Specifications

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measure that is really a string that wraps just like a muscle but does nothing else than measure its own
length. These objects can be used outside the muscle definition for various purposes in the model, for
instance for definition of springs or rubber bands.
After you load the model with the added InitWrapVectors, try using the Step button rather than the run
button. This will show you how the system uses the InitWrapVectors to pull the muscle to the other side of
the cylinder:
If you keep pressing the step button you will see how the muscle now wraps on the other side of the
cylinder.
With the kinematics of muscles well under control, we can proceed to another important and interesting
topic, Lesson 5: Muscle models
.
Lesson 5: Muscle models
Muscle model is a description of how a muscle behaves under different operating conditions. There are two
schools of thought within this area.
• The first school pursues phenomenological models based on the classical work by A.V. Hill. These
models are usually based on a description of a muscle as a contractile element in combination with
a number of elastic elements. While these models make no attempt to directly model the
microscopic mechanisms of muscle contraction, they do reproduce many properties of muscle
behavior quite well, and most models of this class can be implemented with great numerical
efficiency.
• The second school attempts to directly model the microscopic physical phenomena of cross bridge
activity in muscle contraction. The origin of these models is usually attributed to A.F. Huxley
, and
they lead to differential equations and consequently to much more computationally demanding
models.
The AnyBody Modeling System requires muscle models because it must take the strength of different
muscles into account when distributing the load over them. A traditional muscle model is one that takes an
activation signal and a present muscle state as input and produces a force as output. But inverse dynamics,
as it is used in the AnyBody Modeling System, does not work quite like that. Instead of taking an activation
signal as input, AnyBody produces the muscle active state as output. This means that typical muscle models
from the literature must be mathematically reversed before they can be used in the AnyBody Modeling
System. Depending on the complexity of the muscle model, this may be more or less difficult.
AnyBody has three muscle models available differing in complexity and accuracy of their representation of
physiological muscles. All of these are phenomenological, i.e. they make no attempt to capture the
complexity of cross bridge dynamics. You may ask why we would want three different models? Why don't we
just use the better of the three models? The answer is that accurate models are good, but they are never
more accurate than the input data, and it is often difficult to find the detailed physiological data that the
complex models require. Instead of basing a computation on data of unknown accuracy it is often preferable