ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

A simple computational framework to predict patient-specific gait
disorders
W. Kim 1 , A.P. Veloso 1 , V. Vleck 1 , and F. João 1
1
Biomechanics Laboratory
1
CIPER (Center for the Interdisciplinary Study of Human Performance)
Faculty of Human Kinetics
Technical University of Lisbon
Estrada da Costa, Cruz Quebrada, Portugal
1. ABSTRACT
Researchers have reported on the generation of instantaneous axes of the knee (IAK)
utilizing the screw theory to describe the motion of the knee between two generally
disposed axes. However, how geometric change to the dynamic alignment of IAK may
affect the stance phase of foot loading has not yet been fully explained. From the
principle of virtual work applied for a small displacement about IAK, we know that the
reaction torque on the knee joint will be zero if the ground reaction force (GRF) vector
lies along the screw that is reciprocal to it. The reaction forces of the stance phase are
then taken by the body as a whole. We tested the simple computational frame through
readily accessible benchmark data [1]: passive muscle contraction and GRF are
compounded into a wrench which is reciprocal to the IAK and resolved into component
wrenches belonging to the reciprocal screw system. We found that a knee that is at or
close to a reciprocal configuration can exert a wrench of substantial intensity on the
corresponding reciprocal screw, without knee torque overload. The aim of this paper
was to establish the framework for estimation of constraint forces, a process much
simplified by judicious generation of IAK for the arbitrary virtual displacement in
equilibrium. We found that the resulting external force on this reciprocal system must
be neutralized by the reaction of the constraints, thus resulting in heavier pay loads on
constraints.
2. INTRODUCTION
Researchers have reported that osteoarthritis of the knee is associated with deformation
of the lower limb and malalignment of the limb segment. Current modeling algorithms
for human gait emphasise the use of knee adduction moment (KAM) [2], i.e. a risk
factor for medial knee osteoarthritis because of varus knee alignment. Consider the
stance phase of a gait, where the leg is nearly fully extended and the foot is in contact
with a reaction surface. Let the knee receive a twist of small amplitudes around an
instantaneous axis of the knee (IAK). It is proposed to find an expression for the energy
required to effect the displacement and this new method could be a useful addition to
the existing ones.
From the principle of virtual work [3] applied for a small displacement about the
IAK, we found that the reaction torque on the knee joint will be minimum if the ground
reaction force (GRF) vector lies along the screw that is reciprocal [4] to it. Then some
new knee modeling applications are then possible using the principle of aligning a foot
in the stance phase with the knee joint to take advantage of the reciprocal