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

3.3. Muscles
Muscles are modelled as 1D wires. A chain of 2-node quadratic truss elements is used. At the origin,
the muscle wire is attached to the corresponding bone. To generate muscular forces, the muscle is
linked to its insertion point using a connector element. This connector element has the ability to
generate the desired muscle force. For correct muscle wrapping on bony structures, the contact
areas between muscles and bones are added as cylindrical respectively spherical surfaces. The contact
surfaces are defined to be frictionless.
3.4. Optimization
To obtain the muscular forces of the overdetermined system, an optimization algorithm has been developed
[19]. It is based on an inverse dynamic approach coupled with pseudo-inverse null space
optimization. The optimization cost function f is the minimization of mean square muscle
stresses. Input parameter is the trajectory of the planed movement to be followed by the humerus.
Output is the set of computed muscle forces. The optimization routine solves the following minimization
problem:
min f (F )
s.t.0