Your muscles are not able to keep up with their high-velocity animated bones.

This may be due to the Damping parameter settings on your muscle attachment constraints. For any Vellum constraint, as a point’s position is subjected to various influences (forces, collisions, velocity changes, and so on), the Vellum solver tries to satisfy all these factors to determine where the point should end up and whether or not the constraint is successful. The damping ratio that is applied to the muscle attachment constraints is designed to reduce the energy or contribution of the constraints during each substep of their muscle simulation. If the damping ratio is set to too high a value, then the contributions of the attachment constraints will be impeded.

First, try the following:

• Decrease the Damping value for the Muscle End attach constraint.

  For example, if your damping value is 0.01, then try decreasing it to 0.001.

If that does not solve the issue, then also try the following:

• Decrease the Damping value for the Muscle To Bone attachment constraint.

• Increase the number of Vellum Substeps and Vellum Collision Passes on the Muscle Solver.

You are seeing long triangles, tissue surface non-uniform stretching, and tetrahedral tangling in the areas where your tissue creases or folds in on itself (like the elbow and knee bend areas of bipeds and quadrupeds).

This is caused by the solver not being able to resolve an extreme tissue constraint vs. collision configuration (for example, like when the iteration frequency for the tissue’s constraints and collision passes are set to drastically different values). The visual result of this error is an uneven distribution of energy in the tissue.

The default Constraint Iterations value is 100 and the Collision Passes value is 10, which gives you 1 collision pass for every 10 constraint iterations. So with these settings (or with even stiffer constraints), the constraints will always win over the collision passes as the constraints get to update their points 10 times as often as the collisions get to.

To generate a much better result in the crease/fold areas of your tissue, try the following:

• Increase the number of Collision Passes. However, please note that collisions are expensive to compute.

• Make sure that the ratio between your tissue’s Constraint Iterations and Collision Passes values is much closer to a 1:1.

• Alternatively, you can also try increasing the number of Substeps instead of the number of Collision Passes.