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

defined between each meniscus element and every tibia cartilage element that might
come in contact with the meniscus element during movement. Deformable contacts
were also defined between the patella cartilage and femur cartilage geometries for the
natural knee and between the femur component and patella component geometries of
the prosthetic knee. Contact parameters were derived using Hertzian contact theory
(patello-femoral joints), elastic foundation theory (prosthetic tibio-femoral joint), and
parameter optimization to match a finite element solution (natural tibio-femoral joint).
3.2 Musculoskeletal Models
LifeMOD (LifeModeler, Inc., San Clemente, CA) was used to develop body level
musculoskeletal models. LifeMOD is a virtual human modeling and simulation
software add-on to MD.ADAMS. A generic musculoskeletal model was created and
then scaled based on the GeBOD anthropometric database library in LifeMOD [5] using
the subjects weight, height, gender, age, and if available, leg bone geometries. The
prosthetic model included the upper body while the natural model only included the
lower body (Table 1). For both models, the right knee was a mirrored model of the left
knee. The menisci were not included in the mirrored natural right knee. Also, the
subject for the prosthetic simulation had a natural right knee, but geometries of the right
leg were not provided with the grand challenge data set, so the prosthetic knee was
mirrored. Tri-axis hinges combined with passive torsional spring-dampers were
employed to model the hip and ankle as well as joints of the upper body. Femur and
tibia bone geometries and marker positions from motion capture measurements were
used to position the body and align the knee models relative to the hip and ankle joints.
The natural or prosthetic knee models were then rigidly attached to the upper and lower
leg segments of the generic musculoskeletal model. Forty-five muscle elements were
placed on each leg based on the relative positions of the hip, ankle, and knee. The
default attachments of the quad muscles were modified to insert on the patella. The
interaction of the foot with the floor was simulated using eleven ellipsoids representing
the foot. The contact law of Eq. 1 was defined between the foot ellipsoids and the force
plate geometry (k = 87 N/mm 1/3 and n = 3).
a.) b.)
Figure 1: Prosthetic model (a.) and natural model (b.)
during forward dynamics simulation of a squat.
Measured squat motion
was used to move each model as
constrained by the hip, ankle, and
upper body joints, knee
geometries and ligaments, and
foot-ground contacts. During
this inverse dynamics simulation,
the shortening/lengthening
patterns of each muscle element
were recorded through their
respective via points. In
addition, the joint motion of the
upper body was recorded. For
forward dynamics simulations,
all motion constraints were
removed and the muscle
elements served as actuators.