THE USE OF A MECHANICAL MODEL TO DESCRIBE THE STIFFNESS AND DAMPING CHARACTERISTICS OF THE KNEE-JOINT IN HEALTHY-ADULTS

Background and Purpose. Stiffness is a common clinical complaint but i s rarely quantified by clinicians. The purpose of this study was to de termine the reliability of a relaxed oscillation test that yields stif fness and damping coefficients of the knee. These coefficients describ e the knee joint's resistance to bending and the time-dependent nature of that resistance Subjects and Methods. Effects of age, gender, and knee position on these coefficients were assessed in 96 healthy volunt eers aged 20 to 79 years Measures were based on the premise that the k nee joint can be modeled as a damped spring. Oscillations of the knee were recorded using an electrogoniometer with the knee oscillating thr ough about 45 and 75 degrees of knee flexion. Results. Intraclass corr elation coefficients revealed moderate to high reliability in the meas urements taken three times in the same test session and on three separ ate days Analysis of variance showed significant increases in stiffnes s and damping coefficients in the male subjects as compared with the f emale subjects Analysis of variance also suggested an age effect on st iffness coefficients at the 75-degree test position, with decreasing s tiffness with age. Both stiffness and damping coefficients were signif icantly smaller when measured at the 75-degree test position compared with the 45-degree position (Student's t test). Conclusion and Discuss ion. These results demonstrate a reliable method of measuring knee joi nt stiffness, and they correlate well with known morphological differe nces related to age and gender This measure may prove to be more usefu l in evaluating the function of the knee than more commonly used asses sments. It may also lead to a better understanding of bow the knee fun ctions in such activities as locomotion.