Specifications

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Notice that in this case we have used the same strength (muscle model) for both joints. However, the
maximum joint torque in physiological joints varies a lot. The knee extension strength, for instance is
significantly larger than the elbow extension strength. If you perform this type of modeling you can define
joint torque muscles with strengths comparable to the available joint torque and the system can give you an
estimate of how many percent of each joint's strength is used in a given situation. You can also define
different strengths of extension and flexion muscles in a given joint and thereby take for instance the
difference in strength in the knee in these two directions into account.
Another useful property of the general muscles used at joint torque providers is that you can handle closed
loops and other statically indeterminate situations, which are not treatable by traditional inverse dynamics
because the equilibrium equations do not have a unique solution. The muscle recruitment algorithm will the
distribute the load between joints according to their individual strengths, and it is therefore important to
have reasonable estimates of joint strengths for this type of situation.
Contact and other boundary conditions
One of the characteristics of muscles is that they are unilateral, i.e. they can only exert force in one
direction. Mathematically this behavior creates a significant amount of problems, but many mechanical
phenomena have the same characteristics, namely any kind of contact phenomenon. Biomechanics is full of
contact problems:
• The contact between a foot and the floor
• The contact between the upper thighs and the seat of a chair
• The contact between two articulating surfaces in a joint.
There is another less appreciated similarity between muscle forces and contact forces: neither is without
limit. Muscle forces are obviously limited by the strength of the muscle. Contact forces to the environment
may seem like they are only limited by the strength of whatever is supporting the body, but it can also be
limited by friction and by the pressure on the contacting tissues; if you have a stone in one shoe you will
very likely put less weight on that foot than on the other.
So the muscles of the body in addition to creating equilibrium are constrained by the available contact forces
to the environment, and these often have different limits in different directions, typically a high limit in
compression perpendicularly against the supporting surface, a smaller limit for friction tangentially to the
surface, and no reaction available in tension. Mathematically and mechanically this is very much how
muscles work, and the conditions therefore affect the mechanics of the entire system much like muscles do
and can be mimicked by means of general muscles.