefficient was a nonlinear function (approximately quadratic) of the knee joint angle. In conclusion, it appears that the resistive mo ment and the damping offered by the passive tissues spanning the knee joint varies considerably across subjects and can reach relatively sig nificant magnitudes. Researchers may consider including such sources i n their biomechanical models to enhance their fidelity. (C) 1998 Elsev ier Science B.V. All rights reserved.