Specifications

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For use in this tutorial, a slightly modified version of the model is provided here:
demo.diffmusarm2D.any
. Please download it, save it, and load it into the AnyBody Modeling System.
How does a real human body handle this situation? Actually, not a lot is known about how the human body
distributes force between redundant muscles. Compared to other scientific achievements, such as charting
the human genome, it may seem like a simple matter to figure out how much a given muscle is pulling on a
bone. But it is far from simple.
• Measuring force in the first place is not easy. It is always based on measurement of a deformation
of something.
• Measuring force in the human body is almost impossible because it requires insertion of
measurement devices into the body. This is subject to obvious ethical restrictions, and even if a
harmless measurement device could be inserted, it would be difficult to rule out the possibility that
it would influence the function of the body and thereby the results.
• Measuring muscle force is possibly the most difficult of all because it involves very large forces in
soft tissues.
• Even if we could measure a muscle force, it could only be done for particular situations, and it may
not reveal the overall strategy behind the body's recruitment of muscle forces.
So what do we know about muscle forces in the human body? Well, the following is generally agreed on:
• Although infinitely many different muscle activation patterns can produce a given movement and
balance given exterior forces, the recruitment is not random. For repeated movements there seems
to be a consistent pattern of muscle activation. In other words, it seems to be based on some
rational criterion.
• When several muscles are spanning a joint, they tend to collaborate. Although it may be enough to
activate one or a few muscles, the body tends to use all the available muscles.
• In many movements it can be observed that some muscles seem to work against the movement or
the exterior load. These are called antagonistic muscles.
• Large muscles provide more force than small muscles.
With the possible exception of the antagonistic muscles, all of this indicates that the body is trying to make
the best of its resources. In fact, it is also known that idle muscles quickly lose their strength. Similarly,
muscles that are exercised will build up strength. This is really the body's way of making the best of its
resources, and it leads to the suspicion that the recruitment of muscles is also based on some sort of
optimality criterion. The interesting point is that if the insufficient system of equilibrium equations is
augmented with an optimality criterion involving muscle forces, then the problem can have a unique
solution. This is precisely the basis of muscle recruitment in the AnyBody Modeling System.
The basic optimality assumption in the AnyBody Modeling System is that the body attempts to use its
muscles in such a way that fatigue is postponed as far as possible. This leads to the idea of minimizing
maximum muscle activity.
Before we proceed, let us investigate the concept of muscle activity. In the AnyBody Modeling System,
muscle activity is defined as muscle force divided by strength. Simple as that may seem, activity depends on
our definition of muscle strength. You may think of muscle strength either as a constant property of a
muscle or as something that changes with the operational conditions of the muscle. For instance, it is well
known that muscle strength decreases with contraction velocity, so that muscles contracting quickly have
less strength than muscles contracting slowly. This only depends on the kind of muscle model you choose,
and it does not change the fact that the AnyBody Modeling System will recruit muscles according to the
following criterion:
Minimize
(maximum muscle activity)
+ e1*(sum of activities)
+ e2*(sum of squared activities)