  It is well recognized that the relative position of the center of mass (pCOM)
with respect to the base of support (BOS) is a determining factor in the
maintenance of balance. However, during gait the dynamic nature of the BOS is
not well defined and in most studies is completely ignored. Prior work tends to
focus on the variability in the position of the center of mass (COM) with
respect to the laboratory reference frame to attempt to quantify dynamic
balance. We propose a modified method whereby the position of the COM in an
end-effector based coordinate system may be used as an improved estimate of
threat to balance during gait. The distance (DN) of the projection of the COM
(pCOM) from the reference axis, normalized by half the foot length for
inter-subject comparison, is an estimate of the gravitational moment arm. It
can be shown that in concordance with theory, balance impaired subjects with
spastic cerebral palsy (CP) have a larger DN than their typically developing
peers. Furthermore, when compared with variational methods, DN shows better
discriminative ability in characterizing groups.
