Specifications

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muscle model.
• EPOTmt is the total elastic potential energy in the muscle-tendon unit.
• Pt is not relevant for this muscle model.
• Pm is the mechanical power exerted by the muscle's contractile element.
• Pmt is the mechanical power of the muscle-tendon unit on the skeleton.
• Pmet is a crude estimate of the metabolic power consumption of the muscle taking different
efficiencies for concentric and eccentric work into account.
We have seen how the length of the muscle affects its strength, but what about the velocity? Well, the
specified values of -8 m/s is a reasonable estimate for many physiological muscles, but let us try to
decrease it and thereby make the muscle more sensitive to contraction velocity:
AnyMuscleModel2ELin Model2 = {
F0 = 200;
Lfbar = 0.3;
Lt0 = 0.5;
Epsilonbar = 0.05;
V0 = -0.3;
};
A value of V0 = -0.3 is close to the contraction velocity of the muscle in the beginning of the simulation.
This, this decreases the strength of the muscle significantly as we can see by reloading, rerunning and
plotting the Strength variable again:
Instead of being monotonically decreasing, the muscle strength now improves slightly in the initial part of
the simulation, but it is all through the simulation significantly weaker than before. The initial increase is due
to the beneficial effect of the decreasing contraction velocity, so this muscle model in spite of its simplicity is
capable of balancing several of the effects of real muscle physiology.
Another of the important input parameters in this example is the nominal tendon length, Lt0. This is a
parameter that has a very large influence on the muscle's performance. The total origin-insertion length of
the muscle-tendon unit depends on the size and posture of the body. The muscle spans this length with the
sum of muscle length, Lm, and tendon length, Lt, such that Lmt = Lm + Lt. Both Lm and Lt change during
the movement of the body. Lt is given by its initial length, Lt0, and the elastic deformation. Lm has to take
up whatever rest of Lmt that is available after Lt has been subtracted. In some cases, the tendon is
significantly longer than the muscle, and this means that a relatively small variation of the tendon length
results in a large relative variation of the portion of Lmt that the muscle has to fill. Obviously Lt0 plays a
significant role for Lt and hence influences the working length of the muscle. Let us investigate this effect by