Specifications

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The parallel-elastic force sets in when the muscle is stretched beyond its optimal fiber length. In the
movement of this example, the elbow starts at 90 degrees flexion, and as the graph shows, this gives rise to
about 10 N of passive force at the beginning of the movement. This indicates that the tendon we have
specified is too short. If the movement was extending the elbow instead of flexing it, the passive force would
rise sharply. This means that the result of the simulation depends a lot on having the correct length of the
tendon. If it is too short, too much of the load will carried by passive muscle forces. In this example where
we have only one muscle with a complex model, it would not be too difficult to adjust the tendon length
manually until we get it right, but in models with many muscles, this can be a very tedious task, particularly
since it has to be repeated every time the dimensions of the model are changed. Instead, the answer is to
let AnyBody calibrate the tendon length automatically.
AnyBody's advanced muscle model, the AnyMuscleModel3E, is basically a phenomenological model based on
the classical works of Hill. It presumes that each muscle has an optimum contraction in which its fibers have
their best force-producing capability. If we knew the set of joint positions corresponding to this optimum
fiber length for each muscle, then those joint positions were the ones we would be calibrating the muscle in.
Finding joint positions corresponding to optimum muscle fiber lengths is an active and rather young area of
research, and the correct values are only known for a few muscles in the body. However, it is not surprising
that we seem to have been built in such a way that our muscles attain their optimum fiber lengths in the
joint positions where they do most of their work, and if you are unable to find the information about
optimum joint positions you need, then your best choice may be to calibrate the muscle in the joint position
where it primarily works.