The use of surface markers to calculate kinematics of diarthrodial joints is subject to error from relative motion between the markers on the skin and the underlying bones. It has been demonstrated that markers placed over the knee and hip joints could move 10–30 mm with respect to the skeleton which can have significant effects on the accuracy of calculated knee kinematics (Cappozzo et al., 1996, Manal et al., 2000). Several methods have been developed to calculate knee kinematics from marker locations and decrease error resulting from skin motion. The Helen Hayes method uses markers attached to the skin over anatomical landmarks and on wands to calculate the locations of the hip, knee, and ankle joints (Vaughan et al., 1999). The location and orientation of the thigh and shank segments can then be calculated based on anthropometric correlations. Other methods, such as that developed by the Cleveland Clinic, record the position of the femur and tibia using triads of markers attached to each segment (Manal et al., 2000). Additionally, a Point Cluster Technique was developed using clusters of markers attached to each segment to reduce error from skin motion (Andriacchi et al., 1998).

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