Specifications

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Model data uncertainties
In general, the models in the AnyScript Model Repository
are based on data reported in the literature. They
often come from studies of one or few subjects or cadavers, and the data has little or no statistical
significance. You will find the references of the data listed in the comments in the individual AnyScript files.
However, the fact that a specific fiber length or muscle insertion point has been found in an individual
cadaver does not mean that the value is valid for every individual or even typical.
In conclusion there is no guarantee that the values in models from any library are valid for the case you
may want to analyse, and the best advice is to approach the matter with a critical mind. If results look
suspicious in some part of the model, consider whether this can be due to the model input. Some typical
cases are:
• The muscle primarily responsible for carrying the load over a joint does not have sufficient strength.
This can happen even in well-tested models if an unusual loading, posture or support condition that
was never tested before is imposed.
• The model has attained a posture in which the moment arm of a primary muscle erroneously
becomes zero or negative. This can happen, for instance, if a wrapping muscle slides off its
wrapping surface. The variation of muscle length with the joint angle reflects the moment arm, so if
the moment arm is too small, then the muscle will have little length variation when the joint is
articulated.
• If the model makes use of a muscle model with stength/length variation and passive stiffness, then
a tendon length that is poorly calibrated to the model can cause malfunction of its muscle. A too
long tendon will cause its muscle to have little or no strength in its usual operation interval. A too
short tendon will cause a muscle to excert passive force and likely cause muscles on the other side
of the joint to work more than they are supposed to. Please notice that AnyBody has facilities for
calibrating tendon lengths. The muscle modeling tutorial
has in-depth information about these
issues.
Boundary conditions
Input to inverse dynamics is movement and boundary conditions, and these can have more influence on the
result than most inexperienced modelers would expect. In fact, they are the principal source of error in
many models. The human body is remarkable in its ability to make the best of the available supports, and
this often creates the illusion that supports are solid while they really are not.
Consider a hand gripping a handle firmly. Apparently, the hand is rigidly connected to the handle, and you
might be inclined to define a model connection between the two elements reflecting this notion. However,
hands have limited strength to hold on with, and handle surfaces have limited friction to offer. If the model
contrary to reality offers an effortless connection between the hand and the handle, then the model is likely
to exploit this as we shall see later.
Movement data
Recorded movement input such as motion capture data is usually in the form of positions over time. But
inertia forces in the model are derived from accelerations, and to obtain accelerations, the positional data
must be differentiated twice thus increasing noise and inaccuracies by two orders of magnitude. This is the
topic of the first lesson of this tutorial, starting at the bottom of this page.
Errors sources in the basic assumptions
As mentioned a couple of times already, the AnyBody Modeling System is based on inverse dynamics. This
means that - for a given point in time - the system solves the equilibrium equations and resolves the interior
muscle and joint forces. Since these time steps are solved independently of each other, the state can in
principle shift abruptly from one step to the next, whereas, in reality, a change of muscle tone requires a bit
of time. Force development in a muscle is the result of an electric signal from the central nervous system,