Joint and segment coordinate systems revisited.

The joint coordinates systems (JCS) of the hip, knee, and ankle have been extensively described [1, 2, 3]. The principle is to embed a flexion-extension (FE) axis in the proximal segment and an internalexternal rotation (IER) (or adduction-abduction (AA)) axis in the distal segment, following anatomical or functional directions. For the joint displacements, the origins of the proximal and distal segments are also superimposed in a neutral joint position. These definitions apply perfectly to each joint one by one but become problematic when considering simultaneously the hip, knee, and ankle. In fact, there cannot be a unique orthogonal segment coordinate system (SCS) for the thigh and leg that matches with the JCS axes and origins. This paper proposes (1) to redefine the thigh and leg SCS starting from the hip, knee, and ankle JCS, (2) to deduce the computation of the joint angles and displacements, and (3) to apply these proposals to running data measured with pin markers [7].