To set up a functional Spline IK system, follow these primary steps as detailed by Autodesk Help :
Drag the from the Asset Browser’s Constraints folder into the viewport. Define the Chain : Motionbuilder Tutorial - 18: Spline IK
: While MotionBuilder generates a default curve, you can also use a custom 3D Curve from the Elements folder of the Asset Browser for more specific pathing. To set up a functional Spline IK system,
: You can adjust the Control node twist axis , Reverse twist , and Twist interpolation settings to manage how the skeleton rotates along its length as the curve bends. Motionbuilder Tutorial - 18: Spline IK