Specifications

123
F0 = 0;
Lfbar = 0;
Lt0 = 0;
Epsilonbar = 0;
V0 = 0;
};
Let us briefly review the parameters:
Parameter Function
F0 In the simple muscle model, F0 is simply the strength of the muscle. In this two-parameter
model, F0 is the ideal strength, i.e. the strength of the muscle at neutral fiber length and zero
contraction velocity. F0 is measured in force units, i.e. Newton.
Lfbar The neutral fiber length, i.e. the length of the contractile element at which the muscle has the
strength of F0. Lfbar is measured in length units, i.e. meters.
Lt0 The muscle's total length from origin to insertion can be divided into two parts: the length of
the muscle's contractile element plus the length of the tendon. The tendon is considered in
this model to be linearly elastic, and Lt0 is the slack length of the tendon, i.e. the length
when it is taut but carrying no force. Lt0 is measured in length units, i.e. meters.
Epsilonbar This parameter controls the elasticity of the tendon. The physical interpretation is that it is
the tendon's strain when subjected to a force of F0. Prescribing a strain rather than an
ordinary spring stiffness is based on the idea that the tendon thickness must be related to the
strength of the muscle: strong muscles need strong tendons. Hence, Epsilonbar can be
presumed with good accuracy to be the same for a wide variety of very different muscles.
Epsilonbar is measures in fractions and is therefore dimensionless.
V0 This model presumes that the muscle's strength depends linearly on its contraction velocity.
V0 is measured in absolute velocity, i.e. m/s.
We can study the significance of the parameters in more detail, if we formulate the strength mathematically:
You can probably recognize the variable names in the table above from the symbols in the equation. As you
can see, this is really a bilinear model, where the variables are Lm and Lmdot. The strength of the muscle
vanishes is any of the two parentheses becomes zero. This can happen if either Lm, i.e. the current length
of the contractile element, becomes half the length of Lfbar, or if Lmdot becomes equal to V0. Please notice
that Lmdot is negative when a muscle is contracting, so meaningful values of V0 must also be negative. The
system automatically truncates negative values of the strength expression to zero.
In a few moments, when we start playing around with the muscle model in AnyBody, you wll recognize
these properties in the available muscle variables in the Chart View.
Let us assign a name and some reasonable parameters to our two-element muscle model:
AnyMuscleModel2ELin Model2 = {
F0 = 200;
Lfbar = 0.3;
Lt0 = 0.5;
Epsilonbar = 0.05;
V0 = -8.0;