Specifications

142
Detailed calibration
There is a more accurate and detailed way of calibrating tendons, but it requires additional information.
More precisely it calibrates the tendon lengths at user-defined joint postures. So each muscle is associated
with a certain set of joint postures for which the muscle is presumed to be at its neutral position. Please
notice that the set of neutral joint postures is in principle different for each muscle in the system. In practice
the calibration usually takes place on sets of muscles at a time, where all the muscles in a set is calibrated
at the same joint postures. This way, all the muscles in a complicated system can be calibrated with a
reasonable number of operations.
Detailed calibration of tendon lengths is covered in the "Study of studies" tutorial
. Calibration of ligaments is
much the same type of process and is described in detail in the Ligament tutorial, Lesson 7
.
But before we come to ligaments we must cover one last aspect of muscle modeling, namely General
Muscles. They are the topic of the next lesson, Lesson 6 (coming soon).
References
Hill, A.: The heat of shortening and the dynamics constants of a muscle. Proc. Roy. Soc. B., 126, 136-195,
1938.
Huxley, A.: Muscle structure and theories of contraction, Progr. Biophys. 7, 255-318.
Zajac, F.E.: Muscle and Tendon: Properties, Models, Scling, and Application to Biomechanics and Motor
Control. Critical Reviews in Biomedical Engineering, 17, 359-410, 1989.
Lesson 6: General muscles
Physiological muscles are truly amazing machines, and despite many attempts it has not been possible to
make technical actuators that are as light and efficient as natural muscles. As you may have seen in the
preceding sections, the mathematical modeling of muscles is not an easy task either. But once it has been
done, we can use some of the properties of muscles to our advantage. We would like these "muscles" to be
able to have a slightly more general formulation than physiological muscles, which are confined to acting
along strings.
The solution is the AnyGeneralMuscle class. This type of muscle is capable of acting on Kinematic Measures.
Kinematic measures is an abstract class representing anything you can measure on a model, and there is in
fact an entire tutorial lesson devoted to the subject
in the section on The Mechanical Elements. Some
examples are:
• A general muscle working on a distance measure between two points becomes simply a linear force
provider, or in fact a reaction provider in the sense that the force is not predetermined but will
become whatever equilibrium requires.
• A general muscle working on an angular measure, for instance a joint angle, becomes a torque
provider.
• A general muscle working on a Center of Mass measure becomes an abstract force working on all
segments of the body contributing to the center of mass.
This lesson demonstrates how general muscles can be used for a variety of modeling tasks.
Muscles as joint torque providers