Technical Artist and Character Rigger

The models shown are weighted to a maximum of 3 influences per vertex.

Below are the expressions to create the joint twists, and modify rotation values to mirror animations on joints.
Joint names, attributes, and values may be different for your scene depending on various factors such as joint orientation, geometry density, etc, and modify as necessary.

/////// Shoulder muscle twists /////////

LeftShoulderTwist.rotateX = LeftArm.rotateX * -.75;
RightShoulderTwist.rotateX = RightArm.rotateX * -.75;

/////// Hip/Thigh muscle twists /////////

LeftLegTwist.rotateX = LeftLeg.rotateX * -.75;
RightLegTwist.rotateX = RightLeg.rotateX * -.75;

/////// Mirror the rotatation values on the left leg over to the right leg /////////

RightLeg.rotateX = LeftLeg.rotateX*-1;
RightLeg.rotateY = LeftLeg.rotateY*-1;
RightLeg.rotateZ = LeftLeg.rotateZ;