Comparing Cardan Rotation Angle and Finite Helical Axis Representations of Talocrural and Subtalar in Vivo Kinematics

INTRODUCTION The joints of the rear-foot are subjected to the highest stresses and are the most commonly injured, as compared the other joints in the human body. Yet, little is known about the in vivo kinematics of the talocrural and subtalar joints. The data available for these joints have typically been presented in terms of the finite helical axis (FHA) and are most often acquired through static cadaver experiments. The FHA representation has been favored over Cardan rotation angles because it lent itself to the measurement tools available, did not have the potential for data singularities and lacked sequence dependency. Thus, the primary purpose of this study was to fully quantify the kinematics of the subtalar, talocrural, and calcaneal-tibial joints in 25 healthy ankles during a volitional task. Using these data, the utility of representing kinematics using the FHA was compared to the Cardan rotation representation. As part of this comparison, the sensitivity of the rotation angles to the selection of the rotation sequence was determined.