Biomedical Engineering Reference
In-Depth Information
Fig. 11.9
Contact handling between two deformable models, shown schematically in 2D, with
inter-penetrating nodes shown in
grey
and the associated constraint directions shown using
arrows
11.4.1 Lip Stiffness Enables Protrusion and Rounding
One motivation for our face modeling efforts was to better understand the
biomechanics underlying speech articulations with the lips. The production of
rounded vowels in speech, such as /u/, /o/, or /y/ in French, requires a small area
of opening between the upper and lower lip. Although a small lip opening could be
generated in a number of different ways, many speakers achieve it by protruding the
lips. The regularity of this speech articulation across speakers suggests that protru-
sion is an efficient way to achieve small lip opening areas. We were interested in using
biomechanics simulation to analyze how the intrinsic properties of lip muscles could
enable small lip aperture through lip protrusion during rounded vowel production
[
9
].
Stiffness is an intrinsic property of muscle tissue. It increases with muscle
activation, which is known as the “stress-stiffening” effect. In order to assess the
role of intrinsic muscle stiffness, we simulated lip-rounding movements with and
without stress-stiffening effects. Simulations were performed by activating the orbic-
ularis oris (OO) muscle in the model and results are shown in Fig.
11.10
. Simulations
showed that a proper protrusion and rounding lip gesture was achieved by including
stress-stiffening in the OOmuscle. A saturation effect was also observed such that for
a sufficient level of stiffness, lip protrusion and rounding was maintained as the OO
activation level increased. Likewise, with a sufficient amount of OO activation, the
lip gesture was maintained as the magnitude of stiffness increased. The differences
in resulting lip shapes for simulations with and without stiffening were sufficient to
affect the spectral characteristics of the speech signal obtained for the French vowel
/u/. This result suggests that a simple strategy to generate protruded and rounded lips
could be to activate the OO muscle while stiffening the tissues [
9
].
