How to animate stretchable muscles? I want to create animations of muscles on the human skeleton like this, especially from 17 seconds in. 

How can I create stretchable objects/muscle with Blender? I've seen x-muscle system but I'm not sure if it's overkill since I basically just need a stretchable material. X-Muscle System creates a character's skin after adding muscles which isn't what I need.

Also does anyone recognise the software used for the linked video? I've seen that little white man character online somewhere but can't remember which software it was for.
I think you should use IK constraints, expecially with this stretch function:



Also "Mantain volume" bone constraint can be useful.