Specifications

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The interesting point here is that with the long tendon and the high load, the muscle no longer contracts
uniformly. In fact, the muscle extends for much of the movement due to the decreasing load which causes
the elastic tendon to contract instead.
While the two-parameter muscle model captures many of the properties of real muscles it also fails to reflect
important parts of muscle physiology, so it should be applied with care. In particular it does not model
passive elasticity. The following section presents a full-blown Hill-type model, which does not have these
shortcomings.
AnyMuscleModel3E
So far we have been focusing our attention on Muscle1 in the demo model and left Muscle2 with the simple
muscle model. Let us briefly study what Muscle2 is actually doing (if you need an updated working model,
you can download it here: MuscleDemo.5-2.any
). Muscle2 wraps about the cylinder and obviously extends
significantly as the arm turns upward. If you run the analysis and plot the length of Muscle2, you will see
that it increases 0.7 to 1 meter. For a normal muscle (actually a muscle of this size would probably be found
in a giraffe) a stretching of that magnitude would almost certainly lead to some passive force in the muscle.
Passive force is what comes from the structural integrity of the muscle. If we disregard the active properties
of the muscle and think of it as simply a piece of material that we can stretch, then the material will proide a
passive resistance depending on how far we stretch it. This is the passive component of the muscle force.
We can easily find passive muscle force in our own bodies: When we bend forward and try to touch our toes
with the straight legs, then most of us will feel the hamstrings getting very taut. This is passive elasticity.
The two-element muscle model of the preceding section handles the presence of this elasticity by increasing
the strength of the muscle, and this works fine if the muscle is supposed to be active in the sense that the
model in such a state would predict a high force with a low muscle activity. But the passive muscle force
cannot be switched off, so it will still be present even if it is disadvantageous, and the two-element model
will not predict this.
The AnyMuscleModel3E is a full-blown Hill-type muscle model that does not suffer from this deficiency. It is
called a three-element model because it has the following components:
1. A contractile element (CE) representing the active properties of the muscle fibers.
2. A serial-elastic (T) element representing the elasticity of the tendon.
3. A parallel-elastic element (PE) representing the passive stiffness of the muscle fibers.