Experiments building an arm rig using matrix nodes and attributes in Maya.
Helper joints at the elbow and biceps are driven via blendMatrix and setRange nodes.
An additional switch, driven by the hand supinating, blends the biceps joint's movement on and off. This recreates the different degrees of volume change seen in the biceps, depending on the rotation of the lower arm/hand.
Roll joints down the length of the arm created with blendMatrix nodes.
Additional, overall controls (to the side of the hand) allow quicker general posing of metacarpals and first phalanges as groups.