10.15.2007 at 05:52 |  
In this section we present three anatomy-based muscle models forsimulating the behavior of skeletal muscles. Before doing so, however,we discuss the representation of muscle bellies.

Muscle bellies

We use ellipsoids to represent muscle bellies. As Wilhelms argues, the ellipsoid is a natural and convenient primitive forrepresenting muscle bellies because it can be scaled along its threemajor axes to simulate bulging. We automatically adjust the dimensionsof the muscle belly when its extremities are moved furtherapart or when they are brought closer together. These adjustmentsnot only preserve the ratio of the belly’s height to its width, but alsothe volume of the muscle belly—an approach justified by consideringthe anatomical structure.

Fusiform muscles

Many skeletal muscles are fusiform and act in straight lines betweentheir points of attachment. For these muscles we use a simplemodel with relatively few parameters, called the fusiform musclemodel. This model provides a convenient mechanism for locatingmuscle bellies relative to underlying skeletal bones. Specifically,since muscles attach to different bones, the origin may be given inthe local coordinate system of the bone where the muscle originates.Similarly, the insertion may be given in the local coordinate systemof the bone where the muscle inserts. Muscles with tendons may bedefined by giving two additional points, as illustrated in Figure 6.The model takes care of transforming all the points to a commoncoordinate system.Like the joint types, the fusiform muscle model isimplemented in a class. We use two class parameters to define thevolume v and ratio r of the muscle in its natural state, and a numberof instance parameters to specify the location and orientation of themuscle.Two fusiform muscles of the same volume are modeled, but only one has tendons.Notice the effect of the tendons on the perceived bulging ofthe muscle belly on the right. Notice also that the tendons retaintheir lengths, an important attribute of tendons which is not incorporatedin Wilhelms’ modeling of animal muscles.
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