I have been experimenting recently trying to create different rigs and always loved the look of the Fallen from Destiny. However I’ve been intrigued into creating a 4 arm animation system in Motionbuilder.

My idea was to setup the main arms in line with the normal human biped standards for HumanIK. This mean aligning the top arms with the X axis and setting up the bone connections to HumanIK character definition.

Once the character has been defined in Motionbuilder the next step was to setup a custom relation to mimic the arm data from the main arms. So it would go:

MainArm -> rotation linked to -> SecondaryArm

This would be replicated for the entire tree of each arm bone. Once these relations have been setup you'll see that the bottom arms will now the top arms motion, however we would want to edit it and change the offset of the rotation in case of penetration or changing the look. The best way to achieve this is to add a Rotation Add node to the relation constraint.

Once all the bones have been linked this way we can apply some test motion capture to see what the result would look like. I applied the Box_Jab previs motion capture from Motionbuilder's previs folder and this is the result so far.

But what if we wanted to add custom animation simply copying the rotation from one arm to the other wouldn't work, say we wanted to create a take with a roar or provoking the player. Let's take a look at how we can achieve that with Chain IK.

Above you can see the Chain IK setup with the first and end joint in the arm - normally the UpperArm and Hand and then we create an effector or where we want the IK to reach to, and then a Pole Vector to control the elbow orientation. You can add a floor in to calculate when the object is passing through it to limit it as well.

This creates a nice looking and easy to control system that will benefit from the IK setup. I've also added another Relation to copy the rotation of the effector to the hand so it's easier to pose without jumping around between bones. Take a look.

That's it for this look at creating a rig with 4 arms in Motionbuilder. There's many more options to consider like damping rotation from the main arms to the secondary arms to create a delay which looks nice for idles. I hope you've found this interesting and if you have please leave a comment down below!

*Destiny Fallen model obtained from CGTrader for free by author 3DGeneralist.