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A.1: Define anterior and posterior halves of the cat

Both halves of the cat have the same inertial properties (mass, mass center, moments of inertia). Nonetheless, we specify different values to setMassCenter() below. For this reason, among others, we hope to clarify what this property means in OpenSim.

Each body comes with a reference frame in which the body is fixed. This reference frame has an orientation and a position, or equivalently, an origin. Now, as modelers, we know where the mass center of each body is, independent of any reference frames. However, the setMassCenter() requires that, when we communicate the mass center to OpenSim, we express the mass center location from the body's origin, and in the basis (orientation) of the reference frame. In doing so, we actually define the reference frame (that is, its origin and orientation in the body). The mass center is fixed regardless of how OpenSim internally represents it; it is the definition of the reference frame that we can vary.  We could specify the mass center to be (0, 0, 0); this would just mean that the origin of the body's frame is at its mass center; it would not affect the inertial properties on its own. What it doesaffect is how we subsequently express the position and orientation of joints and display geometry in the body's frame (which, again, we defined at our discretion).

 // Prepare inertia properties for the 2 primary segments of the cat.
double segmentalAxialMomentOfInertia = JIratio * segmentalTransverseMomentOfInertia;
double Ixx = segmentalAxialMomentOfInertia;
double Iyy = segmentalTransverseMomentOfInertia;
double Izz = segmentalTransverseMomentOfInertia;
double Ixy = 0;
double Ixz = 0;
double Iyz = 0;
using SimTK::Inertia;
Inertia segmentalInertia = Inertia(Ixx, Iyy, Izz, Ixz, Ixz, Iyz);

// Anterior half of cat.
using OpenSim::Body;
Body * anteriorBody = new Body();
anteriorBody->setName("anteriorBody");
anteriorBody->setMass(segmentalMass);
// By choosing the following as the mass center, we choose the origin of
// the anteriorBody frame to be at the body's positive-X extent. That is,
// the anterior body sits to the -X direction from its origin.
anteriorBody->setMassCenter(Vec3(-0.5 * segmentalLength, 0, 0));
anteriorBody->setInertia(segmentalInertia);

// Posterior half of cat (same mass properties as anterior half).
Body * posteriorBody = new Body();
posteriorBody->setName("posteriorBody");
posteriorBody->setMass(segmentalMass);
// Posterior body sits to the +X direction from its origin.
posteriorBody->setMassCenter(Vec3(0.5 * segmentalLength, 0, 0));
posteriorBody->setInertia(segmentalInertia);

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